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Understanding TxDelay

TxDelay is an adjustment to a TNC to establish an adequate delay between keying up the radio and sending the packet. If the TNC were to send a packet and do push-to-talk at the same time, the transmitter would not be on the air and sending the audio soon enough, and the first part of the packet message would be lost. Usually TxDelay is measured in n milliseconds (1/1000ths of a second) or in 10s of milliseconds (1/100ths of a second).

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.
If TxDelay is too short, the receive station will not hear the beginning of the packet, resulting in a failure.

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 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 serial port TNC like KPC-3, PK88 or MFJ1270B

KISS serial TNCs may have a TXDELAY adjustment set in the node software. Use the TXDELAY command for each KISS serial TNC port.

Adjusting TxDelay on a TNC-PI

The TNC-PI TxDelay adjustment is done at the TNC via potentiometer R6.

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

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