Appendix C. Async Overhead Option - Comtech EF Data CDM-550T Manual

Introduction
For many applications the EDMAC channel provides an excellent solution to the problem of
communicating with a modem (and possibly a transceiver) at the distant end of a satellite link.
However, the EDMAC information remains internal to the equipment, and is not presented to the
outside world. There are some applications where it may be useful to transfer async EIA-232 data
over the satellite link, using the overhead channel, and provide external access. This could be
used, for example, for controlling and monitoring external (non-Comtech) equipment, or perhaps
for PC-to-PC file transfer.
Consequently, in Software Version 1.19 and later, a transparent async overhead mode has been
added that provides this functionality. This mode uses the same frame format that EDMAC uses,
and continues to add 5% to the transmitted bit rate.
In order to provide external access, without using the main Remote Control connector on the rear
panel, the Auxiliary Serial connector is used. This is the small 3-pin connector found on the rear
panel, to the left of the Remote Control connector. This is connected internally to a UART
(independent of the UART used for normal remote control and EDMAC functions) which is
normally used for inter-unit communications in a 1:1 pair, and for FSK communications with a
Comtech Transceiver.
For these reasons, if the transparent Async Overhead mode is used, the following features are not
available:
1) EDMAC is not available (the framing bytes reserved for EDMAC are now allocated to
async transfer).
2) 1:1 inter-unit communications (the UART is now assigned to async transfer).
3) FSK communications with a Comtech Transceiver (the UART is now assigned to
async transfer).
However, is should be noted that AUPC is still available in this mode.
Rev. 1.3
C
Async Overhead Option
Appendix C.
C–1