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CDM-600 Satellite Modem Revision 7
EDMAC Channel MN/CDM600.IOM
11–3
Apart from the round-trip satellite delay, the M&C Computer does not see any difference
between local and distant-end units - it sends out a packet, addressed to a particular unit,
and gets back a response. It can be seen that the EDMAC Master simply acts as
forwarding service, in a manner which is completely transparent.
This approach does not require any additional cabling - connection is made using the
normal M&C remote port. Furthermore, the user does not have to worry about
configuring the baud rate of the M&C connection to match the lowest data rate modem in
the system. The M&C system can have mixed data-rate modems, from 2.4 kbps to 2048
kbps, and still run at speeds in excess of 19,200 baud. It should be pointed out that at 2.4
kbps, the effective throughput of the overhead channel is only 11 async
characters/second. For a message of 24 bytes, the time between sending a poll request
and receiving a response will be around 5 seconds. (Note that when either of the BPSK
Turbo rates is in use, the overhead rate is reduced by a factor of three, and therefore the
response time will be around 15 seconds.)
11.3 Setup Summary
To access a distant-end unit:
Designate a Master/Slave pair - Master at the local-end, Slave at the distant-end.
On the local-end unit, enable framing, and EDMAC, define the unit as MASTER,
then enter the bus address. This is constrained to be ‘base 10' meaning that only
addresses such as 10, 20, 30, 40 etc, are allowed.
Choose a unique bus address for the distant-end. This should normally be set to
the ‘base 10' address + 1. For example, if the MASTER unit is set to 30, choose
31 for the distant-end unit.
On the distant-end unit, enable framing, and EDMAC, define the unit as SLAVE,
then enter the bus address. The orange EDMAC Mode LED should be
illuminated.
Set the local-end unit to RS485 remote control, and set the bus address of this
local unit. The orange Remote Mode LED should be illuminated.
Once the satellite link has been established, connect the M&C Computer, and
begin communications, with both the local and distant end units.
NOTE: EDMAC modes are fully compatible with AUPC modes.