Rockwell Automation DAG6.5.8 APPLICATION GUIDE SCADA SYSTEM - Manual (Page 349)

Publication AG-UM008C-EN-P - February 2005
Basic DF1 Protocol Troubleshooting B-3
5. Use the receiving processor’s RS-232 LED to verify that
characters are being transmitted out of the serial port. If
troubleshooting the MicroLogix 1000 controller with a modem
connection through the AIC+, then the AIC+ TX LEDs provide
this indication. If no characters are being transmitted, the
receiving processor may not be replying to the message because
it was not properly received. Check for:
•
incorrect cable between the modem and receiving processor.
•
improper modem cable or modem configuration. (Required
modem handshaking signals based on the processor serial
port Control Line configuration are not present at the
processor’s RS-232.)
•
baud rate mismatch between modems and processors.
•
error detection (BCC or CRC) mismatch between sending and
receiving processors.
•
destination address mismatch between MSG and receiving
processor (DF1 half-duplex protocol only).
6. Use the initiating modem’s receive data LED to determine
whether data packets are being received back by the modem. If
not, determine problem with modem link.
If you are unable to determine the cause of the MSG error after going
through this list, then further analysis with a serial line analyzer may
be required. Use the following sections to understand the DF1 hex
codes you should expect to capture with your serial line analyzer.
DF1 Protocol
Allen-Bradley products send data asynchronously and serially over the
RS-232 interface, one 8-bit byte at a time. The transmission format
conforms to ANSI X3.16, CCITT V.4, and ISO 1177, with the exception
that the parity bit is retained while the data length is extended to 8
bits.
The following illustration summarizes the transmission format.
start bit bit 0 bit 1 bit 2 bit 3 bit 4 bit 5 bit 6 even parity bit
(optional-normally not used)
stop bit
data bits
bit 7