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: