Odd factory mod on TRS-80 26-3612 (Sharp CE-158) RS232 interface

[Jeff_Birt]

If you have a Sharp CE-158 RS232 interface for the PC-1500 or the Radio Shack version, 26-3612 have you ever noticed bodge wires like this inside. I have two 26-3612 and both have this modification, the one CE-158 I have does not.

One 26-3612 has two diodes added by soldering them in the bodge wires. The other 26-3612 has locations for the diodes on the PCB as well as pads for the bodge wires.

The diodes form a diode OR of UART Pin22 and an inverted copy of the UART serial output. This is fed to Pin11 of the DB25. The other wire connects pin 18 of the DB25 to a 2.2.K resistor which goes to pin 3 of the LH5811 IO chip. Both pins 3&4 of the LH5811 have a 2.2K resistor connected to the pins but nothing on other side of resistor.

The odd thing is that these are TTL level signals, not RS232 level. I can't figure out what they might have been used for.

With diodes in bodge wires.
   

With diodes on PCB:
   

Mod wiring:
   

[BobVA]

Perhaps to support some planned peripheral Tandy never released? Pin 11 is unassigned in the RS232 spec Pin 18 is the (seldom used) "local loopback." Those would seem like good choices for an input/output pair to perhaps provide some kind of control to non-standard device, in addition to the normal serial data? The wired-or is strange. Maybe some kind of "keep alive" that either looks for data transitions, or can be nailed up by the 5811 if required?

(I've seen a 2.2k resistor used to "convert" incoming RS-232 to a TTL input. This is a little sketchy as it counts on the chip having I/O protection circuitry that sorts out the negative voltage swings, etc. but it's not uncommon.)

Interesting find!

[Chr Yoko]

Looks like that RS232C interface is common between PC1500 and PC1600.
But as the PC interface wiring is different between the 1500 and 1600 (2 pins are inverted if my memory is still ok). So there should be some kind of circuitry to switch between both.

If interested i can share some USB-PC1500/1600 interface circuit design.
You can find some designs onto Sharp Forums.
There are even BlueTooth interfaces circuits available now.

[Jeff_Birt]

I don't think this mod is related to the PC-1600 but it is a good idea. Firstly, the 26-3612 was introduced and phased out long before the PC-1600 was available. Secondly it seems that this mod was only done on the TRS-80 version. The PC-1600 also has RS-232 built in, but I suspect it might be able to use the CE-158 as well. The mod wires are also on the DB25 side, not the 60-pin connector side.

I have not looked very closely for differences in the 60-pin expansion connector. Sharp did make a few mistakes in the pinout diagram in the PC-1500 technical manual. The PV and PU pins are swapped in the pinout and one Vcc pin is mislabeled as frame ground. There are a few unused pins on the PC-1500 that they might have used on the PC-1600.