Opens a communication port for access. The board and port need to both be defined in the database (using the Boards and Ports forms from the Communication menu).
If you try to open the same COM port twice with ComOpen(), the second open will not succeed and return -1. If this is passed without checking other Com functions, the COM port may not do anything. For this reason, do not open COM ports twice, and always check the return value from ComOpen().
The communication system should be used for low speed communications only. You should not use the communication functions to communicate with high speed PLCs - the performance may not be adequate. If you need high speed communication (for communicating with PLCs, etc.), you should write a protocol driver. Refer to the CitectSCADA "Driver Development Kit".
This function can only be called from an I/O Server.
The port name as specified in the Ports database.
The mode of the open:
0 - Take control of the port from CitectSCADA. In this non-shared mode, you have complete access to the port - CitectSCADA cannot use the port. Communication will be restored when the port is closed.
1 - Share the port with CitectSCADA. In this mode, you can write to the port, and CitectSCADA can also use it. Please be aware that ComRead will be unreliable if the communication port is opened as shared.
A communication port handle if the communication system is opened successfully, otherwise -1 is returned. The handle identifies the table where all data on the associated port is stored. You can use the handle in the other communication functions, to send and receive characters from the port.
ComClose, ComRead, ComWrite
hPort = ComOpen(sPort, 0);
IF hPort < 0 THEN
Prompt("Cannot open port " + sPort);
TaskNew("SerialRead", hPort, 0);
TaskNew("SerialWrite", hPort, 0);
WHILE 1 DO
! put data into buffer and set length
SerialWriteError = ComWrite(hPort, buffer, length, 2);
IF SerialWriteError THEN
Prompt("Error Writing port");
total = 0;
WHILE 1 DO
length = 128; ! need to set length as read modifies
SerialReadError = ComRead(hPort, buffer, length, 2);
IF SerialReadError THEN
Prompt("Error from port " + SerialReadError : ####);
! get data from buffer, length is set to number read