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DMD15/DMD15L IBS/IDR Satellite Modem User Interfaces
TM051 - Rev. 5.8 4-51
MDPARM_MAPNUMBER_ERROR 0x0A00
MDPARM_TIME_ERROR 0x0A01
MDPARM_DATE_ERROR 0x0A02
MDPARM_TERMINALEMULATION_ERROR 0x0A05
MDPARM_TERMINALBAUDRATE_ERROR 0x0A06
MDPARM_EBNO_THRESHOLD_ERROR 0x0A07
4.9.1.5 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 DMD15 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 then returns a response packet to the remote M&C.