RCS20 M:N Redundancy Switch
User Interfaces
TM056 - Rev. 2.3
4-15
RESPONSE OPCODE DESCRIPTION
OPCODE
Good Message
00h
Bad Parameter
FFh
Bad Opcode
FEh
4.7.5 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 devi ces 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:
Summary of Contents for RCS20
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