Mocomtech CIM-550 Manuel D’Utilisation

CiM-550 IP Enabled Satellite Modem 

Rev. 2 

EDMAC 

CD/CIM550.IOM 

107 

Data to be transmitted to the distant-end is sent to a local unit via the remote control port. 
A message for the distant-end is indistinguishable from a ‘local’ message - it has the 
same structure and content, only the address will identify it as being for a distant-end 
unit. 

Before the M&C data can be successfully transmitted and received, pairs of units must  
be split into EDMAC Masters and EDMAC Slaves. Masters are local to the M&C 
Computer, and Slaves are distant-end. 

Now, a unit which has been designated an EDMAC master not only responds to its own 
unique bus address, but it will also be configured to listen for the address which 
corresponds to its EDMAC Slave. When a complete message packet has be received by 
the EDMAC Master, it will begin to transmit this packet over the satellite channel, using 
the overhead bytes which become available.  

Note:  The ‘normal’ protocol for the message packet is not used over the satellite path, as it is 

subject to errors. For this reason, a much more robust protocol is used which 
incorporates extensive error checking. 

At the distant-end, the EDMAC slave, configured for the correct address, receives these 
bytes, and when a complete packet has been received, it will take the action requested, 
and then send the appropriate response to the EDMAC Master, using the return overhead 
path on the satellite link. The EDMAC Master assembles the complete packet, and 
transmits the response back to the M&C Computer. 

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:  When either of the BPSK Turbo rates are in use, the overhead rate is reduced by a factor 

of three, and therefore the response time will be around 15 seconds.)