Instructions
Chapter 18
I/O Message and Communication
18–2
This is an output instruction that allows you to transfer data from one node to
another on the DH–485 network. The instruction can be programmed as a
write message or read message. The target device can be another SLC 500
processor on the network, or a non-SLC 500 device, using the common
interface file (data file 9 in SLC 500 processors).
When the target device is SLC 500, communication can take place between
two SLC 5/02 processors or between a SLC 5/02 processor and a fixed or
SLC 5/01 processor. The instruction cannot be programmed in the fixed or
SLC 5/01 processor.
The data associated with a message write instruction is not sent when you
enable the instruction. Rather, it is sent at the end of the scan during service
communications of the operating cycle or at the time an SVC or REF
instruction in your ladder program is enabled. In some instances, this means
that you must buffer data in your application.
The processor can service only one message instruction at any given time,
although the processor may hold several messages “enabled and waiting.”
Waiting messages are serviced one at a time in sequential order (first in first
out).
Related Status File Bits
Two status bit files are related to the MSG instruction:
•
Bit S:2/6, DH–485 Message Reply Pending – Read only. This bit
becomes set when another node on the DH–485 network has supplied the
information or performed the action that you have requested in the MSG
instruction of your processor. This bit is cleared when the processor
stores the information and updates your MSG instruction status bits.
Use this bit as a condition of an SVC instruction to enhance the
communications capability of your processor.
•
Bit S:2/7, DH–485 Outgoing Message Command Pending – Read only.
This bit is set when one or more messages in your program are enabled
and waiting, but no message is being transmitted at the time. As soon as
transmission of a message begins, the bit is cleared. After transmission,
the bit is set again if there are further messages waiting, or it remains
cleared if there are no further messages waiting.
Use this bit as a condition of an SVC instruction to enhance the
communications capability of your processor.
You may also be concerned with the function of status file bit S:2/15,
DH–485 Communications Servicing Selection Bit. Refer to chapter 27.
