On Mon, 20 Mar 2017, Win Heagy via cctech wrote: Some owners replaced the 1771 with a daughterboard that contained a 1771 plus a better data separation circuit. The 1771 included internal data separation, which WD advised not to use. "EVERYBODY" used one of those. Before selling the Expansion Interface, the previous owner might have removed the data separator and sold it separately. Some owners replaced the 1771 with a daughterboard that contained a 1771 plus a 179x, called a "Doubler". That added double density (MFM) support. The Doubler included a 1771 because the 179x was incapable of writing some of the address marks that Model 1 TRSDOS used in its disk format. "EVERYBODY" used one of those. Before selling the Expansion Interface, the previous owner might have removed the Doubler and sold it separately. Both the Data Separator and the Doubler COULD HAVE used the 1771 that was taken out, but usually came with one already installed. 1) it made for one less prying out, bending pins and reinstalling with bent pins or backwards 2) it meant that there was a known good chip going in. Some owners replaced the 1771 with an aftermarket daughterboard that contained a 1771 plus some circuitry to do 8" SSSD. Often that was accompanied by a sandwich board for the Z80 in the CPU to remap memory for CP/M, and a CP/M for it. (such as Parasitic Engineering, and Omikron Mapper) "not EVERYBODY" used one of those. Before selling the Expansion Interface, the previous owner might have removed the adapter and sold it separately. (There also existed a "remapped" CP/M for the TRS80 that would work without the hardware remapping, although in general no known commercial software worked with that setup.) In addition to the 1771, does your Expansion Interface include the "RS232 board"? A Radio Shack Computer Store manager asserted that any incompatabilities with the standard were legitimate, "because OUR RS-232 board is RADIO SHACK 232" Before selling the Expansion Interface, the previous owner might have removed the adapter and sold it separately. There were a few modifications to the EI over time. The first version had a simple straight through cable. Then they made some changes and switched to a cable with a circuit box in the middle of it. Then they made some changes and switched to a cable with a circuit box in the middle of it, plus a small second cable. Then they made some changes and switched to a simple straight through cable. Make sure that you use the appropriate cable(s) for the level of changes that were made to yours. To keep reliability, do not plug/unplug and move the CPU and the EI. If you are going to do so, then bolt both pieces to a piece of plywood. RIV-NUTS or NUTSERTS into holes drilled into the bottom of the EI and CPU. You did not HAVE TO put the power supply for the CPU into the compartment in the EI. If you do, cutting a notch in the case means that you can tuck the excess cord in there to have 3" of power cord between the EI and CPU, instead of a couple of feet going out the BACK and around to the front. The RCA TV set that Radio Shack used as a monitor had a great big empty space where the tuner used to be. Plenty of room for a disk drive, or a pair of half-height drives. Mu-metal surround was kinda important. The CPU needed a trivial modification to add lower-case. Often accompanied by adding a "Control key", and sometimes a mod to give reverse video. Remember to put glyptol on the screw after making modifications, since warranty was void if there wasn't any glyptol. -- Grumpy Ol' Fred cisin at xenosoft.com