Installation manual

Publication 1747-RM001G-EN-P - November 2008
12-30 SLC Communication Instructions
MSG Instruction Control Block
Limitations for Manipulating the Control Block Bits
Do not manipulate the MSG instruction control block values except as noted
below. For example, do not clear the first word of the control block, do not
unlatch the time-out control bit (except in an SLC 5/02 MSG instruction), and
so on.
The only MSG instruction control bits that may be manipulated by the ladder
program without adversely affecting the operation of the instruction are the
CO, EN, and TO bits. The enable bit (EN = bit 15) may be unlatched, but
only when the done bit (DN = bit 13) or error bit (ER = bit 12) has been set,
indicating the successful or unsuccessful completion of the previous message.
In addition, when a MSG is in progress and the ladder program wishes to
terminate it for any reason, this may be done by enabling the time-out bit (TO
= bit 8). The next time the processor scans the MSG instruction with the TO
bit set, it will error the MSG (ER = 1). The MSG instruction may then be
re-enabled with a false-to-true transition on the next program scan.