GE GFK-1004B user manual Monitoring the Communications Channel, Monitoring the Commreq Status Word

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Monitoring the Communications Channel

Once you have a working ladder program, you can use the status data to monitor your communications activity and take the desired action upon certain events. The primary indicators of a communications channel are the Channel Status bits: Channel Error bit and Data Transfer bit. In addition, the CRS word and the DCS words can be used to more precisely troubleshoot any problems that may occur.

Monitoring the COMMREQ Status Word

It is critical to monitor the CRS word for each COMMREQ function you initiate. First, zero the associated CRS word before executing the COMMREQ function. Then when the CRS word becomes non-zero, you know the Ethernet Interface has updated it. If the CRS word is updated to a one (1), then the Command Block was processed successfully by the Ethernet Interface. If the CRS word is updated to a value other than 1, then an error has occurred in processing the Command Block. See Section 4 of this chapter for CRS word error codes.

Do not use data received from a server until the CRS word for that channel is 1 or the Data Transfer bit goes to 1.

Monitoring the Channel Error Bit

This bit (normally 0) is the primary indicator for an error on a channel. It indicates any channel error, fatal or non-fatal. It does not necessarily indicate that the channel is down (idle). You may want to monitor this bit and simply reinitiate the Read or Write command if the bit indicates an error. Or you may want to execute the Retrieve Detailed Channel Status Command to find out if the channel is down and possibly why it went down. Keep in mind, however, that the status code may change from between the time the Channel Error bit indicates an error and the time the Retrieve Detailed Channel Status Command retrieves the code.

The Channel Error bit for a channel is not meaningful until after the Ethernet Interface updates the CRS word confirming the Read or Write command for that channel. In the case of an Establish Channel command, the CRS word is updated before the Channel Error bit is set to 1.

Monitoring the Data Transfer Bit

Typically you will set up a channel to perform repetitive reads or writes. The Data Transfer bit pulses ( 0 → 1 → 0) each time there is a successful read or write. This can be an indicator to the ladder program to move the most recent data to another location.

The Data Transfer bit for a channel is not meaningful until after the Ethernet Interface updates the CRS word confirming the Read or Write command for that channel.

Do not use data received from a server until the CRS word confirming the Read command for that channel is 1 or the Data Transfer bit goes to 1. Do not assume that when the Data Transfer bit goes to 1 that a transfer has just completed during the last scan. The Data Transfer bit is not closely synchronized in time with the transfer. The bit only indicates that a transfer has occurred in a past scan. A rising edge on the Data Transfer bit indicating that a transfer has completed successfully does not guarantee that the next transfer has not begun or completed.

4-38 TCP/IP Ethernet Communications User's Manual ± January 1996 GFK-1004B