Comtech EF Data DD2401 VME - Manual (Page 34)

User Interfaces DD2401 VME L-Band Demodulator Card Installation & Operational Manual
4-10 MN-VME2401 – Rev. B
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 DD2401/DD2401L Demodulators 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 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
DVB3000/DVB3030/DM45/DM240 Video Modulator 23
RCS20 M:N Switch 24
RCS10 M:N Switch 25
RCS11 1:1 Switch 26
DD2401/DD2401L Demodulator 27
Reserved For Future Equipment Types 28-31
Note that multi-drop override IDs 01 or 02 can be used interchangeably to broadcast a message
to a DMD3000/4000 modem, or to a DMD4500/5000, or to a DMD15 modem. Comtech EF Data
recommends that the multi-drop override IDs be issued only during system configuration as a bus