Comtech EF Data QAM256 - Manual (Page 60)

QAM256 Digital Video Modulator and Upconverter Remote Operations
TM077 – Rev. 4.0 C- 5
C.1.5 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 QAM256 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
DMD15 Modem 22
DM45 Video Modulator 23
QAM256 Modulator 32
Reserved for future equipment types 24 – 31
Table 3. Broadcast ID’s
Note that multi-drop override ID 01 can be used interchangeably to broadcast a message to a
DMD-3000/4000 modem, a DMD-4500/5000, a DMD15 modem, or a DM45. Radyne Corp.
recommends that experienced programmers issue the multi-drop override IDs only during system
configuration as a bus test tool, and that they not be included in run-time software. It is also