Strange parallel ROM pinouts

I’ve been working a lot this past year on old computers and arcade machines so that means gaining a familiarity with different rom pin layouts.  Looking up datasheets for each and every one repeatedly got old, so I found some helpful charts.  The first one looks like this:


I have no idea where this came from [EDIT: it’s from here. Much credit to the original author Thomas Scherrer Tangen for who showed up in the comments] , but it’s a great resource and covers all common 27xx ROMs in the 24 and 28 pin packages.  Thankfully there’a someone who liked this as much as I did and made it a little prettier and easier to read:


That’s all good, but for some oddball stuff that isn’t quite food enough.  Commodore PETs, Colecovision carts, plenty of older boards use different style roms than the now-standard 27xx series.  Let me introduce you to one of the strangest ones I found, this is taken from the manual for the Midway 8080 Tornado Baseball manual:


This chart has  a lot of information, and some of it is misleading without understanding the rest of the schematic.  The main takeaway is that this chart shows what various pins on each rom should be connected to in order to function.  Three columns however do not represent pins on the rom, they represent pins on the chip select decoder, those are labeled S6a, S6b, and S6c.  ignoring those for a moment this gives us an idea of how the pinouts differ from one rom to another, but what about how the pinouts are the same? well, by correlating the 2708 (the only chip in both the original diagram and this table) you can see how the remaining pins are the single chip select and data and address pins (as well as ground).  Getting back to the columns of address lines, take a look at the schematic right next to this chart:


I really like this setup, it uses a 7442 as a chip select decoder and you get to choose what size roms you have.  For example if you have the smallest roms, 512B ones, you only get 9 bits of address per chip (A0-A8) so A9, A10, and A11 go into the decoder and that picks which of the 8 roms you are talking to based on where in the memory map you are (breaks the memory map into 512B chunks).  If you have 10 bits of address per chip you then run A10, A11, and A12 into the decoder and it’s now breaking up into 1K chunks.  This datasheet only has jumper settings called out for one 2K rom, the 9216, and with that in place you get 8 chunks of 2K using 13 address bits total.  You can’t hook a 2716 directly up using the jumpers and the table, but with a couple of bodge wires between the jumper blocks I think it would work.


This all led me to make my own ‘universal’ rom tables so I could find equivalents or adapters to make one type read as another.  I broke it up into two sheets because otherwise it’s un-usably huge (as it is it’s pretty bad).  I have 23xx, 25xx, 27xx, 28xx, tms47xx, 93xx all on one page and all the older stuff from tornado baseball on a second page.  I left the 2704 with the older roms because I haven’t had call for one and most people start the 27xx series at 2708.  I added the sizes for the roms I’m not familiar with and will be expanding these spreadsheets as I find more information I want to reference.


These screenshots are just a snapshot in time and are not meant to be kept around as the gold standard, it’s kinda hard to read without a ruler they’re so big.  There are other ROM pinouts not included here like the infamous Intel 1702  or various smaller 4bit roms, but those have other pages elsewhere and really this is a tool for my own reference.  Other links are here:

32pin rom strangeness

Reading Irem MASKROM’s

Post script, sometimes I hate commodore:


4 Responses to “Strange parallel ROM pinouts”

  1. Strange parallel ROM pinouts #Memory #VintageComputing #Gaming « Adafruit Industries – Makers, hackers, artists, designers and engineers! Says:

    […] Techie-Blog has been working this past year on old computers and arcade machines, so that means gaining a familiarity with […]

  2. Vtech Whiz Kid reverse engineering | Evan's Techie-Blog Says:

    […] pin, this allows a TMS4732 like is present right now or a more standard 2732 to be used, see my ROM pinouts for the slight differences. The other interesting thing is the use of a PNP NPN pair of transistors […]

  3. Thomas Scherrer Tangen Says:

    cool page, funny to see my original drawing the eprom pinouts
    see more here :

    • abzman2000 Says:

      oh, sweet. I added attribution above because it is no longer true that I have no idea where your picture came from. I like your use of EPROMs as look up tables for 7-segments

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