Avoiding DTE and DCE Speed Problems

• Check the properties line of your INF for incorrect maximum DTE and DCE speed values. Values that are too high for the modem in use will cause MCT to fail. You can determine the maximum possible DTE speed by trial and error with any communications application and/or by examining the modem event log. See Full Debugging of a Modem INF File Using Modem Event Logs for instructions on how to enable modem event logging and interpreting the logs. To determine the maximum DTE speeds by trial and error, try to transmit init strings to the modem at various DTE speeds and learn by trial and error.
• Having DTE set wrong for a 2400 bps external modem can cause MCT failures. Care should be taken to encode correct maximum DCE and DTE settings into the properties line for your modem, especially if it is 2400 bps modem. For example, an external 2400 bps modem may have a maximum DTE of 2400 bps (modem probably has no compression). A similar 2400 bps modem may have a maximum DTE of 4800 bps (MNP5 compression) or a maximum DTE of 9600 bps (full V.42bis compression). Carefully refer to the manufacturer's documentation as your primary authority on maximum DTE settings. You can also experiment with the modem and use MODEMID.EXE to give you a fairly reliable measurement tool for DCE and DTE. If nothing seems to work with a 2400 bps external modem, try to lower the maximum DTE setting in the properties line to about 9600 or even 2400 (for modems lacking any compression).
• A 2400 bps internal modem may allow a maximum DCE of 2400 (as expected) and a maximum DTE of 115200 bps which is reasonable given inexpensive onboard high-speed UART technology. The manufacturer's documentation may state this and you may confirm this by testing and studying the modem event logs. However, having DTE set wrong for an internal 2400 bps modem sometimes causes MCT failures. Setting the properties line to 2400 maximum DCE and a high maximum DTE (for example, 115200 bps) sometimes causes handshake problems. If nothing seems to work with a 2400 bps internal modem, try lowering the maximum DTE setting in the properties line to about 9600 bps or even 2400 bps (for modems lacking any compression).