Open squelch data carrier detect for Flash Packet Radio Control Board (VK6)

This article documents an open squelch data carrier detect (OSDCD) for a Flash Packet Radio Control Board (VK6) TNC-2.

The TNC normally derives Data Carrier Detect (DCD) from its modem chip (usually a broadband energy detector is used in modem chips). DCD is used to enable decoding of incoming Receive Data (RD), and to block transmission whilst the channel is busy.

There are two problems that the OSDCD addresses:

  • squelch may be slow to open on some receivers, preventing decoding of some packets with short preamble; and
  • if squelch opens because of interference or non-packet signals, the transmitter is blocked.

The OSDCD monitors the RD pin of the modem chip, and tries to recognise transitions characteristic of 1200Bd data transmission. The name derives from the fact that the receiver can be operated with squelch open as the OSDCD differentiates between 1200Bd data and other audio.


The modification of the TNC uses the prototype OSDCD described at Open squelch data carrier detect for TNC-2.

The modification involves:

  • installing J2 (the modem disconnect header) if not already installed, and cutting the track between pins 11 and 12.

OSDCD module connections are:

  1. Ground to ground;
  2. Vccto +5V
  3. /DCD to J2 pin 12;
  4. RD from J2 pin 16; and
  5. NC.