Modem Troubleshooting

Before integrating modems into your applications, check the section Modems-Tips & Cautions.

Note ♦ The modem must reply with either OK or READY to each command entered. If the modem fails to answer, the command has not been processed.

+++	Escape Sequence. This causes the modem to close connections and go back to command mode
AT	This command means Attention; and is used to begin a session
AT&F	Restores factory default settings
ATZ	Resets the modem. This command may take time to implement, so the response from the modem may be delayed
ATE0	No Echo
V1	Enable Verbose (long) response
Q0	Respond
X4	Detailed answers
&D0	Ignore DTR
&D2	Once DTR falls, disconnect and go to command mode
&D1	Once DTR falls, disconnect
&S0	DSR always ON. Since the DSR can be permanently set to ON, connecting it to the RTS causes the terminal always be ready to transmit\receive data
&S1	DSR OFF in command and test modes
&C1	Give the user a signal for the DCD
&C0	Don’t give the user a signal for the DCD (refers to LED indications where relevant)
ATS0=1	Auto-Answer after 1 ring
S0=0	Modem doesn't answer. Forces PLC to answer with ATA (pickup)
S10=15	Sets the time ( in units of 0.1 sec) from the time when CD is not detected, until the string NO CARRIER is shown. If the value is 255, then the CD signal will not fall—even if the modems are no longer connected
S7=30	Time-out: If this time is exceeded, the modem notifies that dial has failed
S12	The modem register that defines the time interval during which the line must remain clear, before and after the +++ command.
&W	Burn the configuration into the modem’s non-volatile memory. Note ♦ This is part of the COM Init FB's modem default initialization strings.

This deals with errors that may result from the PC's modem

Message	Cause
Com Port not open, or modem does not exist	The PC was unable to access the PC port. The port may: -Already be in use -Be damaged.
Modem not connected	The PC receives no reply from the modem following the 'AT' command. Check that: -The modem is connected to the same PC port you have defined in Modem Services. -The PC-modem cable is in proper order.
Modem not initialized	The modem was not successfully initialized. Check the topic: Using Hyperterminal for Modem Troubleshooting
The messages below describe the modem 's status if the PC dial attempt (ATD+ number) fails. Any one of these errors aborts the Dial process.
Modem Busy
Modem Error
No Dial Tone
No Carrier	Note ♦ This can occur if, within the modem initialization strings, the parameter S7 TimeOut, is to short to permit the PLC's modem to answer. For example, if this parameter is set as S7=30, the PC modem will wait for 3 seconds to receive an answer from the PLC's modem. If the PLC modem does not answer before the 3 seconds have elapsed, the S7=30 parameter is exceeded, and the PC modem will return the No Carrier error.
Dial time-out exceeded	No reply was received from the modem within the defined time.
The messages below only relate to unsuccessful GSM modem initialization.
GSM SIM card blocked
GSM SIM card does not exist
Illegal GSM PIN code
GSM Network not found
Time-out exceeded

These errors may result from problems in the PLC-side modem

Message

Possible cause

Recommended action

Modem Busy

Modem is engaged,
or is being initialized

Check that the line is free.
Use the SBs: Modem Initialization Status listed above to check the COM port status; communications cannot flow through the port during initialization. For more information check the topic How the M90 works with a modem.

Handshake between modems complete ('CONNECT'),

PLC does not reply

Modem adapter cable

Check the PLC-to-modem connection and pin-out, particularly that the DSR is connected to the RTS on the modem side.

Problem

SB74

Possible Cause & Recommended Action

Modem fails to initialize

PLC-to-modem cable:

Make sure that the cable is securely connected. Check the modem connection and pin-out of the PLC-to-modem adapter cables.

Incompatible communication settings.

Most modems automatically match the parameters of incoming data: baud rate, data bits, parity & stop bits. You may need to manually change your modem's communication settings via Hyperterminal.

PLC modem communication problems

If your PLC is transferring data via modem, you can begin troubleshooting by entering Information Mode. You can then check the status of relevant System Bits and Integers to help diagnose the problem.

To begin diagnosing the problem, check the error code contained in SI 70. Refer to the error code table in How the PLC works with a modem.

Problem	SI 70 value	Possible Cause & Recommended Action
Modem fails to initialize	2: Modem Did Not Reply	PLC-to-modem cable: Make sure that the cable is securely connected. Check the PLC modem connection and pin-out of the PLC-to-modem adapter cables. Note that if you use cables comprising this pin-out, you must set the RS232 parameter Flow Control to N (none).
		Incompatible communication settings. Most modems automatically match the parameters of incoming data: baud rate, data bits, parity & stop bits. The PLC's embedded modem settings are: 9600, 8 data bits, no parity, 1 stop bit. You may need to manually change your modem's communication settings to match these via Hyperterminal.
	0: No Error	SB 72 OFF: In order to work with a modem, you must select 'Use modem' in the PLC Modem Configuration box . This causes SB 72 Initialize Modem to turn ON when the PLC powers up. Note that if the PLC has also been configured to use SMS messaging, that the PLC will not be able to connect to a modem because the SMS feature overrides the modem. Check too, that SB 72 is not disabled in your program.
	6: Modem Report Error	Check the modem initialization commands. Refer to Configuring the PLC to use a modem.

Problem	Possible Cause & Recommended Action
Cell phone does not receive message	Check the cell phone's SIM card; it may be full.