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Woodstock RAM chips

Posted by Eric Smith on 18 Jan 2006, 5:27 p.m.

There are at least five different RAM-only chips used in the Woodstock/Topcat series; some of the calculators (HP-19C, HP-27, HP-29C, HP-67, HP-92, HP-97) have some combined ROM/RAM chips which I won't describe here.

There are three different "single density" RAM chips, containing 8 registers of 56 bits each:

1. Two 1820-1564 RAM chips are found in the HP-22, HP-25, and HP-91 calculators.
2. Two 1820-1630 static RAM chips are found in some HP-25C calculators.
3. Two 1820-1843 static RAM chips are found in some HP-25C calculators.

There are two different "double density" RAM chips, containing 16 registers of 56 bits each:

1. One of the 1820-1886 static RAM chips is found in some HP-25C caluclators, and three in some HP-95C calculators.
2. One of the 5061-0469 CMOS static RAM chips is found in some HP-25C calculators, two in the HP-19C and HP-29C calculators, and three in some HP-95C calculators.

These chips are all generally pin compatible. However, Viktor Toth reports that if you replace two single RAM chips with a double RAM, you have to add a 100K resistor between pins 8 and 14 of the RAM in order for continous memory to function correctly.

The 1820-1564 chip is probably dynamic (requiring a continous clock), so it will probably will not provide continuous memory if you try to use it in the HP-25C, HP-29C, or HP-95C.

The 5061-0469 is CMOS, while all of the 1820-xxxx RAMs listed above are probably PMOS, so the 5061-0469 probably has lower power consumption for longer battery life.

Edited: 18 Jan 2006, 5:40 p.m.

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