Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 152)

Publication AG-UM008C-EN-P - February 2005
4-14 Configuring SLC 5/03, 5/04, and 5/05 Processors
The timeout must be long enough that after the master has transmitted
the last character of the poll packet, there is enough time for a slave to
transmit (and the master receive) a maximum-sized packet before the
time expires.
To calculate the minimum ACK timeout, you must know:
•
the modem baud rate
•
maximum-sized data packet (the maximum number of data
words that a slave write command or read reply packet might
contain)
•
the RTS/CTS or turnaround delay of the slave modem
•
the configured RTS Send Delay in the slave
•
the program scan time of the slave
Determining Minimum Master ACK Timeout
To determine the minimum ACK Timeout, you must first calculate the
transmission time by multiplying the maximum-sized data packet for
your processor by the modem rate in ms/byte. For an example we
will assume an SLC 5/03 processor (103 data words or 224 bytes total
packet size including overhead) and a 9600 bps modem, which
transmits at approximately 1 ms/byte. Therefore, the message
transmission time is 224 ms. For approximate modem transmission
rates, see the following table.
Next, you need to determine the average slave program scan time. In
RSLogix 500, double click on the Processor Status icon and then locate
Average on the Scan Times tab. For this example, assume an average
slave program scan time of 20 ms. Remember, program scan time will
vary by application.
Table 4.2 Approximate Modern Transmission Rates
modem bps approx. ms/byte
4800 2 ms/byte
9600 1 ms/byte
19200 0.5 ms/byte