Bipolar PROM replacement?

[John Crane]

Just wondering if anyone has repaired a 74S471 Bipolar PROM.
These run hot, so I guess it's not surprising they fail.

Since they are difficult to find and program, I'm considering replacing
it with an EPROM (which I can program), and using only 256 bytes of it.

Anyone ever done this?

I know the PROMs run faster than the EPROM, but this is a vintage
computing project, and the machine runs code from a 2716 just fine.

-J

[Rob]

But is the system running code from that 74S471?
Sometimes those PROMs were used as glue logic, i.e. to implement a
random logic function, like in a PAL or GAL.
For that purpose, you need a faster device than for running code.

[Jeff Liebermann]

Finding chips or equivalents is always easier than re-engineering the
device.

Hmmm... Pricy, but hardly scarce:
<https://www.ebay.com/sch/i.html?_nkw=74s471>

There's also a National 74LS471, which might work depending in output
loading and stay cooler:
<https://www.ebay.com/sch/i.html?_nkw=74ls471>
Cheaper too. Also, think about stick on or clamp on DIP heat sinks.

For equivalent chips, see:
<https://www.mikesarcade.com/cgi-bin/spies.cgi?action=url&type=info&page=PromRef.txt>
Looks like the 74S471 can be replaced by a various devices. Be sure
to compare spec sheets for differences.



--
Jeff Liebermann je...@cruzio.com
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558

[John Crane]

On 7/4/21 12:19 PM, Jeff Liebermann wrote:
> On Sat, 3 Jul 2021 19:44:12 -0500, John Crane
> <john_c...@yahoo.com> wrote:
>
>> Just wondering if anyone has repaired a 74S471 Bipolar PROM.
>> These run hot, so I guess it's not surprising they fail.
>>
>> Since they are difficult to find and program, I'm considering replacing
>> it with an EPROM (which I can program), and using only 256 bytes of it.
>>
>> Anyone ever done this?
>>
>> I know the PROMs run faster than the EPROM, but this is a vintage
>> computing project, and the machine runs code from a 2716 just fine.
>>
>> -J
>
> Finding chips or equivalents is always easier than re-engineering the
> device.
>
> Hmmm... Pricy, but hardly scarce:
> <https://www.ebay.com/sch/i.html?_nkw=74s471>
>
> There's also a National 74LS471, which might work depending in output
> loading and stay cooler:
> <https://www.ebay.com/sch/i.html?_nkw=74ls471>
> Cheaper too. Also, think about stick on or clamp on DIP heat sinks.
>
> For equivalent chips, see:
> <https://www.mikesarcade.com/cgi-bin/spies.cgi?action=url&type=info&page=PromRef.txt>
> Looks like the 74S471 can be replaced by a various devices. Be sure
> to compare spec sheets for differences.
>
>
>

This chip is in a Netronics ElfII, an 1802 based system from 79-80 era.
It runs a monitor program in this chip (load to memory, save to
cassette, etc.) I presume they used it as a cheaper alternative to the
EPROMS of the day. I'm the original owner, and after starting it up
back in 1980 or so, I noticed it was too hot (as in burning finger hot),
so I attached a little chip heat-sink on it. I'm thinking now that
that's what allowed to to last so many decades. I've read reports of
these hot running PROMS in old video games that literally burn their
pins off from heat accelerated corrosion.

As an aside, the monitor is only 256 bytes and addressed at F000 hex.
I'm thinking if I use a 2732, I can claim the remaining address space to
FFFF for other utilities in the 4K EPROM.

-J

[Jeff Liebermann]

On Sun, 4 Jul 2021 17:11:44 -0500, John Crane

Is there some reason you can't just plug in the 74LS471, assuming the
pinout is the same as 74S471? Do you want me to check for you? I
would prefer that you do this yourself (because I'm lazy today).

As I recall, the fastest 1802 clock was 4 to 5MHz. In about 1978, I
had a product with a development system that ran at 3.58MHz and used
Intel 2716 UV erasable EPROM's. I vaguely recall using someones 250
nsec RAM. The 2708, 2716, and 2732 were rather horrible in every way
possible. However, things started to improve with the 2764 which
could be found down to 120 nsec. Looks like your Elf II will take up
to 16Kbits EPROM, and to 64Kbits with an expansion card.
<https://en.wikipedia.org/wiki/ELF_II>
I don't know what kind of socket derrangement your Elf II has, but if
it will accept a 24 or 28 pin DIP, it should work. The ELF II runs a
1.76MHz clock, so you should be ok with most 27xx EPROM speeds.
<http://www.cosmacvip.com/ELFII/ElfII.html>
The above article mentions a 74C471 PROM, which might be another
possibility. However, I couldn't find any other evidence that such a
PROM existed.

Anyway, that should be sufficient to get you started in what I hope is
the right direction.

Incidentally, I should have a tube or two of 2764 EPROMs left over
from the bad olde daze when I had to update mobile radios and PC
motherboard BIOS's with a UV eraser and EPROM burner. Bug me via
email if you can't find any and need some.

[John Crane]

Thanks for the offer Jeff. But I have a few EPROMs laying about just
waiting to be used. I noticed you referenced the Wikipedia entry on the
Elf II. In fact, that is my Elf II in the photos!

I have a source for the 74S471 chips, but the problem is how to program
them. They are 'fusible link' types and all three of my programmers
won't handle them. And even if I found a way, it would still run hot. So
that's why I'm thinking of a daughterboard that plugs into the original
socket and holds a larger, slower, cooler EPROM chip.

And since I have the Netronics Full Basic ROM card, I know the machine
can run code directly from 2716 chips (450ns). I even added a fourth
2716 in the spare socket on the card to hold some utilities: serial
loader, term i/o, the RCA SCRT routines, etc. And they all work just fine.

The 74LS471 you mentioned is a new possibility I hadn't considered.
Lower power should reduce the heat a bit. I'll look into that too.
If I can find a programmer for it, that would look more in 'spec' for
the vintage machine.

Thanks!



-J

[Rob]

John Crane <john_c...@yahoo.com> wrote:
> This chip is in a Netronics ElfII, an 1802 based system from 79-80 era.
> It runs a monitor program in this chip (load to memory, save to
> cassette, etc.) I presume they used it as a cheaper alternative to the
> EPROMS of the day.

Ok in that case it should be possible to use an EPROM with a suitable
adapter PCB to convert the 24pin .5" socket for a small EPROM to the
20pin .3" used by the PROM.

[Jeff Liebermann]

A lot. Look at the data sheets. (I'm still too lazy today).

>If I can find a programmer for it, that would look more in 'spec' for
>the vintage machine.

Ummm... they're all over the web:
<https://www.google.com/search?q=bipolar+prom+programmer+74LS471>

<https://www.arlabs.com/device.html#bipolar_proms>
<https://www.ebay.com/sch/i.html?_nkw=prom+programmer>

Since this is beginning to look like a one time use for the PROM
burner, you might consider having someone with a bipolar PROM burner
do the work. For example:
<https://www.hobbyroms.com>
Otherwise, a 2716/32/64 EPROM might be easier to program.

[John Robertson]

Hi John,

I have a Data IO 29B with Unipack 2B that will program the 74LS471.
Email me for details jrr@@@flippers...com (spam@@@flippers...com is also
valid - the 'bots don't believe it so I get little to no junk email on
that address)

I am in Canada which may or may not be helpful for postage.

John :-#)#

[Sergey Kubushyn]

Just in case someone might need a faster PROM replacement, here is an idea:

https://www.ebay.com/itm/322522213187

45nS 2Kx8 in narrow DIP24. There are even faster ones, like 35nS.

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[John Crane]

Yes it is running code in the PROM. That's the Monitor program for the
computer.

-J