rs232 interface for Radio Shack PT210 printing terminal

Tony Duell ard at
Wed Oct 24 16:59:07 CDT 2007

> I was offered a Radio Shack TRS-80 model one and a model 4. The model 4 
> powers up and
> asks for a diskette. The model one has the expansion box and disk but I 
> don't know which connectors
> to hook the stuff up to. Do you know where I can get a boot disk for the 
> model 4 and manual for the
> model one? Or a web site that shows the connections?

The M4 first : 

If you hold down the BREAK key while turning it on or pressing Reset 
(Reset == the unmarked recessed button on the keyoard), it'll go into 
Cassette BASIC. You'll get a CASS? prompt on the screen. Hit ENTER and 
you'll get a MEMORY SIZE prompt. ENTER again and you're in BASIC. Of 
course you can't use the disk drives, but at least it'll let you test the 

As regards the OS, there were basically 2 options. One was the TRS-DOS 
6.x / LS-DOS 6.x, The other was CP/M. I have no idea where you'd find the 
latter (or indeed _why_ you'd want to find it, LS-DOS being a much nicer 
OS), but I got the former from Tim Mann's site. If you can't find it 
anywhere, I suppose I could send you the image I downloaded.

For the M1 : 

The EI  (expansion interfce) is placed with the power button on the front 
face and the 2 removable covers on top. The left-hand cover is held on by 
4 screws, under that you can put an optional I/O board, normally a serial 
port. It's likely to be empty. The right-hand cover is held on by 3 
screws and covers a space to fit 2 power supplies, one for the EI and one 
for the keyboard/CPU. The 2 supplies are indentical. At the front of this 
space, on the PCB at the left side, is a 5 pin DIN socket. That takes the 
output cable from the PSU used to power the EI.

Now, going round the card edge fingers, starting at the centre front and 
going clockwise, we have : 

Front Centre (40 way) : Connected to the optional I/O board. Normally not 

Front Left (40 way) : System bus input from the CPU. How this is 
connected depends on the revision of the EI. The latest one (which is 
what I have) just uses a straight-through ribbon cable. Earlier ones had 
a special 'buffered cable' with a box containing buffer chips in the 
middle. This cable is marked to say which end connects to what. Some 
early versions _also_ had a DIN cable soldered to the EI and the CPU, you 
just plug those together.

Left Front (40 pins) : Buffered system bus output to other devices. 
Normally not used

Left Rear (34 pins) : Parallel printer. Straight-trhough ribbon cable to 
a 36 pin microribon plug to a Centronics printer

Rear left (34 pin) : Disk drives. Normal SA400 pinout to 5.25" drives. 
Model 1 TRS-DOS only supports single-sided drives (and the normal 
hardware only supports signle density (FM) operation. There were add-on 
boards normally called doublers' to do MFM recording. Other OSes (my 
favourite was LDOS) would use said doublers, and would also let you use 
double-sided drives.

On the rear are 3 DIN sockets. You can ignore these. They connect to an 
internal relay to select between 2 cassette recorders, one to each of 2 
of the socekts and then a striaght-through DIN cable to the CPU cassette 
socket. I cna never remember the order.


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