Near Intersection of I-35 & I-90 Southern Mn. | Pins 2,3 and 5 are involved. Pin 5 is simple, it is ground. Pins 2 and 3 refer to Rx (receive) and Tx (transmit). Rx and Tx can get confusing because of their reference. For example we want the Tx from the output on the GPS side to mate with the input or Rx on the device. side A good share of the time we don't want or care about the Tx from the device side even getting to the Rx of the GPS side.
The reason why is that the only time we want the device to send any information to the GPS is to change its configuration. Once that is correct it does not need to change. OK you say but how do I hook it up. Well Pin 5 goes to Pin 5 since that is always ground. Now you only have two choices Pin 2 - Pin 2 and Pin 3 to Pin 3 OR criss cross them.
The null modem does that for you it swaps pins 2 and 3. It is called null modem because in the early days of teletype machines, these wires were swapped in the modem in the phone system. When two machines were connected directly together without a phone modem, the wiring needed to be switched. Thus the little adapter was born and it was called a null (none) Modem. It had the same pins on both ends to allow the two machines to be plugged together and swapped the Tx and Rx lines (some other pairs were swapped as well which usually doesn't concern us).
So to help us out the following rule has evolved. For the most part (not always), if the two connectors will mate a null modem is not needed. If the two connectors are both male then a null modem gender changer is needed
OK you say I've tried it both ways and neither seems to work. Have you hurt anything by trying it both ways. Very unlikely. So what next? Besides this wiring confusion it is often necessary to configure the GPS properly for the second device. The procedure and settings vary from situation to situation. |
Ag Leader and Raven
