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2708 eprom substitution, 2716, 2816

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kingchops

Experienced Member
Joined
Aug 11, 2011
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179
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Adelaide, AUSTRALIA
Since getting my Altair operational I've been trying to come up with a good solution for transferring software across to it. The front panel switches are all still good and working, but I can tell some of the most used switches like examine next and deposit are starting to get a little worn out. I've got an 8k Bytesaver card in my machine which I've loaded up with the 8k Basic bootloader. I've also loaded the Altair Rom written by John Garza which enables deposit and examine of memory from a terminal, as well as the 4k Basic loader and a few other great features. The Bytesaver card only uses 2708 eproms, but I've managed to create a socket adapter to use 2716's instead. And I've used the Top853 programmer to program the 2716's on the pc, this works fine.

For reference all I did to adapt the Bytesaver for 2716's was to bend out pins 19 & 21 and jumper them to pin 12. I did the hack in a socket between the card and chip. Then I just program the top half of the 2716's and they act as 2708's on the card.

What I also tried was to use 2816 eeproms. In my reasearch I've read that 2816's are pin compatible with 2716's so I figured I could use the same adapter with 2816's. But this didn't seem to work. I can program the 2816's using the Top853, that works fine, but when I use them in the Bytesaver, the Altair doesn't read them. Anyone know why this would be?

2716's used: AMD AM2716B 12.5v
2816's used: Xicor X2816CP-20
 
Hi,

Something doesn't quite look right about your 2708->2716 modification which may be affecting your 2816 implementation.

2708
----

Pin 18 = Program
Pin 19 = +12V
Pin 20 = Not Chip Select (/CS)
Pin 21 = -5V

So clearly you want to break out pins 19 and 21 otherwise the 2716 goes BANG. I would also break out pin 18.

So. Where would I connect these three pins on the 2716 or 2816?

Pin 18 of both the 2716 and 2816 is not chip enable (/CE) so I would connect this to pin 20.

Pin 19 of both the 2716 and 2816 is address line A10 so you can wire this to ground (pin 12) to use the first half of the device or pin 24 (Vcc) to use the second half of the device.

Pin 21 of the 2716 is Vpp (which should be wired to pin 24 (Vcc) for normal operation). Pin 21 of the 2816 is not write enable (/WE) which should be held high to read (i.e. also connected to pin 24 (Vcc)).

Dave
 
Dave,

Thanks for the info, my current 2716 adapter is working. I have a 2k monitor program in the top 1k of two 2716's, in two sockets each with an adapter, and the program runs as 2k of continuous memory. I should also mention that I don't want to program the 2716's or 2816's when they are in the target system, just read is fine.

I think the key is pin 21 on the 2816, this pin doesn't affect reading on the 2716, but the way I have it wired would stop reading on the 2816. I'll make up and adapter with you suggestion and try it out. Thanks.
 
Hi,

The datasheet for the 2708 says that pin 18 (program) should "not be VCC" for a normal READ operation. I would suspect, therefore, that pin 18 is connected to ground by default (logical '0') when it is read mode. I checked the Cromemco Bytesaver schematics and there is a pull-down resistor on this pin. I am not sure what the other logic does on this pin as I only located a partial schematic. On a 2716 this will force /CE to be permanently low (i.e. permanently enabled). Pin 20 on the 2708 is /CS which is wired to /OE on the 2716. All this means is that your 2716 will not enter standby mode (i.e. it would consume much less current if /CE and /OE were tied to the same driving source).

I am still a bit confused by your pin 21. Whilst you say your adapter works (and I am not disputing this) the 2716 datasheet states that pin 21 (VPP) should be connected to VCC for correct read operation. Perhaps there are some 2716 which ignore this pin - or it is a "no connect" pin. Either way - you may find some manufacturer's 2716 devices may not work (i.e. those that are expecting pin 21 to be high).

The other confusing thing is that you say you program the "high half" of the 2716 - but pin 19 (A10) is linked to ground. This should, therefore, mean that you would need to program the "low half" of the 2716?

Dave
 
I have to agree with everything Dave says. ISTR that I have been able to read a 2716 with Vpp inadvertently grounded, but I wouldn't count on it; I'd definitely pull pin 21 high (2716-Vpp, 2816-/WE).
 
daver2 said:
Hi,

The other confusing thing is that you say you program the "high half" of the 2716 - but pin 19 (A10) is linked to ground. This should, therefore, mean that you would need to program the "low half" of the 2716?

Dave
Click to expand...

When I say "top" I meant the first 1k of the chip, bad choice of words on my part. While the 2716 adapter works, I did some tests this morning, I tried reading a 2708 in the adapter and that didn't work. So it seems the adapter as I originally wired it only works for 2716's. BTW, I'm not an engineer as you can probably tell, so I only have a vague understanding of what these pins do. I got the original info from the post below, where the guy seemed pretty confident that the adapter should work, and I have to say it has worked for me. See this post:

http://compgroups.net/comp.os.cpm/question-on-eprom-programmers/945291

I still haven't had a chance to try your suggested wiring Dave, but I suspect it will work better. The ideal scenario would be one adapter that would work for all three chips 2708, 2716 & 2816.
 
kingchops said:
...I tried reading a 2708 in the adapter and that didn't work. So it seems the adapter as I originally wired it only works for 2716's.
Click to expand...
You're trying to read a 2708 in an adapter which converts a 2708 to a 2716??? Am I missing something, or did you mean the 2816?

BTW, I'm not an engineer as you can probably tell, so I only have a vague understanding of what these pins do. I got the original info from the post below, where the guy seemed pretty confident that the adapter should work, and I have to say it has worked for me. See this post:

http://compgroups.net/comp.os.cpm/question-on-eprom-programmers/945291

I still haven't had a chance to try your suggested wiring Dave, but I suspect it will work better. The ideal scenario would be one adapter that would work for all three chips 2708, 2716 & 2816.
Click to expand...
I'm still not getting this: why do you need the adapter to work with the 2708? Isn't that what the adapter plugs into?

Regarding pin 21: Randy (the author of the thread you're quoting) hasn't been around for a while so we can't discuss it with him, but if you don't believe the datasheets that state that pin 21 should be pulled up to Vcc when reading, here's a different thread that does support it:
http://www.vintage-computer.com/vcforum/showthread.php?23603-2716-to-2708-conversion
Looks like you can get away with leaving it floating or grounded on some AMDs (as I also found), but Intel for one does require it pulled up to Vcc (as, I suspect, all 2816s do as well).
 
MikeS said:
I'm still not getting this: why do you need the adapter to work with the 2708? Isn't that what the adapter plugs into?
Click to expand...
I don't, it was just a test (I like to tinker). It's a long story, I've also added zif sockets on top of the adapters, this is so I don't have to remove the card to change proms, and I still want to be able to use 2708's. My Bytesaver card is great at reading 2708's but it can only write a 2708 in slot 2. And to get it to write, one of the IC's needs to be removed. So in write mode only the first two sockets are usable. This is very limiting, so to get full flexibility I need to be able to use both 2708's and 2716's.

I appreciate the input and don't get me wrong I accept what you guys are saying. As I said I will try Dave's suggested wiring and post results (I expect it will work better).
 
kingchops said:
...I appreciate the input and don't get me wrong I accept what you guys are saying. As I said I will try Dave's suggested wiring and post results (I expect it will work better).
Click to expand...
It's not what we're saying, it's what the respective datasheets specify.

I still don't get how you expect one adapter to suit 2708s and 2716s? The pinouts and voltages are drastically different.

I'd put the ZIF socket on the Bytesaver; plug in a 2708, or the adapter with a 2716 in it.
 
MikeS said:
It's not what we're saying, it's what the respective datasheets specify.

I still don't get how you expect one adapter to suit 2708s and 2716s? The pinouts and voltages are drastically different.

I'd put the ZIF socket on the Bytesaver; plug in a 2708, or the adapter with a 2716 in it.
Click to expand...

It's not necessary for the adapter to work with 2708's, only with 2716's and a nice to have would be for it to work with 2816's. I've actually created a pass-through adapter for the second slot which is the only slot I'll be using 2708's in.
 
daver2 said:
Hi,

Something doesn't quite look right about your 2708->2716 modification which may be affecting your 2816 implementation.

2708
----

Pin 18 = Program
Pin 19 = +12V
Pin 20 = Not Chip Select (/CS)
Pin 21 = -5V

So clearly you want to break out pins 19 and 21 otherwise the 2716 goes BANG. I would also break out pin 18.

So. Where would I connect these three pins on the 2716 or 2816?

Pin 18 of both the 2716 and 2816 is not chip enable (/CE) so I would connect this to pin 20.

Pin 19 of both the 2716 and 2816 is address line A10 so you can wire this to ground (pin 12) to use the first half of the device or pin 24 (Vcc) to use the second half of the device.

Pin 21 of the 2716 is Vpp (which should be wired to pin 24 (Vcc) for normal operation). Pin 21 of the 2816 is not write enable (/WE) which should be held high to read (i.e. also connected to pin 24 (Vcc)).

Dave
Click to expand...

Tested this today, with the wiring above the Bytesaver can now read 2816's This is great as they are much easier to use.

I'm also thinking of wiring up a switch across all sockets to switch pin 19 high or low, as this would allow switching between two banks of 8k.

Thanks for your help!
 
Just saw your bit about pin 19. Sure, that should work fine.

Another option would be to wire pin 19 of all of the sockets up to address line A10. Disconnect A10 from the 3 to 8 decoder IC (U8) and replace this with A13 (the 3 to 8 decoder now decodes A11, A12 and A13) and remove A13 from the board selection logic (U10 pin 12) by removing the link connected to U10 pin 2 and connecting U10 pin 2 to +Vcc (+5V). This would double the direct capacity of the Bytesaver board if using 2716/2816 devices without the need for manual switching. The 'order' of the EPROM sockets will go a bit screwy as the address lines to the 3 to 8 decoder are not in the same nice logical order as they were before.

If this sounds of interest to you I will download the schematic and provide some better details.

Dave
 
Just saw your bit about pin 19. Sure, that should work fine.

Another option would be to wire pin 19 of all of the sockets up to address line A10. Disconnect A10 from the 3 to 8 decoder IC ( U8 ) and replace this with A13 (the 3 to 8 decoder now decodes A11, A12 and A13) and remove A13 from the board selection logic (U10 pin 12) by removing the link connected to U10 pin 2 and connecting U10 pin 2 to +Vcc (+5V). This would double the direct capacity of the Bytesaver board if using 2716/2816 devices without the need for manual switching. The 'order' of the EPROM sockets will go a bit screwy as the address lines to the 3 to 8 decoder are not in the same nice logical order as they were before.

If this sounds of interest to you I will download the schematic and provide some better details.

Dave
 
daver2 said:
Just saw your bit about pin 19. Sure, that should work fine.

Another option would be to wire pin 19 of all of the sockets up to address line A10. Disconnect A10 from the 3 to 8 decoder IC ( U8 ) and replace this with A13 (the 3 to 8 decoder now decodes A11, A12 and A13) and remove A13 from the board selection logic (U10 pin 12) by removing the link connected to U10 pin 2 and connecting U10 pin 2 to +Vcc (+5V). This would double the direct capacity of the Bytesaver board if using 2716/2816 devices without the need for manual switching. The 'order' of the EPROM sockets will go a bit screwy as the address lines to the 3 to 8 decoder are not in the same nice logical order as they were before.

If this sounds of interest to you I will download the schematic and provide some better details.

Dave
Click to expand...

Dave, sounds very interesting but I think a mod like that is a bit above my skill level. I can get my head around the bank switch, and how to use it fairly easily I might take that on after I've finished the other adapters. I've only built one adapter so far with the updated wiring, so another seven to go.

Thanks again.
 
The mod is only two wires so it is probably simpler to do than what you have been doing with the EPROMS!

It will take much longer to explain what to do rather than do it.

I will print out the Bytesaver schematic when I get a few spare minutes and document the modification. If you wish to explore it in the future then feel free. If not, the modification will be here and might help someone else in the future.

Dave
 
Just an update on this, I now have 4k Basic loaded on 4, 2816 eeproms in the Bytesaver card and I can load it reliably every time. These adapters work great.
 
Great news.

Now, if I hurry up with my modification for you - you can load the same 4K BASIC onto two 2816 EEPROMS...

Dave
 
daver2 said:
Great news.

Now, if I hurry up with my modification for you - you can load the same 4K BASIC onto two 2816 EEPROMS...

Dave
Click to expand...

I'll definitely give it a go Dave. As a purist, it does bug me that I'm only using half of the potential 2k in the chips. It's just not right to leave it sitting there unused.
 
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