I've been meaning to post this for a while now...

What is the purpose of the two diodes between the reset line (highlighted) and the two pins on the PIA?

 

What is the purpose of the two diodes between the reset line (highlighted) and the two pins on the PIA

You don't give enough context. At first I thought they were being used as diode OR, but reset is low active, so it would make more sense for the diodes to be reversed.

 

Where else does the reset line go?

 

Hi,
When the Reset line is pulled low, by whatever is connected as Reset input.?
PAE and Rly are also pulled Low.

PA2 can pull PAE low without effecting Rly and vice versa PA3 pull Rly without effecting PAE

You could consider them as steering diodes.

E

 

The reset line goes to: Watchdog circuit, MPU (that 'custom' SC44125, which I will replace with a 68HC11), the 6821 PIA in the snippet, and the 6840 timer.

The comment about the reset line pulling PAE and Rly low makes perfect sense.

PAE is the transmitter enable line and Rly puts the unit in transmit or receive mode.

So, it seems pulling both lines low would shut off the transmit circuit and put the unit in receive mode.