Eurorack modular 8085 computer - Page 1
Forum section: electronics / general

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2016, 12:37:44 am
content: Hey! I'm currently setting out to buildthe8085 system, and thought I'd share.The computer is going to be modular, with each card being as universal as possible.I'm sticking strictly to the 19'' 3U subrack Euro Card Bus standard, A) because it's not very widely seen in DIY computers and B) because it's just a convenient form factor, I think.Mechanically that means:- 3U subrack with DIN 4.1612 AC connectors to backplane- PCB sliding rails- Either proper eurorack front panels with handles (a bit pricey) or pcb's without handles- 16cm x 10 cm std. vero-boards for creating the circuits on (probably with vero-wire)Computer wise, I plan on distributing the boards as such:- CPU card: 8085, clk, bus drivers, reset- RAM/ROM card(s): Plan on going for 8k*8 RAM/ROM - will have decoding and type patchable for universality.- Serial card: (8250/8251), baudrate gen, etc- Display/keypad card: Hex keypad and 7seg displays, either with an 8279 or some 8255'sDisplay and keypad won't be on the module plate. I think I'll add a separate 3U front plate above for that. (and other controls, too) - Just neater that way.- Cassette interface card (for when I get a 19''rack reel to reel for it)- 8255 parallel centronics printer interfaceAand in the further future:- CRT card (I havenotlooked into driving a CRT- ASCII keyboard card (not difficult, but not that useful without a CRT as display)- Punched tape interface (Yeah, like I ever find a rack paper tape reader)- 8088 CPU card (maybe)I'll try and stick to 74LS and HCT logic (I have so much LS, but HCT would be easier on the PSU)And 82XX intel peripheral IC's.I'm aiming for using the standard ECB pinout for the bus, but I have to make some modifications. The standard has specified z80 interrupt lines, but no /INTA for intel CPU's.Lastly, I need some bus termination, I think. I plan on just adding that to the bus side of the CPU card, as you'd usually always have that installed. I think passive resistive termination is enough for now.Currently working on the power supply, which will be 5V, at first. Later, a +5 +-12V will probably be needed.I'm not sure if I should have more features on my CPU card than I've planned. Right now, it's literally only the CPU module.Let me know what you think! Has anyone ever worked with the 3U subrack /ECB standard?Is there any cards / functionalities you'd add?--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 21, 2016, 01:13:03 am
content: Surely with specs like that (8085 etc), you may want to try/use CP/M. Hence want at least a (8085 era) floppy, to more easily allow that.I never make jokes, on forums. With your 8088 cpu, and paper tape unit, by creating 8088 compatibility versions, of some of the newer (needed) instructions. You can download/transfer win10, onto 1,472,446,284 miles of paper tape. Then after about 345,347,399,585 hours, to read the paper tape, you can play with win10.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2016, 01:56:17 am
content: It would at one point be nice to be able to run some "real" software, and not just small homemade routines.5 1/4 inch floppy drive (dual?) would be convenient, if not necessary for that. I'll add that to the list!Running win10 from paper tape would be "a bit" ...ambitious... but If I do upgrade to 8088, it'd be nice to be able to run IBM DOS.--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 21, 2016, 02:03:18 am
content: Quote from: ChristofferB on March 21, 2016, 01:56:17 amI think you would find, trying to run IBM DOS (or similar), a MASSIVE undertaking. Because (presumably), you would have to turn the 8088 system, into a COMPLETE (compatible) IBM PC, of that era.E.g. CGA graphics (or whatever standard applies with it), BIOS (compatible with both IT, AND your system), all sorts of other bits and pieces, adding to the major headache.BUT it may be possible to get hold of a FULLY open sourced "DOS", by the time you are ready. Which lets you configure/compile/tweek it to your particular hardware.

author: MK14
author_rank: Super Contributor
timestamp: March 21, 2016, 02:05:07 am
content: Quote from: MK14 on March 21, 2016, 02:03:18 amI think you would find, trying to run IBM DOS (or similar), a MASSIVE undertaking. Because (presumably), you would have to turn the 8088 system, into a COMPLETE (compatible) IBM PC, of that era.E.g. CGA graphics (or whatever standard applies with it), BIOS (compatible with both IT, AND your system), all sorts of other bits and pieces, adding to the major headache.E.g. Good luck with building your "Compatible Chip Set", out of 1,000? raw TTL 74LS/HCT devices.BUT it may be possible to get hold of a FULLY open sourced "DOS", by the time you are ready. Which lets you configure/compile/tweek it to your particular hardware.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2016, 12:15:51 pm
content: It would be a pretty big task, for sure. At this point that's also only some far-fetched dream.But the original IBM PC had only a handful of specialty IC's as chipset: CPU, DMA, Interrupt controller, timer, and some peripheral interface (8255). But I do think CP/M would be the better alternative.--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 21, 2016, 12:34:25 pm
content: Quote from: ChristofferB on March 21, 2016, 12:15:51 pmI completely agree.CP/M opens up a world of opportunity, with a huge range of software. Its programming language interpreters/compilers, for example, give tremendous ability to play with it for hundreds of hours or more.The CP/M software base, will really bring your computer to life, and maybe bring back memories of fun computing times, from a long time ago.I agree the original IBM PC, is NOT that bad, to recreate in hardware. Complicated chipsets, were later.The 8085 (8080), is a good choice, because (in real terms), it (8080), was the first, seriously successful microprocessor. There were some before it, but they never really hit the market big time, especially as regards computer systems.I.e. Apart from calculators and display terminals, 4004/4040/8008 were NOT in many computer systems, from a quantity point of view.The Z80 (a sort of semi-illegal, improved clone of the 8080/8085), has been one of the most successful microprocessors, of all time. With the Arm probably being the MOST successful, or the X86, depending on ones opinion.I hope your vero wire, DOES NOT give off toxic fumes. Someone warned me against it, because it can/does give off highly toxic/poisonous fumes. Assuming you are talking about the stuff, which is covered in insulation, which "boils" away, when soldered at appropriately high enough temperatures.I had bad/poor experiences with my early (long time ago), vero wire constructions, whereby I had TOO MANY non-connecting joints, to viably build stuff.Looking back, it was probably because I had a relatively POOR quality, mains type (SCR/TRIAC in the handle), of temperature controlled soldering irons. I should have got something of better quality (at that time).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 21, 2016, 12:53:41 pm
content: QuoteThat's one of the reasons I'm going for the 8085. 90% of homebrew vintage-ish computer are z80 based. I also really like the intel 82xxx series support chips over the z80 based ones, and I have a lot of them.My vero-wire DOES have the fuming issue (it all does, as far as I'm concerned), so I make sure to have adequate ventilation and hold my breath when soldering it.Vero-wire seems very picky about soldering iron. I've found that a high thermal capacity iron at medium temp. is pretty good. The tips I use are flat-hammered and chisel-ground desoldering tips with my PACE vacuum desoldering station. Works pretty o.k.I finished the PSU enclosure, see attached, it's built from an old 24V euro-rack module.--Christoffer

author: Fungus
author_rank: Super Contributor
timestamp: March 21, 2016, 01:50:41 pm
content: Quote from: MK14 on March 21, 2016, 02:05:07 am

author: MK14
author_rank: Super Contributor
timestamp: March 21, 2016, 02:32:18 pm
content: Quote from: ChristofferB on March 21, 2016, 12:53:41 pmThanks for the information, on how to use the Vero system, more reliably and safely. The powerful/meaty desoldering station, makes a lot of sense. As it (I would imagine) has the raw heating power to reliably melt the insulation. Getting PCBs made (externally), is quite expensive, and time consuming (to design each one).So just quickly vero wiring the board(s), is a quick and convenient solution to that issue.The relative lack of surface mount parts and relatively low frequencies, mean that, that method, is perfectly suitable for such projects.Your power supply, looks VERY authentic. It looks like a genuine 1970s, computer/electronic item, and built VERY robustly. Lots of cooling power, visible.Quote from: Fungus on March 21, 2016, 01:50:41 pmThat's a good point. So sensible creating of the BIOS in the new computer (assuming the software exclusively uses the BIOS etc, only), can allow partly non-PC hardware, to be coped with. Via adapted software (BIOS).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2016, 05:39:55 pm
content: QuoteI think if you lay out your boards properly, and use vero-board with ground plane, you can use vero-wire (and wire-wrap) at pretty high frequencies (for the 80's) - There's a guy actually doing surface mount vero-wiring of boards:http://elm-chan.org/docs/wire/wiring_e.htmlIncredible stuff..I've thought about diagnostics/development cards in addition to the afforementionned cards.-A bus monitor cardmight be nice to have, in the style and function of the altair front panel.-Memory access card:A German Euro Card Bus // eurorack enthusiast made a card to decode the regions of memory accessed, by means of 74138's and latches. Looked very useful.-Single-step and instruction step card:These existed for the S-100 (ALTAIR/IMSAI) systems, allowing single-stepping either one cycle or one instruction (8, 16 cycles i think).Any other debug module one could add?--Christoffer

author: woodchips
author_rank: Frequent Contributor
timestamp: March 22, 2016, 06:26:02 pm
content: Might be worth considering using 6U cards otherwise half the card will be buffer chips.

author: MK14
author_rank: Super Contributor
timestamp: March 22, 2016, 06:27:08 pm
content: Quote from: ChristofferB on March 22, 2016, 05:39:55 pmThat is quite amazing. Offhand, I would have simply dismissed vero wiring, surface mount IC's (excluding 0.1 pitch conversion boards) as being somewhat impossible. Because they don't do 0.1" pitched IC sockets.Ground planes, and improved frequencies, is interesting as well.Some of the parts (IC's), that are required for projects like these. Are becoming increasingly difficult to get hold of, in through hole packages, such as DIPs. So that method (you linked to), could still be useful, even on something like a mostly/all DIP IC packaged computer project. Because the odd chip(s) here and there, may be unavailable to you.E.g. DIP SRAM chips.So having that solution as a backup plan, in case you can't get hold of all the bits you need, is very useful.Quote from: ChristofferB on March 22, 2016, 05:39:55 pmIf you are trying to make me REALLY jealous and want to play with your computer, when it is finished. You are doing a VERY GOOD JOB!Yes, the huge panels, with tons of flashing LEDs and nice quality toggle switches (or whatever), still looks really COOL!Quote from: ChristofferB on March 22, 2016, 05:39:55 pmIf it was me...There are a (somewhat) large number of modern "logic analyser", capability devices available, at reasonable prices, these days that I have heard about.So unless you are trying to build it, only using equipment/tools that were available in the 1980s. Then I would seriously consider giving them a look into.But building dedicated debug stuff, makes lots of sense as well.In those days (and a decade later), there were VERY powerful (but sadly very expensive at the time, such as >$10,000), "proper", stand alone logic analyzers. They were amazingly powerful, and very, very useful.You could capture the code using various triggers, while it was running at full speed, in real time.When it triggers, you could see a fully disassembled listing (scroll-able) on screen, which was very useful for seeing what was going on.Presumably such logic analysers (or their modern equivalents), are available used (or new in the case of the modern ones), on ebay (or test equipment sellers for new/modern stuff). At reasonable prices.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2016, 06:35:07 pm
content: I actually have a nice late 80's 40 ch stand alone logic analyzer. But being able to debug via a couple of LED's would be a lot quicker (and NO vintage computer is complete without TONS of flashing LED's)Another issue iswhichEurope Card Bus pins to let deviate from the standard to make this useful with the 8085 and other 8-bit Intel's. D8-15 can of course go, if you never intend to go 16 bit, but the standard states it was developed for the z80 and 8080/8085, so there must be a standardized way of doing that.--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 22, 2016, 07:08:47 pm
content: That makes perfect sense.Taking the big/massive/heavy logic analyser, is quite a bit of work. Many/most things DON'T needs its capabilities.So a quantity of permanent front panel LEDs and maybe seven segment displays (depending on what you want, I think you prefer the LEDs by the sound of it), should work out just fine.I find hanging a number of LEDs (or at least one), onto I/O pins (or PIOs or whatever in your case), useful. Because you can set/clear/flash them in various places in the code. So if it crashes (totally), and you have no idea why/where. You can move their on/off settings, throughout your code, until you find out where it is crashing,Also such outputs, make useful scope/logic analyser trigger signals as well.Some people delight in using serial ports. They then send diagnostic information, maybe continually to it. So if it goes wrong, suddenly. They can trace through (what might be a big file), and see where/how it failed. Using a PC or something to record the data. E.g. "Starting xfer operation". "Finished Ram init.". Etc. Via printf's or whatever.But yes, LEDs and switches, seems to be the direction you are going, which sounds really good.Two birds with one stone. You can use it to debug your system, and then later have some really cool flashing lights, which will look really impressive and authentic (for the 1980s).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2016, 07:22:26 pm
content: Right, that's the plan. I like serial ports too, but software-wise the LED's are faster and easier to deal with. I'd love some numeric displays, but I hate the fact that there are no common available 4 bit to hex display drivers. And the single hex digit displays with built-in 4bit decoder are VERY expensive.So LED's it is.Maybe I should add a jumper to disconnect the CPU clock from the board and have a slow clock module. Though that might cause issues with other boards..

author: MK14
author_rank: Super Contributor
timestamp: March 22, 2016, 07:35:54 pm
content: It is indeed a big pain, there are no commonly, reasonably priced hex displays and/or driver chips.A long while ago, I managed to bag, a modest pile of these:at reasonable cost. As I think they look REALLY cool, and are very quick and convenient, for such projects.Additionally, I got some similar displays, but they are actually alphanumeric (I also managed to bag a modest quantity of those, as well, if I remember correctly, if I'm mixed up, then they are seven segment and/or hex, rather than alphanumeric).I think your idea of slowing or even stopping the clock (as long as compatibility issues and any (if applicable, you could be all sram) Dram refresh issues, are resolvable), is a good one.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2016, 09:16:04 pm
content: Those are the ones I meant!If one were really smart, one would use a MC14411 baud rate generator for the slow clock card, having a switch to select between clock rates, all the way from xtal freq to 300 hz.-This board would be an obvious place to put the single step/instruction step circuit too.Farnell sells the card front panels in kits with all hardware, that seems to be the cheapest option atabout 6£ for the 4 hp one:http://uk.farnell.com/schroff/20818-018/card-frame-plug-in-3u-4hp/dp/1455894I couldn't buy 1,5mm aluminium plate and loose hardware that cheap (unless I planned a LOT of modules)--Christoffer

author: rfeecs
author_rank: Frequent Contributor
timestamp: March 22, 2016, 10:13:56 pm
content: You are probably familiar with the N8VEM project. It sounds similar:https://en.wikipedia.org/wiki/N8VEMThey have done a lot of EuroCard Bus boards, though it doesn't look like any use an 8085:https://www.retrobrewcomputers.org/doku.php?id=boards:ecb:startHere's a project using an 8085 combined with PICs that runs the old Altair software and CP/M,wire wrapped construction:http://mini-altair.tripod.com/

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 22, 2016, 10:46:49 pm
content: QuoteWow, their floppy board and video/keyboard board looks really interesting!Neat looking Altair compatible too! Might just grab some ideas from that.--Christoffer

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 02:49:01 pm
content: I've completed design of the 8085 CPU module.It's pretty minimalist, and only uses one of the 8085's interrupts (it has 5) -and then int acknowledge, thus I can just use the Intel 8259 interrupt controller to get more interrupt options.I've included the HOLD line, that also controls all (most) tri-state gates to the bus, allowing either DMA or punching code in via front panel, Altair-style.-See attached.--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 23, 2016, 03:00:54 pm
content: I hope you don't mind me criticizing the circuit.You seem to have one or more (such as the SID pin), unconnected input pins on the MCU (8085). I believe that is NOT recommended on the 8085 (or most ICs). (obvious solution, to tie them, with resistor or whatever is recommended in datasheets, rather than leave them floating).The reset pins, resistor/capacitor, does not have a diode across the resistor. The possible problem with that way of doing it, is a very fast power on/off/on cycle, may be able to NOT actually properly reset the 8085.The diode forces the capacitor to rapidly discharge when Vcc is switched off, potentially giving very reliable reset operation.Disclaimer:I'm NOT a 8085 guru. So I could be wrong.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 03:11:57 pm
content: Thanks for the feedback!I never quite got the diode seen in power on reset circuits, thanks for clarifying that!And you're right, there shouldn't be floating inputs, I'll work that out too.I'm still undecided whether I should make dedicated RAM and ROM boards, or a combined.Dedicated allows upgrade of the one without messing with the other (could have multiple ROM cards with different software)Combined is obviously more space/board load optimized - but with room for 28 boards that's not very critical.-Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 23, 2016, 03:44:42 pm
content: Having combined RAM/ROM board(s), or separate ones, both have their own, pros and cons.Looking at this page (I know it is different to what you are doing):http://www.s100computers.com/My%20System%20Pages/RAM+ROM%20Board/RAM+PROM%20Board.htmThe apparent high complexity, high packing density and high complexity with configuration switching. Gives me a slight inclination to say, go for separate RAM/ROM boards.There have been times, when ROMs (including Flash(if available/invented in that era?)/EPROM etc) were too slow, to run at full MCU speed, without waitstates. Nowadays, you can easily get very fast ones. Depending on how authentic, you want to go.Off-hand, I'm NOT sure if when the 8085 was introduced, current ROM/EPROM/FLASH technologies, were fast enough for full speed operation.Switching ROM (only) boards around, to change OSs or boot code etc. Is a useful advantage.Expanding memory is also an advantage.Interestingly, the 8085 may actually be qualifiable/used as a spaceworthy cpu. That is a good sign.Apparently there is some talk of "secret" 8085 instructions.https://en.wikipedia.org/wiki/Talk%3AIntel_8085Quote

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 04:24:08 pm
content: That S-100 board does seem rather complex! But it seems it's also intended to be able to run 16bit.I don't think the speed is problematic in my case, if it turns out to be, I can always switch to a bit faster memory - I'll probably stay in the 2-4 MHz clock range.I'm drawing up a 4K RAM board at the moment, featuring little more than a 74138 decoding for 2 6116 2K sram's. I've added a 16 pin socket as jumper between the '138 outputs and the /CS lines you you can map the 4K where it wouldn't interfere with booting.This leaves plenty of room for expansion (even on the same card) if needed.An EPROM card would be almost identical, of course. If one were to create a pcb for these, you'd make them identical and allow ram/rom/mapping with jumpers of DIP switches.I've also added a jumper on the /WR line, to allow for a "memory protect" switch to be placed on the front panel, disabling writing to memory - This might be nice if you've punched in manually and don't want the machine plowing through your code.

author: MK14
author_rank: Super Contributor
timestamp: March 23, 2016, 04:41:53 pm
content: Just a few comments.If (rather than "wasting" 5 inverters) you used 2 of the spare inverters, to (optionally) invert A14/15, you could fully map the memory board on the entire 64K.I.e. You use on board jumpers/switches to choose which of the 4 16K memory blocks (by choosing the non-inverted OR inverted signals), activate the ram board.But that is fine, if you are happy to leave it in that 16K segment.Is 4K enough, there seems to be room for 4 x 2K = 8K. But 4K is fine, if that is what you want ?The write line, probably needs a pull up (or something). Since when the switch/jumper/front-panel-switch is open (to disable writing), it seems to leave the write line, floating. Which could cause it to sometimes corrupt itself, while writing is disabled.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 05:02:46 pm
content: That's a really good idea! The more flexibility, the better!Pullup'ing the /WR line is a good idea. I'd imagined using a switch between /WR and just +5v, thus pulling up that way, but that may not be logical when I find the schematic in a dusty attic in 20 years.(or next week, for that matter)Here's the revision:--Christoffer

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 05:13:10 pm
content: And yeah, 4K might be a little on the small side, but adding in 4K more (even up to 16K) would be pretty easy, even on the same board.The reason I'm using the 6116 2K ram chips is because I have a million of them, and would like to see them used. When I get to ROM, I'll go for 8K (2764) chips, but on a very similar board.--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 23, 2016, 05:22:43 pm
content: Quote from: ChristofferB on March 23, 2016, 05:13:10 pmThat makes sense (2K ram chips). At the end of the day, you had to use something specific. Whatever you ended up using, could have still been criticized.E.g. 1K ==> Too small64K ==> Non-authentic for the eraYou can't really win. I guess you just go with something, and that is just fine.I think there was one early computer, that only had something like a total of 3 bits (approx), of RAM.It is an amazing piece of technology.From my memory, how it works is that there are three or four paper tapes.They are processed simultaneously, 2 are inputs, one is the program, and the final one is the output/answers.Hence it only needs a few internal bits of ram.tl;drIt processes the information, bit by bit, byte by byte, so it does NOT need to store the information inside the cpu.I.e. The adder (or whatever the operation), receives the numbers to add up, directly from the paper tape bits. The output of the adder, goes straight to the output tape.The program tape (loop) or whatever method it used (patch or something), can't remember, tells the cpu what the instruction is, such as add, subtract etc.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 05:28:36 pm
content: Yeah, and I'd rather go with same-size chunks of memory. Decoding for multiple sizes of memory on the same board would be a logical nightmare, I'd imagine.QuoteI'd prefer thinking that I can't really loseQuoteThat sounds strange by todays standards! but there sure were many amazing architectures in the early prototype computers.--Christoffer

author: rrinker
author_rank: Super Contributor
timestamp: March 23, 2016, 05:47:00 pm
content: Quote from: MK14 on March 22, 2016, 07:35:54 pmAre those the HP displays? People in the RCA 1802 crowd will beat a path to your door to buy them from you, a pair of those are what was used on the original PopTronics Elf. My later iteration just uses regular 7 segment LEDs and some 9368's for hex display. Fun part is, a pair of those HP displays, or a pair of 9368s uses more power than the entire rest of the computer. The 1802 is all static CMOS, so it can be clocked to 0 and then restarted at any time. And draws microwatts.

author: MK14
author_rank: Super Contributor
timestamp: March 23, 2016, 06:07:43 pm
content: Quote from: rrinker on March 23, 2016, 05:47:00 pmThe cosmac (ELF, RCA)1802 is a real classic microprocessor. The 1802 + LEDs/Switches/Led-Hex displays in some projects and articles, make a tempting vintage/retro build. In a similar fashion to the OPs endeavors.That photograph (of the display) is NOT mine, it was from the internet.But yes, they are the HP ones, that I have got. (TIL311's I believe).I REALLY, REALLY love those displays, and have no intention of selling them.I can't quiet put my finger on it. But there is something about those displays, that is just so, so nice!A kind of magic or secret source.But there are many nice types of display.Nixie tubes and VFDs (especially the multi-lined alphanumeric ones), are nice as well.You were saying about it being CRAZY that the displays use more current than the rest of the system.Well apparently the original Sinclair miniature TV, complete with tiny/miniature CRT screen. Used LESS electricity for the whole TV, than "just" the speaker and amplifier, of the time. (If I remember correctly, if wrong, it is a big proportion of the total current).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 06:42:02 pm
content: The TIL-311's are really nice! I love those tiny early 70's LED dies. There's a local electronics store in Denmark selling them, at about 8£ each. That becomes a bit pricey- especially for 16 bit stuff!The cosmac ELF is in my opinion the archetypal hobby computer. I managed to find the CDP1802 processor, in a regular electronics store, just like any other component. I thought at the time it mustv'e been one of the very last store sold 1802's, but it turns out they're still being made, for space /satellite stuff.I've finished design of the 8K rom module, almost the same as the RAM card, but I'll post it here for future reference:

author: MK14
author_rank: Super Contributor
timestamp: March 23, 2016, 06:57:41 pm
content: They are available for £2.80 each including worldwide shipping (apparently), on ebay, link below.They may or may not be TIL-311 substitutes, I'm not sure.http://www.ebay.co.uk/itm/NEW-Til311-DIS1417-Texas-Alphanumeric-Hexadecimal-Display-Logic-UltraCart-128/141924967748?_trksid=p2047675.c100005.m1851&_trkparms=aid%3D222007%26algo%3DSIC.MBE%26ao%3D1%26asc%3D20140122125356%26meid%3D270c7facf4974afcae28535ea6150335%26pid%3D100005%26rk%3D1%26rkt%3D2%26sd%3D281809099154One would have thought that some Chineseclone/fakemanufacturer, would be making them by now. I would have thought the technology in them is relatively really simple, by today's standard.You could even use a really cheap, bare die pic (or something) and a pile of tiny LEDs.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 07:03:42 pm
content: So they are! Nice find, definitely bookmarked!If one can live with the 7-seg, I think making a small driver/lookup table in a PROM/ROM/PALwould be a good alternative - probably not cheaper than £2.8, though.--Christoffer

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 23, 2016, 10:51:36 pm
content: Okay this one is a bit more tricky. I've completed design for a serial interface, using the 8250, but I'm not completely certain I got it right, as I've never used the chip before.It still has the 74138 decoder with jumpers (or DIP switch), and else is pretty much just hooked up to the bus and line drivers.I wont bother with a baudrate gen. chip, as the 8250 has a built in programmable divider.I'm using MC1488/MC1489 as RS232 drivers, though Imaychange that to MAX232's, even though that's a bit anachronistic.Any thoughts or design critique would be much appreciated!--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 24, 2016, 01:58:08 am
content: The Wiki seems to think the original 8250 had some bad bugs, and seems to be recommending the debugged version. The 8250B, instead.https://en.wikipedia.org/wiki/8250_UARTQuoteThe +5 and GND power pins are apparently open circuit.I'd be tempted to connect the /RI pin up to the RS232, using the (spare) available gates, for completeness.Transferring long serial files, can take a boringly/annoyingly long time. Especially when you need to do it LOTS of times. Therefore:The 3.072 MHz will work perfectly fine, up to about 38 Kbaud.BUT if you went with the 8 MHz, it will still do all (or just about all), the baud rates up to 38 Kbaud.But then, would also do much higher (such as about 3x faster), rates. Depending on what max baud rate you can sync up with, as regards the other end (PC ?).tl;drSeriously consider an 8 MHz crystal (unless you need/want software compatibility with 3.072 MHz clock divider bit settings). Since it apparently will give you just about all the rates the 3.072 MHz would have given you, AND the options on much faster ones, which go to at least slightly more than three times faster.For quick test development work, it can be more pleasant for the download/upload etc, to take 1 minute, rather than 3 minutes, unless you like drinking coffee, every 5..10 minutes (Joke).Another ideaIf you are planning on developing the code for the ROMS, yourself. I.e. you are going to keep on iteratively changing it, while writing the software. You may want to create a "ROM emulator/simulator" card.Which uses CMOS battery backed up RAM, with write protect, and read(?)/write access via external serial interface.This is to avoid having to continually program/erase (in a UV eraser) and change the ZIP socketed 2764's. On a regular basis.Alternatively you could (temporarily) use flash devices, until your software (for the ROMS), is finalized.But don't worry. Using actual EPROMS is not the end of the world, and perfectly viable. One gets use to keeping a pile of 2764's (in your case) in the UV eraser, and get use to swapping them between the erase, burner and development board(s).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2016, 02:25:12 am
content: Thanks for all the feedback! Yeah, the power lines are obviously an oversight, it's getting late.I actually only have the 8250, but they're pretty easy to find. I'll get the B version, or even spring for a 16550 (pin compatible).You're right, when I have the spare gates, might as well add the /RI. It just weren't connected in the app note I looked up, and it's not so widely used, I believe. Might be wrong, though.Actually, going for 8 MHz clock would be a good idea, even though I might not have the CPU to utilize it now. If I add an 8088 processor card, it would be nice. And it makes no difference softwarewise/componentwise.QuoteThat's exactly what I usually do! The most important step, though is to label EVERY EPROM you've burned to keep track of versions.An EPROM emulator card would be really handy, but I didn't really plan on using the serial port for loading large programs. More just talking to the board via a dumb terminal.QuoteIn the development phase, that's what I'll do. But then I'd like to be able to run other people's software (CP/M, for example).The next card I plan is a centronics printer interface. It'd be useful for debug (same use as serial) but waay easier in hardware and software. I'm using a typewriter as printer, so it basically just needs 8 bit ascii (just a latch) and then a strobe (74121?).--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 24, 2016, 03:32:15 am
content: EPROMS are quite a nice way of developing things, in some ways.E.g. If you create a "test" EPROM, which flashes all LEDs/IO on and off, and continually tests ram etc. You can always put it in at any time, if you are not sure if it is the hardware or software, which has suddenly stopped working correctly.Or keep a previous/stable/known-working version of your software, in case you suddenly want to show someone your computer working and/or re-test the hardware out, to see if the latest problems are software or flakey hardware.EPROMS don't seem to work very well (they are problematic), if you DON'T put a light (absorbing, opaque) label on them. Some cheap paper labels, don't protect enough against light problems (if I remember correctly). When fully enclosed in a light tight case, it does not matter so much.When EPROMS were common place, they were theories and arguments, as to if unlabeled EPROMS, if left in/near direct sunlight, could be erased after a couple of weeks, or so.There were also suggestions, that if you could NOT afford a proper UV eraser. You could leave EPROMS outside in direct sunshine, for a couple of days (approx), to erase them for free. I never did hear (or try/test) if that rumor/theory was true or not. The UK is NOT the best of countries (weather wise) to try that theory out, anyway.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2016, 12:52:41 pm
content: Well most EPROM's you find in gear has these metalized, it looks, labels.I have never had any trouble with accidental erasure, but I'm a bit cautious about photographing a board with flash that has unprotected EPROM's. Might be nonsense, dunno.I've by the way recently aquired nearly 200 intel 1702 EPROMS (the first commercial eprom, if I recall correctly), but they're 256 bytes and impossible to program, requiring 0V, +5V, -9V,+47V and +56V for programming.

author: MK14
author_rank: Super Contributor
timestamp: March 24, 2016, 01:19:55 pm
content: Yes, metalised labels are the way to go.Aren't 200x 1702 EPROMS worth a small fortune ?The gold look ones, seem to sell for a fair bit each on ebay, and the plastic ones a lot less, but at 200x, that would be a fair amount of money.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2016, 01:54:24 pm
content: They might be, I don't know. But they aren't the white/gold ones, unfortunately. They're mid-70's and purple ceramics. I think I'm holding on to them, though, as I'm trying to buy a vintage Intel development kit/system that might be able to use them.

author: MK14
author_rank: Super Contributor
timestamp: March 24, 2016, 02:05:08 pm
content: Quote from: ChristofferB on March 24, 2016, 01:54:24 pmYes, you can make an interesting vintage computer(s), out of them, and stuff.I guess at 256 bytes each, you would need a hefty quantity, to make a somewhat decent amount of memory.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2016, 02:08:56 pm
content: Yeah, but being able to make a set of back-up ROM's for early systems would be fantastic. For me, that's always step #1 before even turning on any very vintage gear I find: back up the rom.Just realized it could be interesting to have an ISA interface card for the system.Shouldn't be impossible, most ISA signals are just regular cpu bus stuff. With an interrupt controller, it'd work fine, I think. They made one for the S-100 bus system, at least.This would allow fairly easy interfacing of a VGA/CGA graphics adapter for getting a CRT monitor running with it, along with other things.--Christoffer

author: MK14
author_rank: Super Contributor
timestamp: March 24, 2016, 02:18:05 pm
content: I agree, if you lost the original ROMS, on the old vintage system. That is the one bit of it, which could be irreplaceable. So backing them up, makes sense.Although you could simply back them up (using an eprom programmer, or something), onto a computer file. Then you can make/remake any ROMS as necessary.i.e. You don't need to go straight onto 256 byte eproms, straight away. But you could do that if you want, I guess.Getting ISA card interface working, would open up a huge number of hardware options. So great, if you can do that.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2016, 02:44:24 pm
content: No you're right. If you have a PC programmer, no reason for making physical backups that might also die. But if you have a non-pc programmer, that only loads from an old EPROM/PROM, and it's an EPROM like the 1702 or 2708, it may be the only alternative. (Some very early eprom burners burned directly from punched tape, I've read)The ISA thing would definitely be handy if I go for an 8088 CPU card at some point. The more complex things like floppy drive controllers or hard drives could be very easily interfaced, you you could find an old ISA card doing the trick for you.

author: MK14
author_rank: Super Contributor
timestamp: March 24, 2016, 02:57:03 pm
content: Quote from: ChristofferB on March 24, 2016, 02:44:24 pmI hadn't thought of that. An ancient EPROM programmer, with no modern interfaces or PC capabilities.But I would have thought that either a somewhat modern EPROM programmer would read them and/or you could hack it by sniffing the address and data lines, to pick up the information.I have heard of some EPROMS which use NON-STANDARD stuff. Such as some HP ones, for some of the HP computers (Very OLD, pre-PC era).But I'm beginning to understand now. So if it is incompatible to vaguely modern EPROM programmers and what programmers you can get hold of, use ancient techniques, such as paper tape. Then short of hacking/sniffing out the data transfers, or something. I can see where you are coming from, now.It's a bit like restoring an ancient, home/business computer. Which has floppies, but they are NOT PC standard ones. So if you lose the boot disks (floppies) and/or software which comes with it (e.g. games), they are potentially lost forever, or would be a major headache to get copies of them again.So you might be forced to use (ONLY) the original machine(s), to make copies of the floppies. Which would be starting to deteriorate, by now, potentially.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 24, 2016, 03:25:15 pm
content: Yeah, exactly. Of course, there'll always be workarounds. I'm sure there's someone out there who's gotten an Arduino to read/write bipolar PROM's, but I just hate being stuck in that "do a project to be able to do a project to be able to.." so on loop.Also, it's not as fun. The salvation is serial communications. A lot of old programmer gear will have a serial port. My bipolar PROM programmer/reader has one, and apparently were designed to talk to a PDP-11, so if you can utilize that port, a lot of solutions becomes possible.But then you need to figure out the format, maybe write some custom hex file transmit/receive routines, and you're doing a project to be able to do a project again.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: March 25, 2016, 02:27:23 pm
content: I did some research on other people's ECB projects. Not that many it seems, but this guy actually made an 8 bit ISA interface, with minimal circuitry:http://www.elektrik-trick.de/archiv.htmApparently, there isn't much to it at all. Makes sense, I just thought it'd require at least some IRQ controller.

author: TonyD
author_rank: Contributor
timestamp: March 26, 2016, 12:16:14 am
content: Quote from: MK14 on March 23, 2016, 06:57:41 pmgreat find (sorry if that's a bit late)

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 08, 2016, 04:01:53 pm
content: Finally have some progress to show off!Got my Schroff 19'' subrack assembled, and backplane figured out, so now it's just a question of wiring the boards.I think one of my first boards should have an IDC connector at the very edge, to connect directly to my logic analyzer. Thus I can check function without having a complete I/O system. The LA (TTI LA4800) has 74HCT input buffers, so I should just be able to run some ribbon cable directly from my bus to the LA.Here's some photos of the system so far (with cameo of a ZX spectrum that I'm gonna use as serial terminal)The empty 3U enclosure beneath the card cage is unoccupied at the moment. Might become floppy station at some point. The flat 4U top box is destined to become a front panel (altair/pdp-11 style)having its own driver card in the subrack.

author: MK14
author_rank: Super Contributor
timestamp: April 08, 2016, 04:23:06 pm
content: I will be VERY jealous, if you make that large top panel, look (similar to) like this:With tons of flashing computer lights and switches.I did not even know you could use the ZX Spectrum as a terminal. I did not think it had RS232 or anything like that. I was never really involved much with the spectrums. Maybe it is a bitbanged port, or add on interface, or something ?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 08, 2016, 04:30:39 pm
content: Wow, that does look incredible. Though having a control/indicator panel that large, one gotta make very sure every light has a function - else it's just like carrying around a slide rule without knowing how to use it.I'd really like using hex encoder wheels or knobs instead of binary switches though. They're just pricey if you don't find them used.You got me there. The ZX spectrum doesn't have a serial port, but the expansion interface does!Still less of a sin than hooking up a modern pc with a usb->rs232 converter to a system that's supposed to be retro.

author: MK14
author_rank: Super Contributor
timestamp: April 08, 2016, 05:08:52 pm
content: Apparently, (from memory), the Cray1 series of super computers (in their day, 1970's), had just such an OPTIONAL flashing lights panel(s). But it was mainly for show, since (if I remember the rumor correctly), management (who PAY for the computer, but DON'T have a clue about it!), need to see something like that (flashing lights panel), to feel they have just wisely spent, $25,000,000 (or whatever the price paid, was).You can get some very nice hex/dec/oct switches. I could not quickly find a good image of them. But the image I have found, is somewhat close/similar.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 08, 2016, 06:42:34 pm
content: Hah! That's amazing. Maybe I should just have a 4U 19'' panel above my machine with blinkenlights and no connection to the machine - just being run by some sequential logic and a clock...Yes that was the encoder wheels I was talking about. But those are strictly in the "scavenge only" section of what is affordable, usually.Maybe just 16 way rotary switches, but they're hard to find too. Cheapest solution would be to go octal with those. But that hurts my brain too much.Cheap sliding switches would be the most economical option, but they're just so nasty to use (and mount).

author: MK14
author_rank: Super Contributor
timestamp: April 08, 2016, 07:21:18 pm
content: Yes, you could make a big string of randomly flashing neons, by using a few passive components. No semiconductors would be needed. Fun to look at, but completely non-programmable!I've had a look around for the switches.These are (claimed to be) brand new, hex, thumbswitches. From the USA, at about $5 each, but you may be able to make a bulk price offer. Offer function is available on page.I don't know what you consider "expensive". Switches like that have *ALWAYS* been expensive (if I remember, correctly), even when they were commonly used/available. Presumably there is a lot of work to manufacture quality ones, and the potential sales are relatively small. Since only test equipment and similar, commonly used them.Most/all mass produced stuff (for the consumer market), DOESN'T use them.http://www.ebay.co.uk/itm/PHGA-3114-CHERRY-ELECTRICAL-SWITCH-THUMBWHEEL-HEX-BLK-MATTE-/111229210264?hash=item19e5c70e98:g:zO4AAMXQWzNSmk5COn the other hand, the following type is what I was originally looking for. As it seems to allow, faster/easier setting. (Keypads are probably quickest, though).

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 09, 2016, 05:57:48 pm
content: Those are pretty nice, even though it looks as if you'd need a stick to hit the up/down buttons.I'm sure they've always been expensive, mechanical components like that often are, and it's difficult to lower that cost without making them really crappy. Those indicators/reduction gear/friction brake knobs for multiturn potentiometers are very pricey too.Actually, if one didn't mind putting some design time/circuitry into it, it wouldn't be too hard to replicate their function with logic. A 4 bit hex up/down counter is only a bit of TTL logic, and a HEX display for that can be accomplished with an EPROM programmed as a look up table.That's actually a really neat solution! and I get the 7segment blinkenlights too!The 74169 would work great for that, actually, having a positive and negative pulse button for the U/D line, and then maybe some ordinary switches to load a value, if one wanted.

author: MK14
author_rank: Super Contributor
timestamp: April 09, 2016, 06:36:48 pm
content: It wouldn't necessarily be so easy to replace the mechanical switches, with mass produced (general purpose), electronics.Because the up/down buttons, would need debouncing circuitry, be able to cope with a huge range of logic voltages (from really low, to high, e.g. 3.3V, 5V, ECL, very low V, etc).If the item was battery operated then mechanical digits, would consume zero power. But bright led displays (to show the switch positions), would probably consume a fair bit.Quality push switches, get high prices as well, which it would still need.Also if there are already other led displays in the item. Having even more for the input switches (hex), may be too over-powering.The modern solution, seems to be LCD screens. But the cheap, dot matrix (such as 2 lines by 16 character) ones, seem to be NOT very clear (in MY opinion). The contrast is usually NOT very good, and they tend to be too small, to easily read. E.g. Compare simple calculators, with relatively good/readable LCDs, with programmable, 2 x 16 char, calculators. The LCD screen (sometimes graphical, as well), is usually of somewhat poor (unreadable), quality, in my experience.The final button set, I showed above, are VERY usable, WITHOUT a stick. But then, I assume, you will be some distance from your rack computer. So it could be different, for you.The keyboards (in my experience), from the 1960's .. 1980's, were of much better quality, than the modern (membrane/cheap) ones. They had a substantial, quality/solid feel to them. (But I guess poor quality ones, were available, then, maybe).Probably the reed relay ones, made nice/quality keyboards, when they use to commonly do keyboards, that way.BUT you still can get quality, mechanical keyboards, such as Cherry (PC). So things are not too bad.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 09, 2016, 06:51:14 pm
content: You're right, it'd consume a fair bit of juice. It'd need 3 EPROMS alone, just as lookup tables.But the 74193 (better than the 169) just has an "up" and a "down" line - add a 74121 to each button gets the necessary debouncing - sure, it'd be TTL or CMOS only, but if other logic levels were needed, adding in a level converter wouldn't be terrible.They're likely usable, but if one has a front panel where you'd need to punch in an entire bootstrap routine each time you turn on the system, ease of access becomes vital.-I agree that older keyboards (with actual separate switches) are much nicer. Though, one of the lousiest keyboards I've ever tried is on an old 1974-vintage dumb terminal. Mechanical plastic plungers with conducting foam pads pressing pcb switch pads. shrieky, creaky and terrible.

author: MK14
author_rank: Super Contributor
timestamp: April 09, 2016, 07:03:01 pm
content: Is there any way you could make an extractable, hex keyboard + led displays/lights ?I.e. You could leave it clipped to the racks, front panel, and it would look cool/nice.OR you could pull it off, and put it near, where you are sitting. For convenient/quick, boot program entry. Until your paper tape, floppy or whatever boot mechanism, you want to use, later. Or Eproms etc. Is ready and in use.Alternatively, you could have two. One for the front panel, and another for placing on your desk. Via a plug in ribbon cable, or something.Just a set of seven segment displays and a pre-made hex keypad, are both low cost and fairly quick to build. If you end up wanting two of them.Part of the real pain in the neck, and TERRIBLE ergonomics, was a feature, of real-life, early computers. (The ones with front panel switches, which MUST have them used, to get the initial boot loader, inputed). Presumably (I think), every time the computer is re-started, you have to put the boot loader in from SCRATCH. Roms/Eproms, were NOT always around, I guess.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 09, 2016, 07:42:20 pm
content: A hex keypad is of cause the most handy, but that requires some interface and a scanning routine. Thus having a hex keypad for entering a bootloader would be silly - you'd need a bootloader for the keypad to work! The original switch front panels accessed the memory directly, halting the processor and tri-stating its busses. That's probably too involved, and will require a lot of circuitry -even worse: a lot of thinking!I have a nice hex keypad with a ribbon cable and DIP header I'll use, I think. Then I'll have just the 7segment displays in rack. I've found an old intel 8279 IC. That's a very interesting keyboard/display controller, the one used on the intel MCS-85 and -88 microcomputer trainer, I might use that. Only issue is I only have the one, so if it's bad, or dies in the process, finding a spare might be difficult.Actually I just found some on Ebay. They're not so rare afterall. I think I'll do that. They have a lot of different modes, from all 64-key keyboard to all 7-seg displays and most combinations.

author: MK14
author_rank: Super Contributor
timestamp: April 09, 2016, 08:07:27 pm
content: There are "tricks" to using raw (unencoded) hex keypads, WITHOUT boot loaders. Typing straight into memory, with few or no latches.There are various solutions, possible.E.g.Diode (steering) from the keyboard switches to create the hex digits. (Simple diode ROM, if you like).Allowing the MCU to create the address pattern, by artificially giving it an I/O map, which is all zeros (or all ones), via pull ups (or downs). While typing in the boot loader program. It relies on the "NOP" or nop like instructions, via the continuous $00, or $FF's, making the MCU reliably increment to the next address location (in a kind of single step mode). How exactly you do this, and its viability, depends on the specific microprocessor.The 4 bits of hex information from the keyboard, can then be written into ram (assuming it has separate 4 bit organisation, otherwise at least one 4 bit latch would be needed). Via some glue logic and/or clever circuitry.E.g.If I remember correctly, some early microprocessor stuff, did just that. To allow simple switches, to enter programs, WITHOUT needing to worry about the addresses. Such as the "SCAMP" computer (from memory, sorry if I'm wrong).BUT you are quite right. It is NOT easy to use the hex keypad without a boot loader program, already being present.Some of these technical challenges are NOT so obvious, as we take so much for granted these days, computing equipment wise.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 09, 2016, 08:19:17 pm
content: Wow, that's actually amazing. Never knew you could do that. Sounds logical enough, but yeah, there must be considerable design obstacles.Here's the chip, in an example circuit, I'll use:http://8085projects.info/KEYBOARD-AND-DISPLAY-INTERFACE-USING-8279.htmlIt seem to have only very few registers, and it handles all 7segment refresh, keyboard scanning and is interrupt driven! no need to waste cycles with reading keypads!The ergonomics might be a hassle, though. I might go with a sloped desktop box instead of a rack unit. Dunno yet.

author: MK14
author_rank: Super Contributor
timestamp: April 09, 2016, 08:43:00 pm
content: The 8279 is an interesting chip. With the right software, it will be useful, for what you want to do.This is part of the reason, why modern day FPGAs and microcontrollers are so useful/versatile. If you were doing this as a modern design. You could just use a simple microcontroller or FPGA/CPLD to "create" the hex keypad and the displays.The next $64,000,000 question is going to be, how small/tiny can you get the boot loader program to be, in order to just load in (or allow to be typed in), a "proper", BIG boot loader program ?

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 09, 2016, 09:17:07 pm
content: Yeah, that's right, but programmable logic kinda goes that step too far in taking all the fun out of it.I plan on using a programmable interrupt controller for function keys. That must be the easiest solution. So a minimal bootloader would be:- Initialize 8279 (couple of bytes)- Initialize interrupt controller (8259 I think) (couple of bytes)- on interrupt from keypad, save data to addr, increment addr, repeat- run from addr when "run" interrupt key is depressedI think that could be accomplished in less than 32 bytes.So two 2x 4-16 decoders and a diode matrix memory could run this without an actual ROM chip.That would be almost a cool enough waste of time to try!

author: MK14
author_rank: Super Contributor
timestamp: April 09, 2016, 09:32:33 pm
content: Quote from: ChristofferB on April 09, 2016, 09:17:07 pmSorry, I was NOT clear enough in my original post. I meant "*IF*" you were doing it in the modern style. Of course, since you are NOT doing it that way, this section of this thread, is working out ways of doing it. I.e. FPGAs/CPLDs/modern MCUs are rules OUT! (Sorry for any confusion).Yes I agree, diode ROMS, are real fun. A long time ago, that is one of the methods, they use to use, to do stuff like that. Also because masks are VERY expensive (so only suitable for high volumes or clients with very deep pockets) and originally EPROMS and PROMS did not exist. Diode matrix/steering ROMS, was the way they had to do it. But there were other solutions, I guess.Another solution. Which would be authentic (time wise, I think). Would be to use a matrix plug board, of the type they use to use in higher end, analogue synthesizers. Hence you would only need to create the boot loader, ONCE, via the plug board. But could experimentally/playfully mess with it, to your hearts content.But don't worry. Being realistic, I'm sure you are much better off, using the steps, you have recently posted about, to solve the problem. I am really just making conversation here.If it had been a computer made out of RAW transistors (i.e. 1950s, and NO integrated circuits, whatsoever!), ONLY. Then maybe the plug board would have made more sense.

author: SL4P
author_rank: Super Contributor
timestamp: April 10, 2016, 06:50:48 am
content: Quote from: MK14 on April 09, 2016, 08:43:00 pmAhhh. I remember back in my days with DEC, working on PDP-11s, we used the front panel toggle switches to thump in about 20 words - then reset the address, and release the RUN switch - to boot the disk controller, and everything from there on was rosy!Slightly later machines had an Intel 4004 based 'front panel emulator' that loaded the boot code with just a couple of switch actions... but no blinkenlights!

author: MK14
author_rank: Super Contributor
timestamp: April 10, 2016, 06:56:21 pm
content: To @ChristofferBWe were talking about your Proms (Intel), and a programmer for them.The following vintage magazine, seems to include a programmer project for them. I'm not 100% sure if it applies to the ones you have, and/or includes enough details to use. But probably interesting, anyway. It goes into a HUGE amount of detail, into these "new" at the time, Proms. (Maybe you have Eproms rather than Proms, and I don't know if that is similar enough ?).http://www.americanradiohistory.com/Archive-Byte/70s/Byte-1976-05.pdfExample post, where we were talking about it:Quote from: ChristofferB on March 24, 2016, 12:52:41 pm

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 10, 2016, 08:22:25 pm
content: Great! that might be useful!Well theysayprom's but they clearly have windows - thus must be EPROMS. Think it's similar enough. I'll keep that in the documentation archive!Using a smaller microcomputer system to bootstrap could be interesting- maybe an intel 8051?-The plug matrix is a cool idea, I actually thought about making one such using 3,5mm mono jacks, with diodes in the jack plugs connecting rows and columns.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 10, 2016, 08:24:04 pm
content: Diode rom matrix boards are difficult to make on strip/hole/veroboard. The layout is awkward.Back in the days, there were dedicated cards, with traces lain out for diodes to be fitted easily.

author: MK14
author_rank: Super Contributor
timestamp: April 10, 2016, 08:44:56 pm
content: Quote from: ChristofferB on April 10, 2016, 08:24:04 pmPossible way of making a cheap matrix/switch plug board, diode array.Could you do it, via headers and 2 pin shorting links, to make a super cheap/quick/easy matrix board.I would imagine the diodes can be done. Maybe you can get SIL/SIP diodes, example shown below:Possibly soldering them on the other (reverse) side of the board, if you are happy about that.

author: MK14
author_rank: Super Contributor
timestamp: April 10, 2016, 08:49:49 pm
content: But a microcontroller, such as the 8051 or something, would also be a good solution. Potentially doable in the 1980's, so your retro/vintage qualifications/requirements, are still met!

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 10, 2016, 11:01:49 pm
content: You're right, that would be a completely viable way of doing it! I think I'll go for actual rom booting for now. Last time I tried diode matrix roms I lost all interest half way through:

author: ChristofferB
author_rank: Frequent Contributor
timestamp: April 11, 2016, 07:40:05 pm
content: I just noticed a rather large design fault with the system. It has very poor handling capabilities for multiple interrupts. I've added four interrupt lines in my backplane bus, deviating from the standard ECB bus pinout, simply by removing unused pins (like D8-15, I'll never go 16bit in this system), called GPINT0-3 that can either be inputs for the 8085's many interrupt lines (all the RSTx.x's) or a 8259 interrupt controller. With that, I could have only one interrupt line going to the cpu.Not having multiple interrupts would be a nightmare. Also, one could just have a 8259 per board, but it'd be hard to mask between them - two boards could still interrupt at the same time. Now I can have a master 8259 on the CPU card, branching to 4 out on other cards, giving up to 32 interrupting peripherals.The other option being having the 8259 on the CPU card, and then have a secondary (ribbon cable with DIP headers?) bus running all the interrupts. It still may be necessary, but it's not very elegant.