Re: Keying interface for PC
I should append a note about using the keying interface with rigs other than the K2...
The 4N25 optoisolator's output transistor is rated at 70 volts C-E breakdown and 50 mA continuous collector current. So the voltage won't be an issue with any solid state rig (tube rig ops beware!), but the current and saturation voltage might be, as Terry had noted.
The K2 draws less than a mA on key-down and has enough voltage margin to accommodate the C-E voltage of the opto plus the diodes used for the "straight-key-sensing" circuit.
Other rigs might sink considerably more current through the key, or require a "harder" ground condition. For instance, a Bayou Jumper passes the full Tx current, about 700 mA, through the key, so no way the little opto could handle that. To test, use your DMM as an ammeter across the rig's key line to measure the "short circuit current." If less than 30 mA or so then no worries.
Worst case - you might have to add a more robust keying transistor stage at the output to handle an beefier keying circuit, or if the opto doesn't pull the line close enough to ground.
I'll make a test with my IC-706 and TS-430S and report back, though I seem to recall using a similar opto isolator with the 706 a few years back with no issues.
I wish I could find a serial BlueTooth module that managed the modem control lines properly, but the ones I've tried, even those with a pinout for RTS, DTR, etc, didn't seem to control them.
I gently prodded N3FJP a couple years back to add program support for Elecraft rigs' ability to send CW via a command over the CAT link, but to date he hasn't had occasion to implement it. Perhaps a "re-prod" is in order :^))
If that feature were included, then my little wireless BlueTooth CAT adapter:
would handle rig control and CW keying over the same link.