Applies To:
  • CitectSCADA 5.00
Summary:
Question: How does the Comx driver detects the end of an incoming stream of characters? 

Solution:
When operating the Comx driver in interrupt mode, the Comx driver should only call back to the protocol driver when it detects the end of a incoming frame. It should not call the protocol driver on every character received. The Comx driver call back to the protocol drivers when there is a pause of [Comx]CharTimeOut number of character time in the incoming stream. One character time is the time it takes to sent one character.

Setting a low value will cause too many call backs to the protocol driver. Setting a value too high may cause the protocol driver to be delayed in responding to the incoming characters.

The default value for CharTimeOut is 5 character time. Which should work satisfactorily for a receive FIFO buffer size of 4 bytes. For receive FIFO buffer size of 8 and 14 bytes the CharTimeOut should be set to 8 and 10 respectively.

A receive FIFO buffer size of 1 will cause comms errors even on a Pentium 100 MHz machine.

For Window 95 the FIFO buffer size can be modified via Control Panel / System / Device Manager / Ports Communication Ports/ Port Settings / Advanced.

 
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