Allen-Bradley MicroLogix 1200 Reference Manual page 314

21-14 Communications Instructions
Publication 1762-RM001C-EN-P
"Control Bits" Parameters
Ignore if Timed Out (TO)
Address Data Format
MG11:0/TO Binary
The Timed Out Bit (TO) can be set in your application to remove an
active message instruction from processor control. You can create your
own timeout routine by monitoring the EW and ST bits to start a timer.
When the timer times out, you can set the TO bit, which removes the
message from the system. The controller resets the TO bit the next time
the associated MSG rung goes from false to true.
An easier method is to use the message timeout variable described on
page 21-12, because it simplifies the user program. This built-in timeout
control is in effect whenever the message timeout is non-zero. It defaults
to 5 seconds, so unless you change it, the internal timeout control is
automatically enabled.
When the internal timeout is used and communications are interrupted,
the MSG instruction will timeout and error after the set period of time
expires. This allows the control program to retry the same message or
take other action, if desired.
To disable the internal timeout control, enter zero for the MSG instruction
timeout parameter. If communications are interrupted, the processor waits
indefinitely for a reply. If an acknowledge (ACK) is received, indicated by
the ST bit being set, but the reply is not received, the MSG instruction
appears to be locked up, although it is actually waiting for a reply from
the target device.
Enable (EN)
Address Data Format
MG11:0/EN Binary
The Enable Bit (EN) is set when rung conditions go true and the MSG is
enabled. The MSG is enabled when the command packet is built and put
into one of the MSG buffers, or the request is put in the MSG queue. It
remains set until the message transmission is completed and the rung
goes false. You may clear this bit when either the ER or DN bit is set in
order to re-trigger a MSG instruction with true rung conditions on the next
scan.
Do not set this bit from the control program.
IMPORTANT
Range Type User Program Access
On or Off Control Read / Write
Range Type User Program Access
On or Off Control Read / Write