<HTML ><HEAD ><TITLE >Configure dumb modem</TITLE ><META NAME="GENERATOR" CONTENT="Modular DocBook HTML Stylesheet Version 1.7"><LINK REL="HOME" TITLE="Remote Serial Console HOWTO" HREF="index.html"><LINK REL="UP" TITLE="Modem configuration" HREF="modem.html"><LINK REL="PREVIOUS" TITLE="Using Minicom to give commands to a modem" HREF="modem-minicom.html"><LINK REL="NEXT" TITLE="Configure modem with AT commands" HREF="modem-hayes.html"></HEAD ><BODY CLASS="SECTION" BGCOLOR="#FFFFFF" TEXT="#000000" LINK="#0000FF" VLINK="#840084" ALINK="#0000FF" ><DIV CLASS="NAVHEADER" ><TABLE SUMMARY="Header navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TH COLSPAN="3" ALIGN="center" >Remote Serial Console HOWTO</TH ></TR ><TR ><TD WIDTH="10%" ALIGN="left" VALIGN="bottom" ><A HREF="modem-minicom.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="80%" ALIGN="center" VALIGN="bottom" >Chapter 12. Modem configuration</TD ><TD WIDTH="10%" ALIGN="right" VALIGN="bottom" ><A HREF="modem-hayes.html" ACCESSKEY="N" >Next</A ></TD ></TR ></TABLE ><HR ALIGN="LEFT" WIDTH="100%"></DIV ><DIV CLASS="SECTION" ><H1 CLASS="SECTION" ><A NAME="MODEM-DUMB" ></A >12.2. Configure dumb modem</H1 ><P >Linux, like most <SPAN CLASS="PRODUCTNAME" >UNIX</SPAN >-like operating systems, expects a serial console to be connected to a dumb modem. Dumb modems are not seen much these days, perhaps only on exotic hardware such as <SPAN CLASS="ACRONYM" >ISDN</SPAN > terminal adapters or satellite ground terminals.</P ><P >A dumb modem is configured using hardware. <A HREF="modem-dumb.html#MODEM-DUMB-FRONT" >Figure 12-1</A > shows the front panel of a fanciful dumb modem. In reality the speed and mode settings are likely to be done using jumpers or DIP switches.</P ><DIV CLASS="FIGURE" ><A NAME="MODEM-DUMB-FRONT" ></A ><P ><B >Figure 12-1. Front panel of a dumb modem</B ></P ><TABLE BORDER="0" BGCOLOR="#E0E0E0" WIDTH="100%" ><TR ><TD ><FONT COLOR="#000000" ><PRE CLASS="PROGRAMLISTING" >+-----------------------------+ | | | SPEED MODE | | [ ] 300 [ ] Originate | | [ ] 600 [X] Answer | | [ ] 2400 | | [X] 9600 | | | +-----------------------------+</PRE ></FONT ></TD ></TR ></TABLE ></DIV ><P >The modem's speed is set to the desired bit rate, in our case 9600<SPAN CLASS="ABBREV" >bps</SPAN >. The modem's mode is set to Answer, that is, to wait for incoming calls and to answer them.</P ><P >If the <SPAN CLASS="ACRONYM" >RS-232</SPAN > control line Data Terminal Ready is low, the modem will not answer a call. The computer is off or the computer's serial interface is not yet initialized. Once <SPAN CLASS="ACRONYM" >DTR</SPAN > is high the modem will answer incoming calls.</P ><P >Once an incoming call is established the modem raises the Data Carrier Detect control line. Only when DCD is high is received data valid (data receieved from a dumb modem when DCD is not asserted is probably line noise). Only when DCD is high is transmitted data passed through the link.</P ><P ><SPAN CLASS="APPLICATION" >getty</SPAN > on the Linux computer has been waiting for <SPAN CLASS="ACRONYM" >DCD</SPAN > to come high, and <SPAN CLASS="APPLICATION" >getty</SPAN > welcomes the user and requests them to log in.</P ><P >Whilst the user is logged in and data is flowing, Clear to Send and Ready to Send are used between the modem and the computer to prevent data being sent too soon. The computer lowers Ready to Send when it is too busy to receive a character. The modem lowers Clear to Send when it is too busy to receive a character.</P ><P >When the user hangs up, Data Carrier Detect falls and the hang up signal is sent to all processes associated with the dial in session.</P ><P >Alternatively, the user can log out. When the shell dies, the computer pulls Data Terminal Ready low, causing the modem to hang up. When the <SPAN CLASS="APPLICATION" >getty</SPAN > brings Data Terminal Ready high again, the modem will accept more incoming calls.</P ><P >We have not yet described Data Set Ready. This line is low if the modem is off or if the modem has not yet initialized. When DSR is low all other signals from the modem are undefined. For example, if DSR is low but DCD "floats" to the high voltage then software should behave as if DCD is not asserted.</P ></DIV ><DIV CLASS="NAVFOOTER" ><HR ALIGN="LEFT" WIDTH="100%"><TABLE SUMMARY="Footer navigation table" WIDTH="100%" BORDER="0" CELLPADDING="0" CELLSPACING="0" ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" ><A HREF="modem-minicom.html" ACCESSKEY="P" >Prev</A ></TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="index.html" ACCESSKEY="H" >Home</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" ><A HREF="modem-hayes.html" ACCESSKEY="N" >Next</A ></TD ></TR ><TR ><TD WIDTH="33%" ALIGN="left" VALIGN="top" >Using <SPAN CLASS="PRODUCTNAME" >Minicom</SPAN > to give commands to a modem</TD ><TD WIDTH="34%" ALIGN="center" VALIGN="top" ><A HREF="modem.html" ACCESSKEY="U" >Up</A ></TD ><TD WIDTH="33%" ALIGN="right" VALIGN="top" >Configure modem with <SPAN CLASS="ACRONYM" >AT</SPAN > commands</TD ></TR ></TABLE ></DIV ></BODY ></HTML >