Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM User Manual
Page 107
Publication AG-UM008C-EN-P - February 2005
Configuring MicroLogix 1100/1200/1500 Controllers 3-39
To continue the example, if Message Retries is configured for 3, ACK
Timeout is configured for 16 * 20 ms = 320, and Reply Message
Timeout is configured for 1* 20 ms, the MSG Timeout value would be:
If five MSG instructions were triggered at the same time, each MSG
would need a timeout value of 5*2.58 = 12.9, which would be
rounded up to 13 seconds.
Standard Polling Mode
For standard polling mode, the transmission of messages by the
master can occur anywhere in the poll scan. This means that the
master might transmit a message to station A just after it has
completed polling station A as part of the poll scan. In this worst case
scenario, it will take almost an entire poll scan before station A will be
polled again and can reply to the master message so that the message
can complete done or in error. In standard polling mode, the MSG
Timeout should be at least as long as this maximum poll scan.
The maximum poll scan = The minimum MSG timeout
Standard Polling Mode With Single Message Transfer
For standard polling mode with single message transfer per poll scan,
the maximum poll scan would be achieved when every slave had a
maximum-sized message packet to transmit when polled. Recall that
the ACK Timeout is calculated as being just long enough for the
master to receive a maximum sized message packet, so the maximum
poll scan time would be approximately:
Therefore, if there are 10 slave stations and the ACK Timeout is
320 ms, then the maximum single message transfer poll scan time
would be:
TIP
Leave the Message Retries at default (3) unless you
have an extremely error free or error prone network.
2 (3+1)
.320 seconds + .02
2.58 seconds
Round up the MSG Timeout value to the nearest second (3)
1
*
ACK
Timeout
The number of
slave stations
=
*
The maximum single
message transfer poll
scan time
=
The minimum
MSG Timeout
10 (slave stations)
.320 seconds =
*
3.20 seconds
Round up the MSG Timeout value to the nearest second (4)