TRS-80 Model 1 Expansion Interface question?

[Fred Cisin]

On Mon, 20 Mar 2017, Win Heagy via cctech wrote:
> But, the FD1771 IC, for some reason, was removed from an otherwise
> apparently functional EI.
> http://imgur.com/a/3NzOh
> Is there any reason why this chip would be removed?  I see a number of them
> on ebay for around $25.  The expansion interface hardware manual indicates
> it is an FD1771B-01, but the service manual indicates a couple
> possibilities....FD1771 A/B -01 -11.  Any considerations to look for here?


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