Tuesday, March 6, 2012

Replica 1 Multi I/O Board

The Replica 1 Multi I/O board kit arrived yesterday and I immediately set about putting it together. It only took me about an hour to assemble.

Parts Laid Out Ready for Assembly

The board has a serial port using a 6551 ACIA, a simple EEPROM programmer, and two 8-bit parallel ports using a 6522 VIA chip. It also has a socket for an optional speech synthesis chip which I didn't opt to get.

It plugs into the single expansion slot on the Replica 1. It has a breadboard area and a connector giving access to the VIA chip ports. A nice touch is a ZIF socket for the EEPROM.

I was able to use the serial port with the supplied modified version of the Woz monitor program that uses the serial port for i/o and confirm that I could load files through it.
View of the Multi I/O Board
I was also able to program the included EEPROM. It simply appears as memory and is writable so no special software is needed to write to it. A nice enhancement to the EEPROM programmer circuit would be a write protect switch or jumper. It is a little too easy to write to the EEPROM and a buggy program might corrupt it. I might add one.
Installed in the Replica 1 Expansion Slot
I'll have to do some experiments with the 6522 VIA. It is more sophisticated that the 6821 chip on the Replica 1 (and Apple 1) having some extra goodies like timers that can generate interrupts.

A bit of trivia: the 6500 processor series from MOS Technology competed directly with the Motorola 6800 series. The short-lived 6500 chip was electrically pin-compatible with the 6800 but resulted in a lawsuit that took it off the market. The Apple 1 could optionally accept a 6800 processor instead of a 6502 with a few circuit changes that were documented in the schematic. Woz chose the 6502 over the 6800 because it was substantially less expensive. The 6500 and 6800 series are similar but not software compatible.
Note on Apple 1 Schematic Describing 6800 Support

Motorola sold 68xx series support chips like the 6821 used in the Apple 1 and Replica 1. MOS Technology offered their own line of 65xx support chips including the 6551 and 6522 found on the Multi I/O board.

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