Are you sure that 256Kb EPROMs were readily available at the beginning of the 80s? Apparently the 2732 only came out in '79.
Whatever the reason for the original pinout, Tandy did upgrade the M100 and later Model Ts' system ROMs to the JEDEC standard in late '83, but presumably kept the option ROM pinout for compatibility with all the existing option (EP)ROMs. On Wed, May 19, 2021 at 8:33 PM RETRO Innovations <go4re...@go4retro.com> wrote: > On 5/19/2021 7:33 AM, Jeffrey Birt wrote: > > You are absolutely correct about the cost of a masked ROM w.r.t. an EPROM > or PROM. The option ROM socket was not intended for the main system > firmware, it was for adding new functionality. Many times this added > functionality was in the form of general productivity applications or > sometimes it was specific to a particular company for insurance agents, or > controlling laboratory equipment. > > These aftermarket applications would never be released in sufficient > quantity to justify a masked ROM and I’m not aware that Tandy ever released > any software itself that way. However, both EPROMS and PROMs have been > available since the mid-1970s and would have been the choice for these > smaller scale software distributions so making the pin out match an > industry standard 27C256 would have made more sense, IMHO. I think Kyocera > figured this out fast though as all the subsequent machines ditched the > funky option ROM socket and used a standard DIP socket with a standard > pinout. > > > I agree technology at the time of this product included JEDEC compliant > EPROMs. Based on masked ROMs in other product offerings (MOS Technology, > of 6502 fame), I don't think MASK ROMs had to differ in any substantial way > from EPROMs. The 1970's era 2364, for instance, matched the 2732 EPROM > pinout almost exactly, with the single inclusion of a top address bit in > lieu of the extra select pin. This allowed the manufacturer to use a 24 > pin carrier which was cheaper, and the buyer could use a 24 pin socket, > which was also cheaper. But, it doesn't look like technology forced the > pinout differences. > > I always felt the non standard pinout was a vendor lock-in move, but > looking at the pinout, it feels more to me that the designer wanted to > avoid the possibility that someone would cram the part in backwards and > hurt it, rare as that might be to put it in backwards. Maybe the carrier > design was not finalized when the schematic/PCB design was done. Namely: > > Putting VCC on 1 and 14 means that putting the ROM in backwards would not > put VCC to GND and GND to VCC. It would put VCC on D3 and GND on A14, but > that would probably be no worse than having stray voltages on an unpowered > ROM. > > Jim > >
