Rockwell Automation Publication 1766-UM001O-EN-P - September 2021 63
Chapter 4 Communication Connections
Construct Your Own Null Modem Cable
If you construct your own null modem cable, the maximum cable length is
15.24m (50 ft) with a 25-pin or 9-pin connector. Figure 47 shows a typical
pinout.
Figure 47 - Typical Null Modem Cable Pinout
Connect to a DF1 Half-Duplex Network
Table 8 shows available parameters for a communication port that is
configured for DF1 Half-Duplex Slave.
DF1 Half-Duplex Master-Slave Network
Use Figure 48 for DF1 Half-Duplex Master-Slave protocol without hardware
handshaking.
Optical Isolator Modem
9-Pin 25-Pin 9-Pin
3TXD TXD2 3
2RXD RXD3 2
5GND GND7 5
1CD CD8 1
4DTR DTR204
6DSR DSR6 6
8CTS CTS5 8
7RTS RTS4 7
Table 8 - DF1 Half-Duplex Configuration Parameters
Parameter Options
Baud Rate 300, 600, 1200, 2400, 4800, 9600, 19.2 Kbps, 38.4 Kbps
Parity none, even
Node Address 0...254 decimal
Control Line no handshaking, half duplex modem (RTS/CTS handshaking, no handshaking (DH-485 network)
Error Detection CRC, BCC
EOT Suppression
enabled, disabled
When EOT Suppression is enabled, the slave does not respond when polled if no message is queued. This saves modem transmission
power and time when there is no message to transmit.
Duplicate Packet (Message) Detect
enabled, disabled
Detects and eliminates duplicate responses to a message. Duplicate packets may be sent under noisy communication conditions if the
sender’s Message Retries are not set to 0.
Poll Timeout (x20 ms)
0...65,535 (can be set in 20 ms increments)
Poll Timeout only applies when a slave device initiates a MSG instruction. It is the amount of time that the slave device waits for a poll from
the master device. If the slave device does not receive a poll within the Poll Timeout, a MSG instruction error is generated, and the ladder
program needs to re-queue the MSG instruction. If you are using a MSG instruction, it is recommended that a Poll Timeout value of zero
not be used. Poll Timeout is disabled when set to zero.
RTS Off Delay (x20 ms)
0...65,535 (can be set in 20 ms increments)
Specifies the delay time between when the last serial character is sent to the modem and when RTS is deactivated. Gives the modem
extra time to transmit the last character of a packet.
RTS Send Delay (x20 ms)
0...65,535 (can be set in 20 ms increments)
Specifies the time delay between setting RTS until checking for the CTS response. For use with modems that are not ready to respond with
CTS immediately upon receipt of RTS.
Message Retries
0...255
Specifies the number of times a slave device attempts to resend a message packet when it does not receive an ACK from the master
device. For use in noisy environments where message packets may become corrupted in transmission.
Pre Transmit Delay
(x1 ms)
0...65,535 (can be set in 1 ms increments)
• When the Control Line is set to no handshaking, this is the delay time before transmission. Required for 1761-NET-AIC physical Half-
Duplex networks. The 1761-NET-AIC needs delay time to change from transmit to receive mode.
• When the Control Line is set to DF1 Half-Duplex Modem, this is the minimum time delay between receiving the last character of a packet
and the RTS assertion.
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