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roaming | If you do a search on this forum (either using the built-in search or use Google's site search) for "null modem cable", you should find a great explanation (I think written by Tedbear). In essence, a null modem cable was originally designed to connect two modems directly. If both are transmitting on pin 2, then connecting them via a standard serial cable will not provide communication, because the Tx lines are connected to each other, rather than to the corresponding Rx line on the other device.
In precision ag, a null modem still serves a similar purpose. You need to know which pin on each device is Tx and Rx, and then figure out if a standard (straight-thru) or nulled cable is required. If you have only a standard cable and you need a null cable, you can just add a null modem adapter to switch pins 2 and 3. You can get the null adapters in various gender configurations to suit your need, or you can add a straight-thru gender changer if needed. I like www.cablewholesale.com for cables and adapters, but there are lots of other local and online sources.
You are correct that joining two null modems = a straight-thru.
For MOST serial communication purposes in precision ag, you only need to worry about pins 2, 3 and 5 (http://en.wikipedia.org/wiki/RS-232 or Google "RS232" if you want more info). HOWEVER, Ag Leader long ago bastardized the RS232 standard to suit their own needs, so you have to be careful when working with Ag Leader devices/cables.
Oh, and don't get hung up on whether or not the connector has thumbscrews or nuts. This really doesn't mean anything as far as whether the connector "should" be connected to another like/opposite connector.
Edited by torn 12/6/2008 13:43
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What does a null modem cable do? Will I fry my Insight or Mojo?
