Identify function of a diode

Hi,

I made a PIC programmer (Velleman kit) but I am having some stability problems with the device.
LED's which are powered by the main 5v bus aren't lighting when switched, so I'm trying to find a cause...

My question is... what is the Diode D7's function in this circuit?
Is it to stop negative going pulses from reaching T4?
The input through R13 is direct to a serial cable, pins 9 AND 3.

Yes, it's to stop negative signals to reach the transistor...

a serial port outputs a positive voltage for a logic 1 and a negative voltage for a logic 0.

The diode will short out the negative signals

Great, thanks... discussion has moved to the following message now:
https://www.electro-tech-online.com/threads/hello.10429/#post-52336