Connecting the Interface

How to connect your PC and Sounder/Key to the Interface:

NOTICE: There have been reports of unexpected operation when using a PC and DC power distribution bus, such as those commonly found in amateur radio set-ups. Apparently, an unintended ground loop through the PC can be set up that bypasses the negative lead out of the interface back to the distribution bus. Any current limiting resistors must be placed on the POSITIVE side of the interface to avoid a ground loop shorting out resistors that are placed in the Negative or Return line of the Interface/Power Supply. This allows full loop voltage and HIGH current to flow through the sounder. The best place for current limiting resistors is on the positive side of the supply as it feeds the key/sounder. This will protect the sounder from high current due to ground loops with the PC and power distribution busses.

This is not a concern if you have an isolated supply/battery (one that is not connected to anything but the telegraph loop), but it is still good practice to current limit the positive path of the supply as it feeds the sounder.