One of the ways in which a TS2068 can be put to use in an unconventional manner is to replace the internal ROM chips with EPROM chips containing the program for the custom application. There are two of these ROM chips in the TS2068. The first figure shows where in the 64K byte address space of the Z80 cpu these ROM’s are mapped.
The devices involved are a 23128 and a 2364 for the 16K byte (128K bit) and the 8K byte (64K bit) ROM’s respectively. Direct replacement of these devices with 27128 and 2764 EPROM’s is not possible because pins 1 and 27 must be maintained in the high state for those devices.
To replace U16 and U20 with 27128 and 2764 EPROM’s respectively requires the main circuit board to be reworked as described below. A circuit diagram showing these changes is given on the next page.
Step 1. Cut the input to pin 27 on each chip.
Step 2. Wire +5V to pins 1 and 27 on each chip to pull these pins high.
Step 3. If U20 is to be a 21128, then replace the RD input to pin 26 with address A13 from pin 26 on U16.