Honeywell BacNet protocol, Инструкция по установке и эксплуатации

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9
vacon • 37 Appendix - additional information
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9. A
PPENDIX
-
ADDITIONAL INFORMATION
9.1 Max master
This parameter represents the value of the Max_Master property of the node's Device object.
The value of Max_Master specifies the highest allowed address for the master nodes. The val-
ue of Max_Master shall be less than or equal to 127. If Max_Master is not writable in a node,
its value shall be 127. The drive has a readable-only value, thus 127.
Example of use:
We have 4 devices on the bus, given addresses from 1 to 4. A token is passed in the MS/TP
allowing each device to send in turns. Device no. 1 passes the token to device no. 2, no. 2 to
device no. 3, no. 3 to no. 4 and no. 4 to no. 1.
Max master is set to 127:
Having received the token and sent its data, the device with the highest address on the bus (in
this case
4
) will check the bus for new devices by sending a Poll for master message. After the
first token-passing round, the Poll for master message is sent to address 5, after the second
round time to address 6 and so forth until address 127 is reached. Because there are no de-
vices at addresses 5 to 127, the sender of the message must wait for a reply to each
Poll for
master message after each token-passing round.
If there is on the bus another manufacturer’s device that supports Max master property, it can
be used to prevent the
Poll for master
messages by giving it the highest address on the bus
and setting the same value to Max master parameter. The device now understands being at
the last address of the bus and passes the token to the first device.