Comtech EF Data RCS20 - Manual (Page 149)

RCS20 M:N Redundancy Switch Revision 15
Remote Operations MN-RCS20 and CD-RCS20
A–7
pitfall, which is often overlooked, concerns multi-drop override IDs. All too often, multiple devices of the
same type are assigned in a direct-linked ("single-thread") configuration accessible to the M&C computer
directly. For example, if two DMD15 Modems with different addresses (DESTINATION IDs) are linked to
the same control bus at the same hierarchical level, both will attempt to respond to the M&C computer
when the computer generates a multi-drop override ID of 22. If their actual setup parameters, status, or
internal timing differs, they will both attempt to respond to the override simultaneously with different
information or asynchronously in their respective message packets and response packets, causing a
collision on the serial control bus.
To preclude control bus data contention, different IDs must always be assigned to the equipment. If two
or more devices are configured for direct-linked operation, then the M&C computer and all other devices
configured in the same manner must be programmed to inhibit broadcast of the corresponding multi-drop
override ID.
The multi-drop override ID is always accepted by devices of the same type on a common control bus,
independent of the actual DESTINATION ID. These override IDs with the exception of “BROADCAST” are
responded to by all directly-linked devices of the same type causing contention on the bus. The
“BROADCAST” ID, on the other hand, is accepted by all equipment, but none of them returns a response
packet to the remote M&C.
The following multi-drop override IDs are device-type specific, with the exception of "BROADCAST".
These are summarized in Table A-4 below with ID values expressed in decimal notation:
Table A-4. Broadcast lDs
DIRECTLY-ADDRESSED EQUIPMENT MULTI-DROP OVERRIDE ID
Broadcast (all directly-linked devices) 00
DMD-3000/4000, 4500 or 5000 Mod Section, DMD15 01
DMD-3000/4000, 4500 or 5000 Demod Section, DMD15 02
RCU-340 1:1 Switch 03
RCS-780 1:N Switch 04
RMUX-340 Cross-Connect Multiplexer 05
CDS-780 Clock Distribution System 06
SOM-340 Second Order Multiplexer 07
DMD-4500/5000 Modulator Section 08
DMD-4500/5000 Demodulator Section 09
RCU-5000 M:N Switch 10
DMD15 Modulator 20