Near Intersection of I-35 & I-90 Southern Mn. | I believe the term null modem goes back to early teletype equipment that was usually connected to a remote device over a phone line. When each unit was attached to a phone modem the Transmit and Receive functions worked properly.
When it was desired to have two of these devices connected together directly without the use of a modem, a couple of problems occur. Both units have the same type of connectors and the wiring pattern would be incorrect.
Thus a cable was created that usually had the same type of connectors on both ends and switched certain wire pairs around. Since this cable replaced the modems that would otherwise have been used - the term "null modem" was created.
For GPS situations, I found that generally if the two cables mate (one male and one female) then plug them together and a null modem is not needed. If the two devices are the same sex (as is the case when hooking the EZ-Guide+ to a computer) then a null modem gender changer is needed. This is why the female to female cable is included with the EZ-Guide+.
Another common situation that requires the use of a null modem gender changer is in the case of using a generic GPS with certain Ag Leader equipment. You are not able to plug the devices together directly since both connectors are female.
In this situation you need a male to male null modem cable. Since Ag Leader took certain "liberties" with the wiring pattern - 12 Volts is present on one of the pins. This can cause problems in certain situations. For that reason be certain to use the Ag Leader null modem that comes with those units. The null modem gender changer that comes with the Ag Leader products is quite short but a longer version is available.
For most situations, the only pins in the DB9 connectors that are used are Pins 2,3 & 5. Most null modems regardless of sex will just run 2-3, 3-2 and 5-5. Pin 5 is ground and Pins 2 & 3 are the Tx and Rx that need to be reversed. These pins will allow two way communication between the devices provided that each device has its own power source.
Most newer laptops do not come with a true serial port. Some of the USB convertors work for some applications and not others. This is part of the reason that I bought a used laptop with a true serial port for working with these situations.
The updates are generally quite easy, once you've done them. For those who only need to do it occasionally, it might be wise to have your dealer do it for you since they should have experience with the process.
Edited by tedbear 1/20/2008 04:40
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updating ez-guide
