Sharp PC-1500

Back in the early eighties, I needed a programmable calculator for engineering studies, and a relative working at Sharp recommended the PC-1500. In hindsight, it was an excellent choice.

The PC-1500 is more a small computer than a calculator, with BASIC built in, so I immediately started hacking. Wrote some code to examine memory, and before long had the whole thing mapped out. This was before I obtained a copy of the Technical Reference Manual.

Once I had the manual, I decided to add as much RAM as I could. I had the CE-161 16K RAM module, but more is better, right?

This is the PC-1500 memory map from page 93 of the Technical Reference Manual. It contains an (inconsequential) error (Y2 maps 8000H to BFFFH) and doesn't give a full picture -- with much peeking and poking and reverse engineering of the schematic I found that
  • The display chips are mapped to the 7600H to 77FFH block (512 bytes).
  • The display chips wrap (three times) to the 7000H to 75FFH block.
  • The 2 x 2114 memory chips are mapped to 7800H to 7BFFH.
  • The 2 x 2114 memory chips wrap to 7C00H to 7FFFH.

I decided to add a 32k x 8 static RAM chip, mapping that from 0 to 6FFFH. I also removed the 2 x 2114 (1k x 4 each) memory chips and re-mapped the 6116 (2k x 8) to the 7800H to 7FFFH area. This gives me 32k of contiguous RAM, of which I can use the bottom 28k.

I made a PCB, with the 62265 / M5M5256 chip, space for an EPROM in the 8000H to BFFFH range, and a 4023 triple 3-input NAND gate (/CE = ME0 * /A15 * /A12.A13.A14 where the first NAND gate generates /A12.A13.A14, the second NAND gate inverts A15, and the third NAND gate generates /CE from ME0 and the outputs of the first two gates).

In the years since I last used the PC-1500 the little rubber spacer block inside turned to mush.

For what it's worth, here are my notes at the time, also more notes.

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