ADC MDM-TEL
Modem
Document
MDM-TEL Application Note.doc
Page 13 of 16
13
Last reviewed: 03/07/2002
Local
Modem
IBM Compatible
Remote
Modem
PLC
Troubleshooting a Modem Connection
:
1. Remote modem is not answering:
-
Try to isolate the problem to the remote side or local. If you have a known good working
PLC/Modem setup, try dialing into it. If you are successful at this, then you know the problem
is on the local side.
-
Make sure that the remote modem configuration has auto answer turned on. You also need
to have a number of rings selected.
-
Try removing the cable between the remote modem and PLC. If it answers, you know the
problem is in the remote cable connection. Consult the diagrams towards the beginning of
this document for the correct pinout. If possible, try using the programming cables we
provide as well as the null modem adapter that comes with the modem.
-
Try a different cable on the local modem.
-
Verify settings for the modem driver on the PC. Consult the appropriate section of this
document for setting up this driver in Windows.
2. Remote modem answers but the PLC doesn’t connect:
-
If possible, try to connect to the PLC directly from the serial port of a PC.
-
Verify that the PLC port is Ok.
-
Phone line problem. Repeat connection attempt on another phone line if possible.
-
The PLC port has not been configured or improperly configured. Consult the section in this
document that describes the port configuration.
-
Verify the remote modem configuration. Follow the steps in this document again and re-
download the configuration to verify the correct setup.
-
The modem driver is not configured properly. Consult the steps in this document to verify
modem driver setup.
-
The wrong modem may be answering. This can happen on a phone network in some
facilities. Have someone power off the remote modem and attempt to connect again. If there
is an answer, there is a problem with the phone network, which needs to be corrected.
-
The remote modem is not on a dedicated line. PBX (Private Branch Exchange) lines will
cause problems with many different software packages.
3. The PLC connects but the connection is lost intermittently when program status is turned on
or during a program transfer.
-
Poor phone line quality. Sometimes there is no way to resolve this. Be sure that the phone
line on the remote end is not run around devices that cause high RF signals, such as
frequency drives. You can often tell the quality of a phone line by actually calling someone
and talking on this line. If it is very noisy, then it is most likely that you will have problems
using it for PLC connectivity.
-
Poor cable quality between PLC and modem or PC and modem if using external modem. If
the cable between the modem and PLC is routed around devices that generate high RF
signals, you will most likely have problems with your connection. They will need to be
isolated. Distance between the PLC and the modem could also be a factor. 50 ft. is the
maximum length for RS232. You may achieve this distance depending upon your
environment (high RF signal environment).
-
Raise the timeout and retries in the link editor in DirectSOFT32:
-
If you are still experiencing problems after following these steps, contact AutomationDirect
tech support at (770) 844-4200.
