The CHEWEY (looking) NULL MODEM
 Unfortunately pocket computers usually have their serial connectors wired as DCE (receiver) because then they will connect directly to a PC (DTE). To puzzle us more, TNCs are wired as receivers too despite sending information to the computer - the TNC is a modem and modems are DCE. To connect DTE - DCE your wires go 'straight through'. To connect DTE with DTE you must flip the connections with an in-line NULL-MODEM (or just alter the wire itself). Likewise to connect a handheld computer (DCE) and a TNC (DCE) you need a Null Modem. I made a null modem - well a lot of them in fact - before the advent of USB and 'firewire' but wiring two 9-D sockets together and filling the space with hot glue. The effect looked rather like a mint. The following NULL MODEM only swaps the RX and TX lines. A full NULL MODEM swaps lines 5..6 and 7..8 but the THD7e doesnt use pins 5-8 and the GPS doesn't either so I cut my gluing time down. Looking at the diagram below. With the THD7 NOT plugged in, S is CLOSED and the unit is a basic NULL MODEM adaptor with two DTE wired 9 pin sockets. When the THD7 is plugged in, S is OPENED and anything from the TNC is lost, however the GPS continues to feed both the TNC and the THD7, and the waypoints from the THD7 can get to the GPS.  Its always possible I have this back to front of course..... it was a while ago now... Still, an amazingly versatile collection of connections at one point! The acid test of this set up was when I quickly altered the station from being a simple GPS/TNC/RADIO setup to running UIVIEW on a laptop connected to the TNC now in KISS mode. The GPS continued to feel positional data to the THD7 which beaconed at a few hundred milliwatts. UIVEW tracked the THD7 giving me an on screen scrolling location marker. The TNC digipeated the THD7 out of the car into the network! What a hobby!  |