CompuServe Messages

Most Hayes modems use commands to control the behavior of RS-232 circuits. Among these are the &C command, which controls the behavior of the "Carrier Detect" circuit, and &D, which controls the behavior of the "Data Terminal Ready" circuit. &C0 (the current default) causes the Carrier Detect circuit to be constantly in the ON condition. &C1 causes the circuit to be ON only when carrier is actually present on the line. The &C0 default was selected because some software, particularly those which use the PC ROM BIOS serial port routines, are unable to even send commands to the modem if CD is in the OFF condition. However, virtually all recent comm software expects CD to reflect the actual line condition. This means that users of such software very often must change to &C1 and save this in the modem's non-volatile memory. &D0 (the current default) causes the modem to assume that DTR is always ON from the terminal or computer (DTR is ignored). &D1 causes the modem to return to command state on an ON-to-OFF transition of DTR; &D2 causes the call to be disconnected and auto-answer to be supressed; &D3 causes the call to be disconnected and the modem to be reset to its stored profile. Most recent comm software expects a drop in DTR to result in the call being disconnected, which again means that users often have to change the stored profile from the default &D0 to &D2. The Systems Engineering department at Hayes is studying whether the factory defaults should be changed to &C1 and &D2. The decision will be based on whether or not selection of these defaults would still, as it has in the past, make it impossible for some people to use or configure our modems. If acceptable workarounds would exist, however, the greater convenience for the majority of users of not having to change these defaults would justify the change. If you have an opinion on this matter, particularly if you are aware of programs or applications which would be negatively affected by this change, please contact me by email. Thanks! — Toby Nixon, Principal Engineer, Hayes Microcomputer Products Inc.