Elektronika MK-90 memory cartridge

Description

The cartridge is designed as removable storage medium for the Elektronika MK-90 pocket computer. The RAM inside the cartridge is accessed by the computer through a serial interface with a transfer rate of ca. 100kbits/s. Although there's only 10kB RAM on-board, the built-in controller chip apparently seems to be capable of addressing up to 64kB of RAM.

What's inside

The cartridge consists of 5 static RAM chips KA537RU10 with battery backup and a KA1835VG2 controller.

Connector pin functions

RAM cartridge connector

Pin	Symbol	Function
1	BATT.	plus pole of an internal 3V lithium battery
2	VCC	positive supply voltage +5V
3	CLOCK	serial clock input
4	DATA	serial data input/output
5	SELECT	chip select input
6	GND	ground

Bus communication protocol

Data written to the cartridge change at rising edges of the CLOCK pulses, and are shifted-in on falling edges of the CLOCK pulses beginning with the most significant bit. The first transmitted byte after the SELECT high to low transition is the command op-code. Following the op-code, any addresses and data are then transferred.
The waveform below shows an example of Write Address command (signals SELECT, CLOCK, DATA).

write command bus timing

Data read from the cartridge change at falling edges of the CLOCK pulses, and are sampled at rising edges of the CLOCK pulses.
The waveform below shows an example of Read Data command.

read command bus timing

Commands

Op-code	Command name	Function
0x00	Read something	This unknown command returns 0x00. It's issued by the MK-90 before reading the loader code.
0x10	Read Postdecrement	Read any amount of data bytes from the cartridge starting from the location pointed to by the address register. The address register is decremented after each received byte. This command is not used by the MK-90.
0x20	Write Postdecrement	Write any amount of data bytes to the cartridge starting from the location pointed to by the address register. The address register is decremented after each transmitted byte. It's used by the MK-90 to clear the cartridge RAM with spaces when formatting with INIT.
0x80	Write something	Unknown command not used by the MK-90. Writing random data disables access to the cartridge RAM. Perhaps memory paging?
0xA0	Write Address	Write the 16-bit address register, the most significant byte first.
0xB0	Read Address	Read the 16-bit address register, the most significant byte first. This command is not used by the MK-90.
0xC0	Write Postincrement	Write any amount of data bytes to the cartridge starting from the location pointed to by the address register. The address register is incremented after each transmitted byte.
0xD0	Read Postincrement	Read any amount of data bytes from the cartridge starting from the location pointed to by the address register. The address register is incremented after each received byte.
0xE0	Write Postdecrement	It seems to be an equivalent of the 0x20 command. Not used by the MK-90.

The lower four bits of the command op-code seem to be "don't care" values. All other op-codes not listed in the above table appear to be ignored along with any following data.