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Debugging Communication Between an I/O Server
and a PLC
This section provides you with simple steps to diagnose and correct server to PLC
communication problems. The debug techniques listed below address both serial and
board servers. Disregard any information that is not applicable to the server type that
you are using.
When attempting to establish communication between a server and a PLC, if no data
appears in the server's program window and the data items are not updating in
WindowViewer, switch to the Wonderware Logger and check for error messages.
For more information on the Wonderware Logger, see your online “FactorySuite
System Administrator's Guide.”
For example, some of the most common errors that may appear in the Wonderware
Logger for serial servers are:
Response Timeout
WCRET =-2
WakeUp=-2
Receive Overrun
Framing Errors
Note
Unless specified otherwise, most serial communication based servers are full
duplex. If you require a server for half duplex (one that monitors the CTS and RTS
lines) or if you are not sure whether the PLC's protocol is full or half duplex, call your
PLC supplier.
Also, during in-house server testing, we have found that the communication cards that
use the National 16450 and 16550 UARTs seem to be less susceptible to level and
timing problems. Cards based on other chips may work, but we recommend using the
National cards. Some of the highly integrated UART chips (most notably, Winbond
and UMC) have a tendency for their transmitters to hang, requiring re-initialization of
the UART. If this occurs, you may have to restart the server or execute the Reinitialize
I/O command from the Special menu in WindowViewer.
Check your cabling to the PLC.
Is it wired correctly? Check for shorts, loose wires, broken wires, crossed wires, and so
on.
A continuity tester can be helpful here.
Verify the I/O Server’s serial configuration settings (Parity, Stop Bits, Baud Rate,
Handshaking and so on) against the settings in the hardware device.
Verify the communication port is working properly in Windows.
1. Close the server program.
If you are using a server that requires a TSR, you will not be able to verify that
the port is functioning properly while the TSR is running. Stop all TSRs then
continue with this procedure. If you confirm that the port functions properly
without the TSR running, change your software interrupt (IRQ) to another
number, for example, change 60 to 62.
Also, if you are using an AT type computer, two devices cannot share
interrupts. Verify that the communication port you are using has a unique
interrupt setting.
