Another EPROM addressing question - Page 1
Forum section: electronics / projects

author: metertech58761
author_rank: Frequent Contributor
timestamp: January 21, 2025, 06:36:07 am
content: When I began accumulating the boards for one system to document their schematics and dump the firmware as needed, there was one board I narrowly missed acquiring.As I understand it, this company had an ongoing exchange program where end users could send back the old-style CPU board and its companion memory board in exchange for the new-style CPU board.So, opportunities to acquire that board are now VERY few and far in between.The main difference between the old board set and the replacement board (besides the firmware rev) is that the original eight 2516s were consolidated into a single 27128.(my gut feeling is that an end user got the EPROMs mixed up when upgrading the firmware and likely caused a small industrial accident)With that out of the way, the three boards in question are:290009 / Old-style CPU290051 / Old memory expander board290077 / New style CPUI managed to acquire examples of the 290051 and 290077 boards.The one I missed out on was the 290009, and the picture I have of that board was from the reseller's site before they took the listing down.I did ask the reseller if they had pictures of the back, but unfortunately, both the 290009 and 290077 boards have a backing plate, and their pictures of the back side are with that plate on.Anyway, I have managed to document the schematic of the 290051 and 290077 boards.For completeness on the schematic side, I would like to try and work out the last few details of the 290009 schematic...I was able to work out the differences component-wise between the 290009 and 290077 boards:The upper half of the ROM space, the timer, and the 25LS2521 (74LS688) address decode circuitry all relocated to the 290051 board.The lower half of the ROM space remains on the 290009, and instead of the 74LS139, a 74LS138 is used for the address decode instead.So far, so good.The part where I run into trouble are a set of pads by each of the ROMs and the RAM, and also by the 74LS138 for the ROM chip select.So, I'd like to see if the brain trust is able or willing to take a crack at this puzzle.

author: moffy
author_rank: Super Contributor
timestamp: January 21, 2025, 07:29:55 am
content: "The part where I run into trouble are a set of pads by each of the ROMs and the RAM, and also by the 74LS138 for the ROM chip select."If you are talking about the five sets of 6 pads in the first photo, they might be links to select the upper bits of the address space for the ROMs.

author: metertech58761
author_rank: Frequent Contributor
timestamp: January 21, 2025, 07:37:34 am
content: Correct. Thing is, the only other EPROM that uses a single 5V supply (which is all that's available) would be the 2732, and that's just one address line different.

author: metertech58761
author_rank: Frequent Contributor
timestamp: January 30, 2025, 11:20:08 pm
content: Hmm.

author: bson
author_rank: Unknown
timestamp: January 31, 2025, 09:25:33 pm
content: What do you actually need help with?

author: m k
author_rank: Super Contributor
timestamp: February 02, 2025, 06:53:57 am
content: You shouldn't charge CR2032.pin 2316 2332 2516 2532 2716 273221 CS3 CS2 Vpp Vpp Vpp A1120 _CS1 _CS1 _CS _OE _OE _OE18 _CS2 A11 _OE A11 _CE _CE2332 is stated to be 2732 compatible.What with the ROM access?ROM is low and RAM is high.

author: metertech58761
author_rank: Frequent Contributor
timestamp: August 14, 2025, 06:15:57 am
content: Somehow I missed the notification for this thread.I was hoping for some reasonable guesses as to the purpose and connection of the sets of pads by the EPROMs on this board...