192K SRAM for TANDY 600

The original option RAM banks for TANDY 600 are 96K, and the machine can accept a maximum of 2 cards, for a total of 192K possible add-on ram plus the built-in 32K for a total of 224K.

This single board plugs in to both CN1 and CN2 and provides both 96K banks from a single 256K or 512K chip and maxxes out the machine.

The original TANDY 26-3910 option ram cards are actually constructed as 3 banks of 4 8K chips, total 12 8k chips, on each card. Each of the 2 memory bus connectors has 3 bank-select lines, and the original ram card decodes the bank-select lines and the two highest address lines to select one of the 8K chips.

The address and data lines are common across both ram connectors, but the bank-select lines are seperate, so there are a total of 6 bank-select lines on the bus across both connectors.

This card takes one of each address and data line from whichever connector makes the easiest routing, and uses a 74HC148 to combine all 6 bank-select lines from both connectors into 3 additional address bits, and all possible addreses then come from a single large chip.

The board can accept either a 256Kx8 or 512Kx8 chip. 5-volt 256Kx8 SRAM are no longer common, although you can still get them from Mouser. But 512K AS6C4008 are much more common. So if you want to get all the parts from DigiKey or somewhere else instead of Mouser, you may substitute AS6C4008. If looking for AS6C2008A elsewhere, be careful not to get AS6C2008 without the A. You need 5 volt, and AS6C2008 is not 5V. AS6C2008A and AS6C4008 are 5V.

When soldering, take special care to make the pin headers exactly parallel to each other and perpendicular to the PCB, or else they won't be able to insert into the motherboard connectors, or worse they may strain the motherboard connectors and break the solder joints in the future.

The easiest way to ensure the pins are spaced and aligned correctly is just to set the pin headers and PCB on top of a standard breadboard. Just lightly so the tips of the pins are held in the right position by the breadboard holes. You don't need to push the pins into the breadboard, just resting on top with the tips in the holes. Tack a couple pins from each connector while the breadboard holds the pins in place to lock in the alignment.

It's easiest to work from the center of the board outwards. Do the SRAM first, then the other chips, then the caps, and the pin headers last.

If the board is working correctly, the main menu will display 220528 bytes free after a cold reset.

(hold BKSP+ALT+LABEL, press POWER, release all)

TANDY 600 info & software: http://tandy.wiki/Model_600