From what I can tell, if you are using both in the same 16-bit PDP11

On Wed, 7 Nov 2001, Tom Uban wrote: Ok, it's newbie time. Is there any functional difference between the -C and -D? That is, can either do things the other cannot? -- Jeffrey S. Sharp jss(a)subatomix.com

Presumably this is directed at me? I've never had to replace one. If I did, I'd try to get a similar bipolar PROM from any manufacturer (there are still quite a number around, even if they're not being made) and program it from the tables in the printset. Other than that, I'm not sure. This sort of logic is not going to fit well into an FPGA or CPLD, I think. Are there fast enough EPROMS now (I'm out of touch with the latest devices, but I think I remember seeing some fairly fast devices in the databooks). -tony

> > Just how fast do these devices have to be, and how deep are they? I've got

10's of ns at worst. And typically 256*4 bits or so.

I suspect those would be fine for many of the PROMs in old minicomputers... > configurations are out there as well. These are contemporaries of the 25ns > 22V10 UV-erasable PALs from CYPRESS. That would date them about 1990. > > Small bipolar PROMs were often used like PALs, which were not yet popular,

In the case of the 11/45 (1972), I don't think PALs were even available at the time it was made. > were more expensive than the PROMs. I've not seen EPROMs that small,

> but it's likely a big EPROM would work if it's fast enough. In a case where

Of course it would. EPROMs never need a refresh (for obvious reasons), so it's safe to tie unused address inputs low and to program the required pattern into the first few words of the EPROM. I don't see how that could ever not work. Well, if you want to take a dump of the PROM, work out the logic equations from it, then fit them into a PAL, and get it working, good luck :-)

Yes, some PROMs were used as logic functions rather than (say) microcode store. But it's not that easy to replace them with PALs (it's possible in some cases to fit the logic into a small-ish PAL). It's a lot easier to use a PROM/EPROM device.

-tony

> > > Just how fast do these devices have to be, and how deep are they? I've

> some > > > > 10's of ns at worst. And typically 256*4 bits or so. > > > So those would be 82S129's or the like? There are some such devices that

That sort of thing, yes... Exactly. Of course you want to be sure that any you buy surplus have never been programmed. There is no way to erase these devices. Ditto for PALs, of course. > > Well, if you want to take a dump of the PROM, work out the logic > > equations from it, then fit them into a PAL, and get it working, good > > luck :-) > > > The PROM is listed as a truth-table, right? The only ABEL version I used

Basically, yes. It's a bitwise dump of the PROM, not always formatted in the obvious way (the microcode PROMs have the dump displayed as the values in particular fields, which may be 1 bit long, 2 bits long, 3 bits long, etc). You'd have to turn that into a pure binary dump. If the logic will fit into a PAL and if you have suitable software. > > Yes, some PROMs were used as logic functions rather than (say) microcode > > store. But it's not that easy to replace them with PALs (it's possible in > > some cases to fit the logic into a small-ish PAL). It's a lot easier to > > use a PROM/EPROM device. > > > If one doesn't have a suitable bit of software, that Quine-McLuskey (?)

Which is, of course, how most PAL compilers do logic optimisation. > that I learned about in college some 30+ years back (and promptly forgot)

> automated easily enough to produce reduced equations. Reducing the prom

> to a set of equations by isolating each bit in the output word and OR'ing

> locations at which those bits are true is one reasonably way to do the job.

Some of the free stuff doesn't. I have a freeware (I think, it came from National Semiconductors via Elektor as part of the software for their GAL programmer project) GAL compiler that does no optimistations at all. It's simple-minded, but at least it never does something that you don't expect! > > PROMs are programmable-OR-fixed-AND devices, while PALs are programmable > AND-fixed-OR devices. The process of generating them is, therefore

Of course... > for a given logic function, but, expanded and re-reduced, the result should

> the same. Of course a registered PROM is needed to produced registered

Yes, but we are replacing PROMs with PALs, not the reverse. There are registers PROMs (82S114 and 82S115 spring to mind), but they weren't used in the 11/45 CPU. In any case, registered PALs are easy enough to find. > I'd say a 16L8 would do for most PROM-based applications. A 16L8 is capable

> generating any logic function of 16 inputs. A 256x4 or 32x8 PROM has less

Absolute rubbish. There are plenty of 16 input, 1 output, functions that will not fit into a 16L8. The obvious way to see that is that to specify an arbitrary 16 input function takes 64K bits (there are 2^16 lines in the truth table, you need one bit to specify the output for each one). The programming data for a 16L8 is much less than 64K bits (I seem to remember it's around 3K). Therefore there are functions you can't stick into the 16L8. As a trivial example, suppose have 16 inputs (allowing 2 outputs). And that we're already using one of the outputs for a function we can fit. Then the following function will not fit in a 16L8 AFAIK : Output = (I0 + I1 + I2 + I3 + I4 + I5 + I6 + I7 + I8 + I9 + I10 + I11 + I12)/ The final / is there because the OR matrix in a 16L8 is really a NOR matrix. That expression needs 13 inputs to the OR matrix, IIRC there are something like 7 or 8. Demorgan doesn't help because you can't invert the whole exprrssion in a 16L8. Even with a 16V8, where you can invert the whole thing and turn it into an AND expression that will fit, it's not hard to think up expressions that can't be simplified into < 8 terms ORed together. Expressions with multiple XOR terms are notoriously difficult to fit into most PALs. You run out of OR terms very quickly. There were some special PALs (16X4?) with XOR gates in them to get round this problem, but they are not common, they're hard to find blanks for now, and most GALs can't be configured to replace them. Not so. A 256*4 PROM effectively has some 256 input OR gates in it. These do not exist in PALs. -tony

But let's face it, I've also seen a hack that added functionality that had a TTL chip epoxied back-to-back with pins sticking up in the air and wires going all over. Looked like a nasty spider. On the same token I have seen a daughter card plugged into a chip slot with a few wires going to a pin row header and all daughter card wires neatly aligned. The latter looked so good, in a way it looked better than a machine made board.

That sound like my interpretation of the Claus Buckholtz 256k Atari800xl 'dead bug' upgrade. Dead bug because the ic's were upside down, legs splayed.... After looking at his world-famous upgrade and the ICD 'Rambo 256k' upgrade which was on a nice little board, I realized they are one-and-the-same. I made a cross between the two I called the 'Sly Stallone 512k 800xl'. It used a peice of perf board and a wire-wrap socket. I filed the pins on the socket a bit so it would plug into a 16-pin (74ls158?) socket on the xl's motherboard. It looked a LOT nicer than the dead bug version and used fewer parts than ICD's version. Saved two headers and a bit of ribbon cable. Regards, Jeff In &lt;3BE2162E.F173AE12(a)mail.verizon.net&gt;et>, on 11/01/01 at 10:42 PM, Eric Chomko &lt;vze2wsvr(a)verizon.net&gt; said: -- ----------------------------------------------------------- Jeffrey S. Worley Asheville, NC USA 828-6984887 UberTechnoid(a)Home.com -----------------------------------------------------------