PROM alternatives

[netdudeuk]

Hi

I'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 moment

IC54 (74S287)   256x4 bits
IC70 (74S472)   512x8 bits

Even 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

[Andy Chee]

I would bite the bullet and make an adapter PCB for a 2732/2764 EPROM.

[netdudeuk]

I would bite the bullet and make an adapter PCB for a 2732/2764 EPROM.

That would be easy thanks but, looking at the specs, I don't think that they would be nearly fast enough.

[edavid]

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/

[PCB.Wiz]

IC54 (74S287)   256x4 bits
IC70 (74S472)   512x8 bits
Please does anyone know of more modern / cheaper / easier to program alternatives.

The 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.

[netdudeuk]

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.

[PCB.Wiz]

There are parallel flash chips like W27C512-45Z and SST39SF010A-55 that are probably fast enough, and can be programmed easily.

I 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

[Postal2]

Thanks for all the replies.

You can easy emulate your PROMs with cheap old and widely used at school GAL PLD.
https://github.com/sirrosh/prom2pld

[netdudeuk]

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/

Thanks, 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 

[coppice]

IC54 (74S287)   256x4 bits
IC70 (74S472)   512x8 bits
Please does anyone know of more modern / cheaper / easier to program alternatives.

The 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.

I 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.

[netdudeuk]

Thanks for all the replies.

You can easy emulate your PROMs with cheap old and widely used at school GAL PLD.

I'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.

[edavid]

I 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.

The 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.

[bson]

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).

[Postal2]

.... one of the PROMs holds a graphics set bitmap.  Is there an alternative for that ?  ....

PROM for graphic bitmaps not need to translate to PLD. Very slow access used - 450 ns.

[PCB.Wiz]

I'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.

If 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 to
3.4.3.4.3.4.4 product terms.

As #13 mentions, there are CPLD if that does not fit 22V10, the ATF1502 would be another candidate. (JTAG Serial program 5V )

[Andy Chee]

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?

[Postal2]

Sorry fellas. I've confused you a little. Access time of 450 ns allows to use any flash for graphics.

[netdudeuk]

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).

Yes, 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 ?

[edavid]

I've already bought the 2102-2 RAM chips as they're dual port.

They are not dual port.

[netdudeuk]

I've already bought the 2102-2 RAM chips as they're dual port.

They are not dual port.

Maybe my poor use of the terminology.  There's a DATA IN and DATA OUT rather than a D0.

[shabaz]

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 ).

[edavid]

I've already bought the 2102-2 RAM chips as they're dual port.

They are not dual port.

Maybe my poor use of the terminology.  There's a DATA IN and DATA OUT rather than a D0.

That's called separate I/O.  You could use higher density RAM plus a buffer IC and still come out way ahead.