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User Interfaces
DPARM_NOTIMPLEMENTED_ERROR
DPARM_DATAINVERT_ERROR
DPARM_SUMMARYFAULT_ERROR
DPARM_EXTERNALEXCSOURCE_ERROR
DPARM_BUFFERSIZEMSEC_ERROR
DPARM_BUFFERSIZEBYTES_ERROR
DPARM_ASYNCINTERFACETYPE_ERROR
DPARM_BUFFERSIZEMSECBYTES_ERROR
MDPARM_TIME_ERROR
MDPARM_DATE_ERROR
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 DMD2401 LB/ST
Modems 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:
4-34
DMD2401 LB/ST L-Band Satellite Modem and ODU Driver
0x0622
0x0623
0x0624
0x0625
0x0629
0x062A
0x0630
0x0631
0x0A01
0x0A02
TM075 – Rev. 1.3
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