New 8085 project - Page 1
Forum section: electronics / projects

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 10, 2016, 12:14:35 am
content: Hey! I was going to start a small 8088 conputer project, turns out they're difficult. Don't wanna. So I've started an 8085 based project.I've never really worked much with the intel 82XX family of chips, but I'd like to, so this project will feature nearly only those.Now, as mentioned, I don't really have experience with these IC's (or the 8085), so I wouldloveif someone would take a quick glance at the schematic to see if there's something I've completely missed.By the way, as i draw this (which I do in ExpressSCH, I'm creating a lot of custom components (the 82xx's and others) - If anyone uses ExpressSCH, and would like this pack-throw me a PM)The entire project will be Vero-Wired on protoboard - does anyone remember VeroWire? Haven't found much info about it, but it's surprisingly easy and cheap to use!Future plans include the 8250 or 8251 UART, and the 8279 display/keypad interface.Cheers!--Christoffer

author: Tomorokoshi
author_rank: Super Contributor
timestamp: January 10, 2016, 02:28:16 am
content: Look at the Radio Shack TRS-80 Model 100. It uses the 8085 along with several of the peripherals you are using.https://en.wikipedia.org/wiki/TRS-80_Model_100http://www.club100.org/http://www.eevblog.com/2010/10/05/eevblog-116-retro-notebook-teardown-tandy-model-100/The technical reference and service manuals are linked to from Dave's video.

author: calexanian
author_rank: Super Contributor
timestamp: January 10, 2016, 02:29:14 am
content: Not to dissuade you cause i think this would be a fun project but is there any particular reason you want to work with the 8085? If you want to write assembly for an 8080 its a good option cause it only needs a single 5 volt supply, but you stil have to deal with the requirement for latches on the address line, etc. The 6502 KIM board would do a lot of what you want I would think and is readily available in a nifty modern from from Briel electronics. Z80 is also a good option cause you have easier implementation of DRAM refresh. That is if you just want to play with old chips. Otherwise have fun and carry on.Just some thoughts.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 10, 2016, 03:01:07 am
content: Tomorokoshi:Wow, I was pretty sure it was z80 based! I'll check that out as design reference! Thanks!calexanian:Well It's a mix of wanting to use some specific chips, tailored for the INTELs, and just wantingto do something 8085-ish. I have done a z80 project in the past, as well as some messing around with the motorolla 68xx's and MOS 65xx, so I wanted to try something Intel. The 8080 is hell, with strange clocks, lotta voltage rails, non-TTL levels, etc. 8088/8086 is definately gonna be a project some day,but i'd rather not at this point.Thanks, anyways!--Christoffer

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 10, 2016, 03:02:27 am
content: Another major factor in choosing the 8085 is that I have some example code for a lot of the support chips, in the mcs-85 user manual.

author: promacjoe
author_rank: Regular Contributor
timestamp: January 10, 2016, 06:05:21 am
content: It looks like you've got a good start on the project. if I may make a suggestion, Make provisions for the ROM and RAM chip to use the 512 Kb version. It takes the same basic pin out, And with a little bit of modification to your address select, you can overlay the to types of memory. This will allow you to maximize the usable memory that you have. Basically anything that is not addressed to ROM, gets addressed to ram.

author: rob77
author_rank: Super Contributor
timestamp: January 10, 2016, 07:41:04 am
content: seemes to be a fun projecti would probably change the eprom for a flash - there are chips pin compatible with eproms - you don't need a UV eraser for flash.and don't forget to add tons and tons of decoupling capacitors - each chip 10uF tantalum + 100nF ceramic as close as possible to the chip.and good luck with the project - you can learn a lot from it

author: jwm_
author_rank: Frequent Contributor
timestamp: January 10, 2016, 08:07:06 am
content: It might be cool to just use a single FRAM chip to subsume both rom and ram, you can get them in standard pinouts. They do have an address latch built in, but I think that may help here.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: January 10, 2016, 08:45:18 am
content: Small tip: take a look at the 74xx573 as a replacement for the 74xx373 you have in your schematic. It has the exact same function but a different pin-out which might fit better in your strip boards...

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 10, 2016, 01:13:15 pm
content: promacjoe:Not a bad idea, I had just considered adding some more slots for additional 64Kb chunks, or maybe EPROM/SRAM selectable sockets.rob77:I should, but I have so many EPROMS that it's no issue. I always have a bulk of 10+ erased chips, and then an antistatic foam sheet for each project where I have all the versions of my ROM.Works fairly well.I'll have loads of decoupling!jwm_:I didn't even know FRAM were commonly available!obiwanjacobi:I knoow! But I'm stuck with a trillion 74HCT373's, and I have to keep using them!Thanks for all the feedback!--Christoffer

author: SL4P
author_rank: Super Contributor
timestamp: January 10, 2016, 02:36:07 pm
content: Quote from: ChristofferB on January 10, 2016, 01:13:15 pmThat sounds like you found my parts bin from the 80s !

author: rob77
author_rank: Super Contributor
timestamp: January 10, 2016, 04:12:15 pm
content: i just noticed something in your schematic....why is the output of address latch driven by ALE (address latch enable) connected to a data bus ? it must be the address bus. the CPU's AD0-7 holds the bottom half of the address during ALE active.- AD0-7 should go to the 74373's inputs and to the data bus at the same time (there is neither RD nor WR active while it holds the address - so it's safe to attach directly to data bus).- output of the 74373 is the A0-7 and A8-15 from the CPU forms the address bus.btw.. it might be not an issue with 8085 but with early AVRs (AT90S8515) the ALE signal was too short for the 74373 , had to widen it by a 33pF capacitor to ground

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 10, 2016, 04:44:19 pm
content: D'oh! Hadn't even noticed that! Thanks, I guess that could have caused a ...few... hedaches in the debug phaseEither I read "Data Latch Enable" or I just forgot what I was doing.-I've seen others use the 373, but if it doesn't latch properly, it's worth a try!--Christoffer

author: jwm_
author_rank: Frequent Contributor
timestamp: January 11, 2016, 02:17:59 am
content: Quote from: ChristofferB on January 10, 2016, 01:13:15 pmYup, it can be hit or miss on digikey new for the parallel type you want that are in stock, Ramtron pulls/NOS are pretty common on ebay, maybe $4 for a fm1608. They can be treated almost the same as SRAM except that when you strobe the CE line the address is latched in place, you can probably use this to get rid of a buffer.I have an 8085 and some support chips in my parts bin somewhere I have been meaning to use for something, was thinking of making an altair 8800 or IMSAI clone but with FRAM and the 8085 for simplifed wiring.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 11, 2016, 08:49:43 am
content: Quote from: jwm_ on January 11, 2016, 02:17:59 amNeat, Ill have to remember that!Sounds like a nice idea with the Altair clone. I'm pretty sure that you could actually get an 8085 CPU card at some point- even an 8088 one! But don't think it ever met an FRAM card--christoffer

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 11, 2016, 08:53:38 am
content: Intel did a book in the 1980s on the 8085 including supporting hardware and software that was very nice. It taught me how computers worked. For the life of me I can't remember what it was called but it was A4 sized, about two inches thick and blue.AFAIK 8085 has a multiplexed address/data bus and two phase clock which is a bit of a bugger. Not the easiest start. Personally I like the Z80 better myself. Easier to use and much more literature around.

author: ChristofferB
author_rank: Frequent Contributor
timestamp: January 11, 2016, 09:47:33 am
content: I think I have seen the book you're talking about, wasn't it the one that came with the mcs-85 trainer?The 8085 does have multiplexed data and address bus, to free up some pins, which allowed it to have both serial port and multiple interrupt lines, which is rather nice.It can run directly from an xtal like any microcontroller!I think I will work on an 8-bit ISA interface for it, would allow some very cool features...

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 11, 2016, 09:54:05 am
content: That's the one!I may have been confusing myself with the 8080 there!Good luck - these projects are always fun. Please post back with any progress

author: rdl
author_rank: Super Contributor
timestamp: January 11, 2016, 02:56:49 pm
content: Elenco still sells the Micromaster MM8000K computer trainer kit, which uses the 8085. I'm surprised it's still available. I bought one back in the 90s. I built it and it didn't work right but I didn't know enough to figure out why. I still have it somewhere, maybe I'll dig it out and try again.

author: jbix
author_rank: Newbie
timestamp: January 17, 2016, 09:11:48 pm
content: I love seeing these retro computer projects! They look like fun, maybe one day I'll take one up. Good luck, and please post progress!

author: xtech
author_rank: Regular Contributor
timestamp: January 18, 2016, 03:00:59 pm
content: Add 64KB RAM (two 62256 or 621024) and serial port like 16C550 (or version without fifo: 16C450, both have build-in baudrate generators) and your machine will be CP/M 2.2 ready. Don't forget about 1-bit register (with reset) in I/O space which makes you possible to unmap ROM (CP/M system needs RAM available from address 0x0000) - you can store whole CP/M in ROM, copy it to RAM and then unmap ROM. As a mass storage CF card can be used (even in 8-bit mode, only half of capacity will be available).

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 18, 2016, 04:45:42 pm
content: If you're going to do anything with an SBC/small computer, CP/M is a good win!My first computer was a CP/M 2.2 one - an RM380Z my father discarded when he got his hands on an IBM XT. Loved that machine and used it until the power supply exploded in about 1987. Cashed in my savings from some inheritance and bought an Acorn A310 from Beebug.Anyway enough flashbacks from the 1980s from me...