MFJ-1278B MULTI-MODE

ADVANCED OPERATION

1278B will assume is required for the repeater to drop. If the MFJ-1278B has detected channel activity recently enough that the repeater transmitter should still be on, it will wait only the TXDELAY time before sending data, rather than adding an AXDELAY time as well.

The commands TXDELAY, AXDELAY, and AXHANG all set times in units of 10 ms. If AXDELAY is in effect the total keyup delay will be

Keyup delay = (TXDELAY + AXDELAY) * 10 in milliseconds.

If channel activity has been heard more recently than AXHANG*10 ms ago, the keyup delay will only be

Keyup delay = TXDELAY*10 in milliseconds.

Packet Timing

The AX.25 protocol provides for re-transmitting packets if no acknowledgment is heard from the intended destination station within a certain period of time. A packet might not be acknowledged due to channel noise or "collision" with another packet transmission, and since there may be other stations on the channel, the receiving station may not be able to acknowledge the received packet immediately. The time lapse before the originating station re-transmits the packet is set by the command FRACK (frame acknowledge time). The maximum number of re-transmissions before the originating station terminates the connection is set by the command RETRY. The maximum number of transmissions of a packet is RETRY+1, since the initial transmission does not count as a re-transmission. Setting RETRY to 0 specifies an infinite number of retries.

The frame-acknowledge time is automatically corrected for the additional time required for digipeating. The time interval before MFJ-1278B re-transmits an unacknowledged packet is:

Retry interval = FRACK * (2*n + 1)

in seconds, where n is the number of digipeaters in use for this connection.

Acknowledgments of digipeated packets are made from end to end, and digipeaters do not acknowledge the packets they relay. If there are several intermediate relays, the chance of either the original packet or the acknowledgment to be lost increases drastically. To reduce this problem, an automatic wait time can be imposed on any station not transmitting a digipeated packet. Stations waiting for a clear channel to transmit packets wait for this time interval after the channel clears before transmitting. This wait does not apply if the station will be transmitting one or more digipeated packets. This usually gives the digipeater a clear chance at the channel.

Page 133
Image 133
Epson MFJ-1278B manual Packet Timing, Keyup delay = Txdelay + Axdelay * 10 in milliseconds, Retry interval = Frack * 2*n +