How should I short the positive input of a PVX-4110 when using it to produce negative pulses?
Use a LEMO connector with the conductor shorted to the shield and no length of cable at all. If you don't have one available we can quote you a price on a shorted connector.
Another option is to use the supplied input cable and jumper the other end. However, we recommend the first option as it limits the cable length on the input.