Sharp PC-1401 Memory Mod

 Projects / Misc  Original post date: 02/04/2015

This is an internal memory mod for the Sharp PC-1401 Pocket Computer (1983) from 4kB to10kB.  This is probably the max memory configuration they intended to sell, but didn't. Instead they moved on the the PC-1402 a year later that has the 8kB chip installed.  http://en.wikipedia.org/wiki/Sharp_PC-1401http://www.old-computers.com/museum/computer.asp?st=1&c=1170

The following show the memory expansion connector with the signals labelled. It is likely to be connected to a daughter card via a conductive elastomer.

Note the /CS pin is shared with the memory chip in the left. They probably intended to sell a 2kB unit along with a 8kB expansion. Memory prices probably fall enough around that time, so they simply given up on the idea. A year later, they moved on with the PC-1402 with a 8kB.

I replace the 2kB SRAM with a low power 8kB SRAM  (-12L = 120ns Low power). The new part has 4 extra pins and some minor differences in the pinout.

The mod needed is as follows:

• 8kB chip: power is at pin 28 and there is an additional active high chip select pin "CS2" at pin 26. CS2 lined up with the VDD pin on the 2kB footprint, so I jumper it to pin 28.
• 8kB chip: /WE is at pin 27 where as it is on the pad that is lined up with pin 23 of this chip. Pin 23 on this chip is actually A11 which is one of the extra address line. I lifted the pin and connect the signal from the expansion connection pin 13 to it. I also routed the pad to pin 27.
• 8kB chip: pin 1 - no connection. pin 2: A12. I connected it to expansion connector pin 12.

There is a second 2K chip south of my 8kB to make up the 10kB (or 4kB of the original memory.)