Miguel:
I do not have too much specific information in hand regarding the details of the design from 12 or 13 years ago. But let me tell you what I can remember.

- The design was 8031 and so the circuit was required to have external EPROM. The circuit was fitted so that a RAM chip could be placed into the same socket so program code could go into the read / write memory for debug. The PSEN and RD signals were OR'd together so that MOVC and MOVX instructions would refer to the same address spaces.

- The circuit also had an external RAM (separate from above code memory) that was preserved by a backup battery.

- Since there were external RAM/ROM there was an address decoder chip to make chip enables. One set of decodes was arranged to provide access to the read/write ports of the Mitel DTMF transceiver. MOVX instruction could be used to transfer data to chip to generate DTMF signal in WR/ cycle. Using MOVX instruction with read cycle permitted the DTMF detection status and possible decode nibble to be read from the part.

I do not have the software source code on any on my "today" computers and cannot guess which one of over 5000 diskettes it is on in this set of boxes I have. (Note that the diskettes used to be sorted out but my office, at the time, went through the January 17, 1994 Northridge, California earthquake. They have never been resorted from the mess left from the EQ !!).

- The program code was debugged and tested using a product called DDT-51 that was made by the people at a company called Micro Mint (They were the people loosely connected to the Circuit Cellar Magazine).

Hope this helps.
Mike