Comtech EF Data DM240XR User Manual
Page 58
DM240XR High-Speed Digital Modulator
User Interfaces
MN-DM240XR– Revision 13
4–36
4.4.5 Collision Avoidance
When properly implemented, the physical and logical devices and ID addressing scheme of the
COMMSPEC normally precludes message packet contention on the control bus. The importance
of designating unique IDs for each device during station configuration cannot be overemphasized.
One 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 DM240 Modulators
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 23. 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 below with ID values expressed in decimal notation:
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
DMD15 Demodulator
21