PROM alternatives - Page 1
Forum section: electronics / projects

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 06, 2024, 06:32:40 pm
content: HiI'm trying to put together the part to build a vintage Transom Triton computer.There are two tricky chips that I'm looking for at the momentIC54 (74S287) 256x4 bitsIC70 (74S472) 512x8 bitsEven if I can find them at a reasonable price, I'd still have an issue with programming them. I have the images.Please does anyone know of more modern / cheaper / easier to program alternatives.Thanks

author: Andy Chee
author_rank: Super Contributor
timestamp: July 06, 2024, 07:33:47 pm
content: I would bite the bullet and make an adapter PCB for a 2732/2764 EPROM.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 06, 2024, 08:40:06 pm
content: Quote from: Andy Chee on July 06, 2024, 07:33:47 pmThat would be easy thanks but, looking at the specs, I don't think that they would be nearly fast enough.

author: edavid
author_rank: Super Contributor
timestamp: July 06, 2024, 08:48:52 pm
content: Do you know what access time you actually need? Can you post the relevant part of the schematic?There are parallel flash chips like W27C512-45Z and SST39SF010A-55 that are probably fast enough, and can be programmed easily.Or you can usually convert the PROMs back into random logic at the cost of a few extra packages. 74ACT logic is more than fast enough.You can also order pre-programmed PROMs from arcade suppliers such as:https://www.arcadechips.com/https://www.arcadecomponents.com/

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 06, 2024, 08:50:49 pm
content: Quote from: netdudeuk on July 06, 2024, 06:32:40 pmThe bipolar proms are usually quite fast, so you may need to measure and qualify parts.I can see parts like CY27H010-30PC are fast EPROMS, likely in larger package, but easier to program.If you need 5V that will restrict your choices too.If you can find the details, one trick used on slower EPROM/FLASH memory was to select some faster address pins.eg a square 64kByte memory will have 8 address lines on output selectors and 8 lines on input selectors, and the output path is faster.It was rare to find that level of detail in the docs, so you need to test for it.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 06, 2024, 09:07:18 pm
content: Thanks for all the replies.I've attached the original magazine article which has the VDU schematic and a circuit description. Maybe they will make the requirement clearer.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 06, 2024, 09:19:57 pm
content: Quote from: edavid on July 06, 2024, 08:48:52 pmI see SST used to make a SST39SF010A/020A/040A-45 version that went EOL in 2013, there may be some floating about ?Addit : I see Aliexpress have what claims to be : 10PCS New SST39SF020A-45-4C-WHE

author: Postal2
author_rank: Frequent Contributor
timestamp: July 06, 2024, 09:23:54 pm
content: Quote from: netdudeuk on July 06, 2024, 09:07:18 pmYou can easy emulate your PROMs with cheap old and widely used at school GAL PLD.https://github.com/sirrosh/prom2pld

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 06, 2024, 09:30:36 pm
content: Quote from: edavid on July 06, 2024, 08:48:52 pmThanks, but in this case, the 512 bytes of PROM hold the graphics pixel data. I'd imagine that we'd need quite a lot of random logic chips to emulate that.

author: coppice
author_rank: Super Contributor
timestamp: July 06, 2024, 09:32:06 pm
content: Quote from: PCB.Wiz on July 06, 2024, 08:50:49 pmI used those fast Cypress EPROMs many years ago as a substitute for Schottky fused parts, and they did a good job. They are just as fast, and program in a similar way to other EPROMs. So, if they can still be found they should be a good fit for the OP's needs.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 06, 2024, 09:35:37 pm
content: Quote from: Postal2 on July 06, 2024, 09:23:54 pmI've got some GAL22V10s but they're pretty simple, set up using a simple truth table. In this case, one of the PROMs holds a graphics set bitmap. Is there an alternative for that ? I'll need to read the description again to see what the other PROM is used for. Thanks.

author: edavid
author_rank: Super Contributor
timestamp: July 06, 2024, 09:37:23 pm
content: Quote from: coppice on July 06, 2024, 09:32:06 pmThe problem is that they aren't supported by the common modern programmers, and of course you need a UV light to erase them. The flash parts are a better solution if the package isn't too large.

author: bson
author_rank: Unknown
timestamp: July 06, 2024, 11:48:10 pm
content: There's no mention of speed grade, so presumably the 50ns variant is plenty fine. The character video memory scan clock isn't going to be all that high. Maybe ~2MHz, so even anything less than 200ns or so should be killing it? Just a wild guess based on an NTSC/PAL clock divided by 5-6 for a character clock. (But I could be wrong, it's been ages.)I'd take that schematic and reenter it in KiCAD. Then modernize the memory, no need to have a pile of 2708's and whatever they used for RAM, just drop in bigger memory parts and rip out the various extraneous support logic, like the superfluous address decoding. (Like maybe a 27C256, 32k*8, as EEPROM. They can still be found.)I didn't look closer, but what kind of keyboard do they expect? Maybe rework it so it has a PS/2 port (which is really little more than a shift register).

author: Postal2
author_rank: Frequent Contributor
timestamp: July 06, 2024, 11:59:38 pm
content: Quote from: netdudeuk on July 06, 2024, 09:35:37 pmPROM for graphic bitmaps not need to translate to PLD. Very slow access used - 450 ns.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 07, 2024, 12:28:22 am
content: Quote from: netdudeuk on July 06, 2024, 09:35:37 pmIf you can get a contents copy, you can enter the PROM into SPLD/CPLD tools, to see how it compresses.I did a quick fetch from the archives and I find a 7 segment 16 line HEX ROM, compresses to3.4.3.4.3.4.4 product terms.As #13 mentions, there are CPLD if that does not fit 22V10, the ATF1502 would be another candidate. (JTAG Serial program 5V )

author: Andy Chee
author_rank: Super Contributor
timestamp: July 07, 2024, 12:30:58 am
content: An even more left field idea would be to make a RAM & uC daughter board to plug into the original PROM socket.The uC would bootstrap the RAM by copying the pixel data on power up (from its internal program or data EEPROM), then connect the RAM to the Triton circuitry, then go to sleep.Actually, with a fast enough uC perhaps you could get the uC to emulate the pixel data PROM directly?

author: Postal2
author_rank: Frequent Contributor
timestamp: July 07, 2024, 12:39:35 am
content: Sorry fellas. I've confused you a little. Access time of 450 ns allows to use any flash for graphics.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 07, 2024, 07:42:07 am
content: Quote from: bson on July 06, 2024, 11:48:10 pmYes, those old memory devices make no sense. I've got loads of AS6C1008-55PCNs and AT28C256s. I was thinking that I could then ditch those silly extra supply rails but the 8080 says otherwise.They used to use an 'ASCII keyboard', which was basically an old in one PCB with the keys and a decoder chip, resulting in 8 data bits and a strobe.I've already bought the 2102-2 RAM chips as they're dual port. I've considered hooking up a Propellor to the other side of them to replace all the video circuitry, as I've done with my Z80 and 6502 projects. This would be moving even further away from the Triton design, which is the first ever computer I used.I've just come across some cheap N82S126ANs. If they were fast enough (50ns), I could double up for one of the replacements to get more capacity. I'd still need to program though. I've got two modern programmers and neither will do them. I've looked at open source type programmers using PICs etc but they're all a big faff for what the job is. Does anyone do them in the UK ?

author: edavid
author_rank: Super Contributor
timestamp: July 07, 2024, 02:16:03 pm
content: Quote from: netdudeuk on July 07, 2024, 07:42:07 amThey are not dual port.

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 07, 2024, 03:25:56 pm
content: Quote from: edavid on July 07, 2024, 02:16:03 pmMaybe my poor use of the terminology. There's a DATA IN and DATA OUT rather than a D0.

author: shabaz
author_rank: Frequent Contributor
timestamp: July 07, 2024, 04:40:10 pm
content: I'd use Flash mem too.. cheap, easy-to-solder/socketable package, in-stock and supports 5V operation if that's what you need.Some Arduino code might exist which supports programming it (I've not checked), otherwise it would be straightforward to write some code, or use a T48 programmer (it supports the example one in the screenshot, SST39SF040-55-4C-NHE-T ).

author: edavid
author_rank: Super Contributor
timestamp: July 07, 2024, 05:16:37 pm
content: Quote from: netdudeuk on July 07, 2024, 03:25:56 pmThat's called separate I/O. You could use higher density RAM plus a buffer IC and still come out way ahead.

author: marcopolo
author_rank: Regular Contributor
timestamp: July 24, 2024, 12:59:19 pm
content: Quote from: netdudeuk on July 06, 2024, 06:32:40 pmI can program 74S287, 74S472, 2708 and supply MH74S287, TMS2708JL-45

author: netdudeuk
author_rank: Frequent Contributor
timestamp: July 26, 2024, 10:43:58 am
content: Quote from: marcopolo on July 24, 2024, 12:59:19 pmThanks for the reply. It looks like I now have the parts coming and someone has kindly offered to program them.