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
