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DM240XR High-Speed Digital Modulator User Interfaces

MN-DM240XR– Revision 13 4–36

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 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 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

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Radyne DM240XR Especificaciones Pagina 58