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. Jeff Birt From: M100 <m100-boun...@lists.bitchin100.com> On Behalf Of Peter Noeth Sent: Tuesday, May 18, 2021 7:30 PM To: Model 100 Discussion <m100@lists.bitchin100.com> Subject: Re: [M100] Other things that used the Molex socket Jeff, You have to remember that the socket was designed for a "Masked ROM", like the Main ROM, which **was** the standard pinout. At that time there were no programable ROMs (PROMs or EPROMs) that had compatible pinouts, due to the circuit changes necessary for the programming function (capacitor "cells" in EPROMS and "fuses" in PROMS) that caused the pinout to change because of internal layout. Computer manufacturers used Masked ROMs to reduce the per chip cost in large quantities, where they didn't expect frequent changes in content. Also I don't remember a 32K EPROM being available in 1983. Most were 8K or less. The initial high cost of producing the "mask" and required minimum quantity order explains why there were not more "option ROMs" from other software houses. The One Time Programable (OTP) PROMs would have been a good choice, but didn't exist in the 32K size, and the OTP EPROMS didn't exist until decades later. I remember this was always a pain in modifying computer "operating systems" in the early days of "turn key" computers. Not a problem with the S-100 Bus systems, as the CPU board was loaded with easy to change EPROM sockets. Regards, Peter ------------------------------ Message: 2 Date: Tue, 18 May 2021 07:36:25 -0500 From: "Jeffrey Birt" <bir...@soigeneris.com <mailto:bir...@soigeneris.com> > To: <m...@bitchin100.com <mailto:m...@bitchin100.com> > Subject: Re: [M100] Other things that used the Molex socket. Message-ID: <019b01d74be2$6b1ffeb0$415ffc10$@soigeneris.com <http://soigeneris.com> > Content-Type: text/plain; charset="UTF-8" Interesting. I think if Kyocera/RadioShack had not chosen to go with the stupid non-standard pinout it would have been a more popular way of distributing custom applications for the machine. The socket itself was a good idea to make putting a chip in more idiot proof. Jeff Birt
