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Understanding TxDelay

TxDelay is a setting and operation in a TNC that causes the TNC to send null information for a period before the start of the meaningful packet messages. Usually it is in milliseconds (1/1000ths of a second) or in 10s of milliseconds (1/100ths of a second).

If TxDelay is too short, the receive station will not hear the beginning of the packet, resulting in a failure.
If TxDelay is too long, time is wasted and the audio heard by the receiver (and anybody else that i listening) will have a noticeable period of flat tone before the start of the random-sounding packet noise.

More detail

Packet radio messages are transmitted by a radio. The radio starts in receive mode, is placed into transmit, and then a packet is transmitted. The process by which a radio changes modes from receive to transmit takes time. The time is usually measured in fractions of a second and can be from 1/2000th of a second, up to half a second.
The TxDelay adjustment needs to be over a wide enough range that a fast or a slow radio can be compensated for.

Adjusting TxDelay on a TNC-PI

The TNC-PI TxDelay adjustment is done at the TNC but via commands sent over the i2c bus from the Raspberry PI. There are application program provided with the TNC-PI called tncpi_setparams and tncpi_getparams. In a proper TARPN install, these programs live in /usr/local/sbin and are called from the tarpn script using the commands tarpn i2c-set and tarpn i2c-set.

See TNC-Adjust-And-Test for proper instructions.

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