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342 Rockwell Automation Publication 2080-UM002N-EN-E - November 2022
Appendix G Connect to Networks using DF1
Minimum DF1 Half-Duplex Master ACK Timeout
The governing timeout parameter to configure for a DF1 Half-Duplex Master is the serial port
ACK Timeout. The ACK Timeout is the amount of time you want the controller to wait for an
acknowledgment of its message transmissions. Set in 20 millisecond intervals, the value is the
amount of time the master will wait for:
• An ACK to be returned by a slave when the master has just sent it a message, or
• A poll response or message to be returned by a slave when the master has just sent it a
poll packet.
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 controller by the modem rate in ms/byte.
For an example we will assume a Micro800 controller (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 Connected Components
Workbench software, double-click the controller variables in the Controller Organizer and
locate the system tag _SYSVA_TCYCYCTIME in the Variable tab. For this example, assume the
program scan time is 20 ms. The program scan time varies by application.
Finally, you must determine the larger of two values, either the configured slave RTS Send
Delay or the turnaround time of the slave modem. The RTS Send Delay time can be found by in
the Configuration screen of the Micro800 embedded serial port. Note that the RTS Send Delay
time is in intervals of 20 ms, so with a value of 3 in the box, the RTS Send Delay time would be
20 ms multiplied by 3. Using this value (60 ms) for our example, and assuming that the
turnaround time of the modem is 50 ms (which will vary by modem), you would choose to use
the RTS Send Delay time of 60 ms for your calculation.
Table 93 - Approximate Modem Transmission Rates
Speed (bps) Rate, approx (ms/byte)
4800 2 ms/byte
9600 1 ms/byte
19200 0.5 ms/byte