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DM240-DVB High-Speed Digital Modulator
4.9.5 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
Broadcast (all directly-linked devices)
DMD-3000/4000, 4500 or 5000 Mod Section, DMD15
DMD-3000/4000, 4500 or 5000 Demod Section, DMD15
RCU-340 1:1 Switch
RCS-780 1:N Switch
RMUX-340 Cross-Connect Multiplexer
CDS-780 Clock Distribution System
SOM-340 Second Order Multiplexer
DMD-4500/5000 Modulator Section
DMD-4500/5000 Demodulator Section
RCU-5000 M:N Switch
DMD15 Modulator
DMD15 Demodulator
DMD15 Modem
DVB3030 Video Modulator, DM240
Reserved for future equipment types
TM083 – Rev. 3.0
User Interfaces
Multi-Drop Override ID
00
01
02
03
04
05
06
07
08
09
10
20
21
22
23
24-31
4-21
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