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// --></script> <a onmouseout="if(MSFPhover) document['MSFPnav20'].src=MSFPnav20n.src"onmouseover="if(MSFPhover) document['MSFPnav20'].src=MSFPnav20h.src"href=JimPrice.Com.htm language=JavaScript> <img align=middle alt=JimPrice.Com border=0 height=40 name=MSFPnav20 src=_derived/JimPrice.Com.htm_cmp_jimpricetheme110_hbtn.gif width=140> </a></p>  </font></table>  <font face="Arial, Arial, Helvetica"> </font> </center></div> <p align=center><font face="Arial, Arial, Helvetica">   </font></p> </table><table border=0 cellpadding=0 cellspacing=0 width=100%><tbody><tr><td valign=top width=1%> <font face="Arial, Arial, Helvetica"> <p>            </p>  </font><td valign=top width=24><td valign=top> <font face="Arial, Arial, Helvetica">  <p align=center><img height=8 src=_themes/jimpricetheme/whblu.gif width=455></p> <h1 align=center><b> Null Modem Cable </b></h1> <p>After all, isn't this why you came to this site?</p> <p>The purpose of a null-modem cable is to permit two RS-232 "DTE" devices to communicate with each other without modems or other communication devices (i.e., "DCE"s) between them.  </p> <p align=left>To achieve this, the most obvious connection is that the TD signal of one device must be connected to the RD input of the other device (and vice versa).</p> <p align=left>Also, however, many DTE devices use other RS-232 pins for out-of-band (i.e., "hardware") flow control.  One of the most common schemes is for the DTE (the PC) to assert the RTS signal if it is ready to receive data (yes, it DOES sound backwards, but that's how it works), and for the DCE (the modem) to assert CTS when it is able to accept data.  By connecting the RTS pin of one DTE to the CTS pin of the other DTE, we can simulate this handshake.</p> <p align=left>Also, it is common convention for many DTE devices to assert the DTR signal when they are powered on, and for many DCE devices to assert the DSR signal when they are powered on, and to assert the CD signal when they are connected.  By connecting the DTR signal of one DTE to both the CD and DSR inputs of the other DTE (and vice versa), we are able to trick each DTE into thinking that it is connected to a DCE that is powered up and online.  As a general rule, the Ring Indicate (RI) signal is not passed through a null-modem connection.</p>  <p align=center><img height=8 src=_themes/jimpricetheme/whblu.gif width=455></p> <p align=center><b> Common Null-Modem Connection </b></p> </font> <center> <font face="Arial, Arial, Helvetica">  </font> <table border=1 bordercolordark=#000000 bordercolorlight=#999999><tbody><tr><td align=center colstart=1> <font face="Arial, Arial, Helvetica"> <b> Signal Name </b>  </font><td align=center colstart=2> <font face="Arial, Arial, Helvetica"> <b> DB-25 Pin </b>  </font><td align=center colstart=3> <font face="Arial, Arial, Helvetica"> <b> DB-9 Pin </b>  </font><td align=center colstart=4> <font face="Arial, Arial, Helvetica">    </font><td align=center colstart=5> <font face="Arial, Arial, Helvetica"> <b> DB-9 Pin </b>  </font><td align=center colstart=6> <font face="Arial, Arial, Helvetica"> <b> DB-25 Pin </b>  </font><td align=center colstart=7> <font face="Arial, Arial, Helvetica">    </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> FG (Frame Ground)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 1  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> X  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 1  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> FG  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> TD (Transmit Data)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> RD  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> RD (Receive Data)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> TD  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> RTS (Request To Send)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 8  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> CTS  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> CTS (Clear To Send)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 8  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> RTS  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> SG (Signal Ground)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> SG  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> DSR (Data Set Ready)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 20  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> DTR  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> CD (Carrier Detect)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 8  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 1  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 20  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> DTR  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> DTR (Data Terminal Ready)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 20  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 1  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 8  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> CD  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> DTR (Data Terminal Ready)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 20  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> DSR  </font></table>  <font face="Arial, Arial, Helvetica"> <p> </p>  <p align=center><img height=8 src=_themes/jimpricetheme/whblu.gif width=455></p> <p align=center>Here's another null-modem connection that I've seen floating around the net.  Some folks say that it's the cable that's shipped with LapLink 4 Pro.</p>  </font> <table border=1 bordercolordark=#000000 bordercolorlight=#999999><tbody><tr><td align=center colstart=1> <font face="Arial, Arial, Helvetica"> <b> Signal Name </b>  </font><td align=center colstart=2> <font face="Arial, Arial, Helvetica"> <b> DB-25 Pin </b>  </font><td align=center colstart=3> <font face="Arial, Arial, Helvetica"> <b> DB-9 Pin </b>  </font><td align=center colstart=4> <font face="Arial, Arial, Helvetica">    </font><td align=center colstart=5> <font face="Arial, Arial, Helvetica"> <b> DB-9 Pin </b>  </font><td align=center colstart=6> <font face="Arial, Arial, Helvetica"> <b> DB-25 Pin </b>  </font><td align=center colstart=7> <font face="Arial, Arial, Helvetica">    </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> FG (Frame Ground)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 1  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> X  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 1  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> FG  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> TD (Transmit Data)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> RD  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> RD (Receive Data)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 3  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 2  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> TD  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> RTS (Request To Send)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 8  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> CTS  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> CTS (Clear To Send)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 8  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> RTS  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> SG (Signal Ground)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 5  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 7  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> SG  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> DSR (Data Set Ready)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 20  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> DTR  </font><tr><td align=left colstart=1> <font face="Arial, Arial, Helvetica"> DTR (Data Terminal Ready)  </font><td align=CENTER colstart=2> <font face="Arial, Arial, Helvetica"> 20  </font><td align=CENTER colstart=3> <font face="Arial, Arial, Helvetica"> 4  </font><td align=CENTER colstart=4> <font face="Arial, Arial, Helvetica"> -  </font><td align=CENTER colstart=5> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=6> <font face="Arial, Arial, Helvetica"> 6  </font><td align=CENTER colstart=7> <font face="Arial, Arial, Helvetica"> DSR  </font></table>  <font face="Arial, Arial, Helvetica"> <p align=left>Personally, I don't think that a null-modem cable built to the above pinout will work quite as well, but a lot of folks appear to have success with it.  In general, it will work with some software packages, such as those that only use RTS/CTS hardware flow control.  However, some packages that rely on the proper assertion of the CD signal will not work with this cable.</p>  <p align=center><img height=8 src=_themes/jimpricetheme/whblu.gif width=455></p> <p> </p> <p>Here's a good set of figures for DB-25 male and female connectors, as viewed from the pin side (not the solder side).</p> <p align=center><img alt="DB-25 Male"border=0 height=65 src=images/db25male.gif width=171></p> <p align=center>DB-25 Male</p> <p align=center><img alt="DB-25 Female"border=0 height=65 src=images/db25fem.gif width=171></p> <p align=center>DB-25 Female</p> <p> </p>  <p align=center><img height=8 src=_themes/jimpricetheme/whblu.gif width=455></p> <p align=center>Here's a good set of figures for DB-9 male and female connectors, as viewed from the pin side (not the solder side).</p> <p align=center><img alt="DB-9 Male"border=0 height=65 src=images/db9m.gif width=83></p> <p align=center>DB-9 Male</p> <p align=center><img alt="DB-9 Female"border=0 height=65 src=images/db9f.gif width=83></p> <p align=center>DB-9 Female</p> <p align=center> </p>  <p align=center><img height=8 src=_themes/jimpricetheme/whblu.gif width=455></p> </font> </center> <p align=left><font face="Arial, Arial, Helvetica"> If you're using a serial cable or null-modem cable for local serial connectivity or for modem replacement, you might consider looking at the <a href=http://www.traversix.com/> Traversix </a>  network connectivity solutions.   You can use Traversix  ethernet-to-serial connectivity products to connect to serial-enabled devices over the Internet, even (especially) when those devices are located behind a third-party firewall.   </font></p> <center> <font face="Arial, Arial, Helvetica">   </font> </center>  </table><table border=0 cellpadding=0 cellspacing=0 width=100%><tbody><tr><td> <font face="Arial, Arial, Helvetica"> <blockquote><blockquote><p style="margin-left: 0"align=left><i> <font color=#000080 size=2> <a href=http://www.jimprice.com/jim-peng.shtml> <img border=0 height=74 src=_borders/peng2t.gif width=38> </a> </font> </i> <font size=2> <font color=#000080> This page and its contents, copyright © 1996-2007 </font> , <a href=http://www.jimprice.com/> JimPrice.Com </a> . <font color=#000080> Here's a link to our <a href=http://www.jimprice.com/jim-privacy.shtml> privacy statement </a> .  If you have any questions about this page, please don't hesitate to e-mail me.  (The address should be obvious to all but spam-bots.) </font> </font><p align=left><i> <font size=2> This site last updated 7/11/2008 </font> <br> </i> <font face="Arial, Helvetica"size=-1> </font><p><p> </blockquote></blockquote>  </font></table>