Dave wrote.... My apologies :( Dave asked me this offlist a while back and I hadn't gotten to it yet. On responding to personal (offlist) email, lets just say I will never die. I tend to respond quickly to things I know off the top of my head, and take eons to respond to anything that I have to dig up in a manual or look up or actually perform some task. On to the info... There are the differences "as they were sold", and then there are the "real" differences. The differences "as they were sold" are more of a packaging thing - those really aren't differences as the computer was certainly capable of having or not having the packaged options, they just were only orderable/sold a certain way at time of initial order. Then you get to the real hardware differences. According to sales price lists... 2100A Computer programmers panel, extended arithmetic instructions, memory parity check with interrupt, power fail interrupt with automatic restart, and memory protect Options: -008 8K memory -012 12K memory -016 16K memory -024 24K memory -032 32K memory 2100S computer programmers panel, extended arithmetic instructions, memory parity check with interrupt, power fail interrupt with automatic restart, memory protect, 16K words of memory, floating point instructions, DMA, time base generator, and tty interface Options: -024 24K memory -032 32K memory So, according to "as they were sold", a 2100A could have 8, 12, 24, or 32kw of core while a 2100S could only have 16, 24, or 32kw. This is pure packaging, the memory controllers in both 2100A and S both have clear jumper settings for 8, 12, 16,24, or 32. As they were sold, the A didn't have the floating point option roms while the S did. Of course the floating point roms were perfectly installable in an A as an option (but see below). The S came with the cards for DMA, TBG, and TTY while the A did not. Of course, you could easily order DMA, TBG, and TTY cards for an A too. So in effect, the only difference between the two is the paint and lettering on the front panel. As a result - it is best to disregard ALL of the above and just consider the machines identical. In other words, just because it's an A doesn't mean it doesn't have FP or FPP... many, perhaps most - did. BUT... see below... One exception. There is some disparity between the two (given certain date codes... given other date codes there is NO disparity between the two) with regards to their capacity to hold add-on firmware roms EASILY. I do not have the exact info at my fingertips, but it basically goes something like this: On the 2100 computers (A&S), addon firmware can go on circuit board A1 and A2. I forget which modules 0-4 go on which boards, but long story short firmware goes on boards A1 & A2. On early models of the A, there are no sockets in some of the locations where you would put addon roms. The etching is there, just no sockets. I don't think the jumper setting to enable those sockets is present. On the S, all microcode module locations have sockets and can be enabled by jumpers. What I can't recall is if the older 2100A boards had zero sockets past the base instruction set, or had some but less than the maximum. I think it was the latter. However, later 2100A machines did (I think) come with A1 and A2 boards that were fully socketed for all modules. And to add to that... I don't remember where, but I definitely remember seeing somewhere (perhaps in the 2100A schematics) that there was a note in the user manual by HP sanctioning user modifications to the A1 &| A2 boards to upgrade it to allow all microcode modules. They basically said - if you want to add microcode in modules X... you must either trade in your A1 & A2 boards for the newer version, OR, you must cut this trace, add this jumper, and solder in chip sockets for the remaining spots on the board. Again - correctness warning - I do not have manuals or systems in front of me as I write this, I'm going from memory. I may well have made errors as to specifics above, but I am sure that the general idea is correct as to the differences. So, that all being said... if you have a later A with all microcode modules socketed or an earlier A that someone added the sockets to, or an earlier A that someone traded in the A1 & A2 boards, there is no difference between the A and the S. If you have an earlier version of A that has fewer microcode sockets, some may say that the fact you have to install sockets means they are different, but others may say that's no difference at all. Your call. Jay West