Model I Version
Model III Version
Michael Wessel and Mark Pruden have developed X-MEM/80, a 512 KB paged memory expansion for the TRS-80 Model 1 and Model III.
According to them:
No permanent modifications are required for installation. The Model 1 version is entirely external (via expansion port edge connector), and the Model III version simply plugs into the existing Z80 socket on the Model III motherboard – the CPU needs to be relocated from the Model III motherboard onto the X-MEM/80 board, but it is already socketed. No motherboard modifications are required, but the RF shielding in the Model III needs to be removed.
Unlike previous memory expansions for the Model I (i.e., SuperMem), this is a minimally invasive memory expansion that does not require any soldering for installlation.
X-MMEM/80 is partially SuperMem compatible, as it allows to “bank” the upper 32 KBs (address range from 0x8000 to 0xFFFF) with the memory on the card.
A jumper allows configuration in two different modes – SuperMem and X-MEM/80 modes:
More info can be found at: https://github.com/lambdamikel/x-mem-80