[aprssig] Trouble with Calculator Serial Cable

[gerheim at cox.net]

It turns out that the serial cable for the HP-50G calculator doesn't respond well to RF.  When the D7 transmits on high power, (5W)  the serial interface locks up and won't function.  It either produces garbage output or doesn't produce any output.  I have to disconnect the cable to get it to "reset".  

If the D7 transmits on low power (couple hundred mW), or if I put some distance between the HT and the serial cable, it'll do OK.  (There are 2 cables in use:  one from the HT to the null modem, and the calculator cable (with integral circuitry) from the null modem to the calculator. )

I'm going on the theory that the cable circuitry (level shifter/inverter) is shielded adequately in its metal enclosure, but that RF is going down the cable(s) to either the calculator or the cable circuitry.  I'll try some Radio Shack ferrite chokes on the cable(s) this evening.  

Any ideas out there? 

I already emailed the cable manufacturer.  

It's especially bad that the circuitry "locks up" after a transmission.  I have no requirement to use the serial interface _during_ a transmissions.  If it fails during a transmission, it's a tolerable fault.  If you have to unplug stuff to get it running again, that's bad.  

--
-Al Gerheim

HP-49G+ Software Page:
http://members.cox.net/above