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#Smart serial to rj45 dte software
DSR doesn't seem to have much utility at all communication software doesn't need it and in null-modem wireing it makes more sense to match DTR with CD than with DSR. This is a good way for software to get the modem back into command mode, although this can also be done by sending "+++" surrounded by a pause, then "ATH\r" to hang up. If the computer lowers DTR, most modems respond by switching to command mode or dropping the call, depending on their configuration. Imagine that if your wiring always asserts CD or if you reconfigure the serial port to ignore CD, then when you lose a phone call another caller may assume your open session!ĭTR and DSR are less important. TTY devices in Unix typically only operate when CD is up, so getty will wait until the modem automatically answers and raises CD before presenting the login prompt, and when the call is terminated, the modem will lower CD, the kernel will hang-up the session (SIGHUP, EOF, and revoke the controlling terminal). It's easiest to set this up by configuring a modem to automatically answer, then configuring getty to offer a terminal on the TTY device.
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When the modem is not configured to automatically answer, the situation is only a little better: after a terminated call, the software will not reset, so next time the modem rings, the software will not understand that the "RING" sent from the modem is not data.Īn important use-case for CD is calling into Unix TTYs. If the modem is configured to automatically answer, this results in a security hole because the next caller will pick up the unterminated session. Without CD, when a call is terminated unexpectedly, the software will keep the session for the call open. While one can often work around lacking CD, detecting CD drop is the only reliable way to detect when a modem leaves data mode, for example when a call is terminated. So RI is not necessary or sufficient to answer a call.ĭropping CD is a common practice, but doing so is problematic. Furthermore, the only time software can get away without understanding the command language is when the modem is set to automatically answer, in which case CD will be the pertinent signal, not RI. Communication software can detect an incoming call without it: Incoming calls only happen when CD is down, and the modem is always in command mode when CD is down, and the modem will send "RING" on RX when the phone rings. RI (Ring Indicator) is the most useless signal and the most obvious one to drop. Also, to accommodate our goal below of making null-modem easy we'll have to select pins carefully. Perhaps you need to run three serial cables over an existing Cat5 cable (three pairs for TX/RX and one pair for a shared ground).
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Not only are we already short one wire, but sometimes fewer than 8 wires are available. Which signals can be left unconnected is a critical discussion.