The challenges most often encountered when using a dial-up remote I/O Device involve communications speed, parity, and control signals from the connected equipment. If changing the speed and parity does not resolve a communications interruption, evaluate the modem's answering codes and command echoing.
Following is a list of settings that might be helpful in resolving dial-up communications interruptions. (Since not every modem supports the same in commands in the same way, this is only a guide. Consult the modem manual for exact details.)
On the modem at the PC end
ATV1 //Enables long-form (verbose) result codes
ATQ0 //Result codes are sent on the RS-232 connection
ATE0 //Commands sent from the computer are not echoed back to the RS-232 connection
AT&C1 //DCD will follow carrier on the line
AT&K0 //Handshaking OFF
ATW0 //Upon connection, only DTE speed is reported
AT%C0 //Compression OFF
AT&D0 //DTR always on
If the modem at the PC end is configured so that calls are automatically answered even when your Citect SCADA project is off, data being reported from the I/O devices may be lost. Therefore, it is recommended that you turn off the PC modem's auto-answer feature before taking your project offline. To do this, set the following parameter to zero:
ATS0 = 0 // Auto answer OFF
Be aware, however, this will also impact applications that might use the modem other than Citect SCADA, as the modem cannot answer a call while Citect SCADA is not driving its functionality.
On the modem at the I/O Device end
ATV0 //Enables short-form result codes
ATQ1 //No result codes are sent on the RS-232 connection
ATE0 //Commands that are sent from the computer are not echoed
back to the RS-232 connection
AT&C1 //DCD will follow carrier on the line
AT&K0 //Handshaking OFF
ATW0 //Upon connection, only DTE speed is reportedAT%C0 //Compression OFF
AT&D0 //DTR always on
ATS0 //Set to greater than 0 (sets the number of rings necessary before the modem answers an incoming call).
Published June 2018