Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 106)

Publication AG-UM008C-EN-P - February 2005
3-38 Configuring MicroLogix 1100/1200/1500 Controllers
For both Point-to-Multipoint and Point-to-Point Link Configurations
•
All MicroLogix 1100/1200/1500 controllers have the capability to
initiate and reply to PLC-5-type read and write messages by
choosing PLC-5 as the Target Device. Use this for both PLC-5
and Logix processors.
•
The maximum read or write message for a MicroLogix
1100/1200/1500 controller is 103 words.
•
The maximum read or write message to a MicroLogix 1000 is
41 words.
Minimum Master MSG Block Message Timeout
Once the master ACK timeout is determined (see page 3-14), then a
minimum MSG block Message Timeout value can be determined. This
value is calculated differently depending on the master polling mode.
Message-based Polling Mode
Message based polling mode messages are executed serially - each
message in the queue must complete as done, or in error, before the
next message in the queue is executed. For this mode, the minimum
MSG Timeout should be:
Realize that if the Message Retries is set for the default (3), then the
number of ACK Timeout periods is (3)*2+1=7 (message, poll, retry_1,
poll, retry_2, poll, retry_3), because a poll packet is sent between
every Message Retry.
However, the slave may still respond to retry 3 with an ACK just
before the ACK Timeout period expires, and the master would poll
after the Reply Message timeout period for the reply...which could
take approximately another ACK Timeout period to receive.
Therefore, for message-based polling mode, set the MSG Timeout
value to at least:
Maximum number of
simultaneously triggered
MSG instructions
The sum of ACK Timeouts
due to the maximum number
of Message Retries for each
queued-up message
=
*
The minimum MSG
Timeout
+
Reply Message
Wait timeout
2*(Message_Retries+1) ACK Timeout + Reply Message Timeout
=
The minimum MSG
Timeout
# of MSG instructions
* *