Troubleshooting PLC Communication with Lookout and Lookout Protocol Drivers

This document discusses steps to troubleshoot typical communication problems between a PLC and Lookout or Lookout Protocol Drivers.

Verify PLC Communication

1. Are you connecting the PLC appropriately? Are you using the right interface and/or the right cables?
Make sure you refer to the PLC documentation for setting up the connection. One thing to keep in mind is there are two types of serial cables: straight and null-modem cable. If using a serial connection with a straight serial cable produces problems, try changing it to null-modem serial cable, or vice versa, as this might fix the problem.

2. Can the PLC communicate with its proprietary software?
For example if you have an Allen Bradley PLC, verify that it can communicate with RSLinx, or that a DirectLogic PLC can communicate with DirectSoft software. If your PLC can't communicate with the proprietary software, then it will not be able to communicate with third-party software such as Lookout. Contact the PLC manufacturer for technical support in troubleshooting PLC communication with its proprietary software. Once you get it to communicate with the proprietary software, write down the configuration settings for future reference.

Common Problems for Communicating with a PLC in Lookout or Lookout Protocol Drivers

1. Are you using the proper configuration such as baud rate, protocol, and parity?
Once you are able to communicate with the proprietary software and you note those settings, use the same configuration to connect to Lookout or to any other software. Write the settings down if necessary, for future reference.

2. Are the Receive Timeout and serial port settings appropriate?
If communication problems are still not corrected, try increasing the Receive Timeout in the PLC object configuration and/or the serial port Receive gap. Receive Timeout is the amount of time Lookout will wait for a response before it produces an alarm. Receive gap specifies the number of empty bytes (or amount of time) a driver receives from a controller before the driver recognizes the end of a message frame and asks for another message. Normally you should leave this at the default setting of 5. However, if you are experiencing garbled communication alarms, you might try increasing this number to allow more dead time before Lookout decides it has received a complete message. For example, with a slow baud rate of 1200, you might have to increase the Receive gap setting to approximately 30.

If your device requires hardware handshaking, reset the serial connection on the serial port settings to be Radio (RTS/CTS).

To access the serial port configuration, select Options>>Serial Ports in Lookout and the following dialog box appears.

Troubleshooting Common Problems for a PLC with Direct Serial Connection

1. Having problems communicating with serial connection?
There are two types of serial cables: straight and null-modem. Make sure you are using the correct cable for the PLC. Check the PLC documentation for which cable to use.
Make sure your serial port can send and receive data appropriately. You can test it by doing a loopback test. For more information on loopback testing, check out Serial Communication Starting Point.
You can also monitor serial communication between the PLC and your computer using DiagnosticFile. For more information about DiagnosticFile, refer to our KnowledgeBase entry Monitor COM Port Activity in Lookout and Lookout Protocol Drivers.

2. Does the PLC communicate well with the proprietary software but not with Lookout, even after following the previous steps?
Try using the Port Monitor freeware from SysInternals to monitor communication between the proprietary software and the PLC, then use the same freeware to monitor communication between Lookout and the PLC, and compare the results.

Troubleshooting Common Problems for a Remote PLC with Dial-Up or Radio Modem

1. Are you having trouble communicating with a remote PLC using Lookout?
If you are connecting to a remote PLC using a radio or dial-up modem, the first thing you should check is whether Lookout has configured the COM port for remote connections. Select Options>>Serial Ports in Lookout. Select Dial-Up for a modem connection or Radio for a radio modem connection then click Accept.
Make sure the two modems can communicate with each other by calling the remote modem from HyperTerminal. Perform a loopback test to make sure the serial port can communicate properly. For more information on loopback testing, check out Serial Communication Starting Point.

2. Is the modem hanging up after the first poll?
Refer to Why does my modem hang up after I polled my remote PLC with Lookout?, which discusses this problem.

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