FlexROM 200

Work in progress. NOT YET TESTED - PROCEED AT OWN RISK

Like FlexROM_102 but for TANDY Model 200.

Adapter boards to replace the main system ROMs in a TANDY 200 with a re-writable EEPROM or FRAM or FLASH, and provide revertable connections for use with the Main ROM feature of the REX_Classic.

The Model 200 needs two chips replaced in positions M15 and M13, each with their own chip-enable wires connected to the REX.
The original directions just say to remove the original ROM chips and install a couple of wires from the empty sockets to the REX.

With that setup, the machine can no longer run without the REX.

The goal here is to provide a way to use the REX main-rom feature while still leaving viable roms installed in the original sockets, inactive while REX is installed, but able to be re-activated without opening the case again. So the REX could be removed, or scrambled by installing a bad rom image, and the machine can boot from the internal roms.

For Model 100 and 102 this is fairly simple.

For Model 200, it's a little more complicated, because there are two chips to replace, and one of those is weird and needs an adapter with extra parts

For the 32K M15 chip

The original chip is standard and you only need a FlexROM_102, and connect it the same way as for 102, /CS_BUS goes to TP1 on the REX.

Follow all the FlexROM_102 directions for programming, installing, connecting to REX.

For the 8K M13 chip

This adapter exists but is not yet tested: FlexROM_200_M13
BOM: https://www.digikey.com/short/q3zm9cf4

To program the M13 board:
set the small jumper to the PGM position
jumper /OE_BUS to /OE_IC
place the board into a programmer

To install:
set the small jumper to the RUN position
If using REX main-rom feature: connect /OE_BUS to TP2 on the REX
otherwise jumper to /OE_IC install board in position M13 on TANDY 200 motherboard

Like the directions for FlexROM_102 show, also run a 2nd dupont jumper wire from /OE_IC out to the option rom bay but don't connect it to anything. Label it "M13_IC". To remove REX and revert to booting from the internal roms, remove the M13 wire from the REX TP2 and connect it to the M13_IC wire.