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 CitectSCADA 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 CitectSCADA, as the modem cannot answer a call while CitectSCADA 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 reported
AT%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).