Using the Message Instruction
13–7
•
Start Bit ST (bit 14) is set when the processor receives acknowledgement
from the target device. This identifies that the target device has started to
process the MSG request. The ST bit is reset when the DN, ER, or TO bit is set
or on a false-to-true MSG rung transition.
•
Enable Bit EN (bit 15) is set only if the transmit buffer is available. If the
transmit buffer is not available, the EN flag remains false. When the transmit
buffer becomes available, the EN flag goes true. It remains set until the next
false rung execution after the MSG completes (DN bit set) or an error occurs
(ER bit set).
Note
The operation of the EN bit has changed since Series C.
The operation associated with a message read or write instruction is sent when you
enable the instruction. Replies are processed at the end of the scan.
Controller Communication Status Bit
When using the MSG instruction, you should also use the following controller
communication status bit:
Active Protocol Bit (S:0/11) – This is a read only bit that indicates which
communication protocol is currently enabled or functioning; where 0 = DF1
(default) and 1 = DH-485. Use this bit in your program to restrict message
operation to the specific protocol in use.
Programming
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