IntroductionWhen troubleshooting an application using a modem or other device, there are three things to consider: the modem or serial device itself, the COM port and cable, and the PC application software.When a redirector and a modem/serial server enter the picture, the number of potential error sources increases.
This technical note suggests a logical approach to the diagnosing problems in this environment.The Software Modem Cable ConceptA redirector has one simple function: to pass data between a remote TCP/IP connection and a virtual COM port on a client PC.The analogy of a "software modem cable" is appropriate, as the redirector's primary mission is to pass data and (optionally) control signals between the PC and the modem.
When suspecting an issue lies with redirector configuration, first consider if the issue would happen if a faulty modem cable were suspected on a local modem.Software Modem Cable LimitationsWhen a redirector is used with a server that does not support COM Port Control, this software modem cable is similar to a hardware modem cable that has only three wires: TD, RD and ground.
There is no provision for control/status lines such as CD, DTR and DSR to pass data between the PC and the modem.Thus, the client PC raising or dropping DTR has no effect on the actual DTR pin on the modem.
In addition, baud-rate and framing information cannot be controlled over the TCP/IP connection. If the client PC opens the port at 38400 8-N-1, but the modem pool's hardware port is configured to be 9600 7-E-1, then the physical modem will not communicate using 38400 8-N-1.None of these limitations exist when the redirector is used with a server that supports COM Port Control. But if COM Port Control is not present in a misbehaving equipment configuration, it is important to understand these limitations and to investigate their impact on the client applications being used.How the Redirector Fits InThe redirector sits "underneath" the COM port API in Windows. It takes data that is directed to a COM port using this API and sends it over the network. Data from the network is passed back through this API as though it came from a hardware COM port.Unimodem is the part of the Windows operating system that maintains the list of modem devices which have been installed in the system. It sits "above" the COM port API in Windows.Using these modem devices it initializes and dials modems by sending data to the COM port API and watching for appropriate responses from this COM port API. When an application uses Unimodem to open a modem device on a local COM port (COM2 for example), it sends/receives data from hardware.When an application opens a modem device on a redirected COM port, it still sends/received data from hardware, but it is up to the redirector to move the data to the actual hardware elsewhere on the network.This raises another important point: it is Unimodem that actually interacts with the modem when it is time to initialize and dial. The redirector is simply a conduit for Unimodem, just like a physical modem cable. If dialing fails, it is often because Unimodem is receiving unexpected responses from the modemThe Auto Configure as a Litmus TestThe Auto Configure in Tactical Software redirector products is a very helpful tool to ensure that the redirector is configured for a clear data path to the modem.If the Auto Configure runs successfully, the chances are high that the redirector itself is configured properly. To continue the analogy, if this test proves the "modem cable" is plugged in properly, then problems from then on are likely to lay elsewhere.