Applies To:
  • CitectSCADA 1.00 1.01 1.10 1.11 1.20 2.00 2.01 3.00 3.01 3.10

Question: I sometimes get the error response bad or garbled with the TIDIRECT serial protocol. Most of the time the protocol works fine and I can get valid data back from the PLC and write commands down to the PLC. I am using the local COM port running at 19.2 Kbaud and the setup is as per the online help. What is causing this problem?

The problem you are seeing is caused by the local COM port dropping a single character. If a single character is dropped then the entire protocol frame will be bad and Citect will report the hardware error "response bad or garbled". You may also get the driver error 1 which is an overrun error.

You most likely have a non buffered UART, this is a type 8550 chip. The 8550 UART can only buffer 1 character at a time, so if Windows is busy for even a short period of time the single character can be lost when then next character arrives. You should be using the buffered UART 16550. The 16550 UART can buffer up to 16 characters so the chance of loosing any characters is greatly reduced. Some low end computers are still shipped with the slower UART (to save a few dollars!). You can check what type of UART your computer has by using the utility MSD. Run MSD and select the COM PORTS to display information on your communication ports.

If you have a computer with the slower 8550 UART you can slow down the baud rate until the errors go away. You may have to slow the baud rate down substantially to make this error go away a better solution is to add a new COM port with higher performance. This could include a Digiboard or a lower cost option is to install a single 16550 UART on a COM port add in board.

There also was a problem with Citect for Windows version 3.0 and 3.01 when Citect checked the software protection key you could looses a character. This problem has been solved in version 3.10. If you are using versions 3.0 or 3.01 and you have a software protection key on the I/O Server then upgrade to version 3.10.