27C256 would be right but you won't be able to use it without a pinout adapter. Not just the physical carrier to fit the chip in the socket, the pins are moved around in the M100 socket.
I would look carefully at the contact between all the pins inside the socket and the rexcpm board. It's easy for a leg to fail to make contact. There isn't really a *cheap* or fast way to make your own option rom. The pinout adapter and the physical socket connection makes it complicated. You could make yourself a http://tandy.wiki/Teeprom. It's easy to do and all the parts are things you can buy new today, no hunting for old no-longer-made stuff on ebay. But it's kind of silly what that all adds up to, ordering a 3d printed part, and the eeprom is a ridiculous $9 for a single 32K chip, and you need to get a soic test clip which is $30 itself let alone a programmer. And it's not fast waiting for everything to ship. Especially if you don't pay extra for rush service for the pcb and the carrier. For quick n dirty testing you could skip the carrier and just glue some cardboard and a ribbon to the bottom, but you're waiting for everything else anyway. I might possibly be able to make an updated version of that which uses a $1 flash part instead of a real eeprom, and staggered rows of pin holes instead of a $30 test clip to make a programmer connection, but not today. So, asking to borrow one is reasonable. I can lend you something. I'll email off list. Meantime really double check the pin contacts between the rex and the socket. See if any of the pins in the socket look compressed. On Wed, Oct 21, 2020, 11:55 AM Steve Glenner <[email protected]> wrote: > Hello > > I’m looking to borrow or buy cheaply any M100 option rom to verify my M100 > works with option rooms. > > I’m trying to get a new REXCPM working on my M100, and running into an > issue where the software will not run after initializing and loading the > REXCPM software onto it. > Basically the initialization program seems to complete without errors, > initializing the directory and transferring the RXC_11_b39 firmware from a > disk emulator, but then when I enter “CALL 63012” the M100 sort of hangs > and various areas of the screen get all black pixels, like the block > character is sent for a few repeats on random parts of the screen, but it > never results in a working Rex. Resetting goes back to the basic menu but > no rexmgr shows up. > > I’ve worked with Stephen Adolph a bit on this, he’s sent me a test program > to test all the memory on the REXCPM which seems to succeed, so I next want > to verify that the M100 option rom circuitry is working under normal > conditions. I don’t need any specific rom, and I don’t even need to keep it > very long if you want it back. > > Alternately, I have several microcontrollers and even an old pic-kit > programmer so maybe I could burn a conventional option rom image to a > physical EEPROM (I guess a 27C256?) > > Thanks, > Steve > >
