Near Intersection of I-35 & I-90 Southern Mn. | A bit of history, when teletype machines are/were used, there was typically a machine on each "end". They were connected by means of modems and a telephone line.
In some situations both machines were close to each other and wired "direct" without the use of the phone line or modems. This meant that their 9 pin connectors would not mate since they were both the same sex. Some type of adapter was needed in between. It allowed the two cables to be connected together. This also required swapping certain pairs of wires. These adapters were given the name Null Modem since a modem was no longer needed.
A similar situation can come about with various combinations of GPS receivers and Displays. That is sometimes the cables can be connected directly to each other but sometimes a null modem gender changer is needed in between.
My experience indicates that if the two cables will mate (one male, the other female) that plugging them together is fine.
If the two cables will not mate (both male or both female) then a null modem gender changer is needed. It allows two male or two female cables to be connected together and crisscrosses the Transmit and Receive signal wires (2 & 3). It runs the signal ground wire (5) straight through. It crisscrosses some other pairs also that are not used.
In either situation the two devices must be set with the same BAUD rate, data bits etc. I view this as being similar to putting hay bales in a barn with a conveyor. The parties on both ends and the conveyor speed must be agreed upon.
If using one of these with an Ag Leader display harness and another brand of GPS, it is wise to clip all pins but 2,3 & 5. Pins 2, 3, & 5 are the only pins necessary for setting and using GPS. Ag Leader uses Pins 4 & 6 in their display cables for GPS power and ground. Those pins can be a problem and cause damage with certain combinations of equipment.
Edited by tedbear 5/21/2026 08:02
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Ez Guide Plus Firmware update
