For some time I’ve been thinking about replacing the IC-706Mk2g in my pickup with a newer radio. The obvious choice was a IC-7100 as it has all the same connectors, including that for the AH-4 auto-coupler. As a bonus the radio is the same size and uses the same mount bracket. I had held off getting the IC-7100 for quite some time as I wasn’t convinced its ‘L’ shaped head was suitable for mobile use. Having owned it for a month I’m still not convinced but do like the radio and its odd shaped head unit is growing on me.
D-Star was not a selection factor for the IC-7100 purchase, but since the radio had this capability it seemed wrong not to give it a go 🙂 D-Star could be a news item in its own right, but suffice to say my experience thus far is not great. Several of the repeaters around the Bristol area have either migrated to alternate digital systems, don’t seem to be operational or are not connected to the D-Star network which undermines usefulness. As the memory channels in the radio are programmed with many, many repeaters, selecting one in the vicinity of my location whilst driving was tricky. But IC-7100 has the ability to receive NMEA data from a GNSS receiver and use geolocation information to identify repeaters in the vicinity of the radio that may be used. I was keen to give this a go but needed a source of NMEA data. Unfortunetly, the IC-7100 does not contain a GNSS receiver but it does present a three-pole, 3.5mm NMEA interface. The pins are: Ground, TxData and RxData.
Locating a suitable GNSS receiver was not as simple as it should have been as most nowadays implement either Bluethooth or USB as the physical interface rather than NMEA. However, some research identified the G-STAR IV from Globalsat. It does have an RS-232 physical interface and as NMEA is compatible with RS-232 at the physical layer it worked fine. However, the G-Star IV used a 6-pin mini-din plug rather than the more conventional 9-pin ‘D’ connector associated with RS-232 plus it required a +5V DC, 80mA power feed.
Clearly, it was necessary to construct an interface between the radio and the G-STAR IV receiver not only because the physical connectors were different but because a power feed was needed. After some investigation, I decided to use the +12V DC power from the truck batteries reduced to the level required using a small voltage regulator, 7805, circuit. This worked great providing a rock-sold power feed. A byproduct of connecting to the truck batteries was that the power was always ‘on’ and thus the GNSS receiver was powered and consuming current. So, my design included a power switch to remove the GNSS receiver from the power when not in use.
The interface unit was quickly assembled using a plastic case, suitable connectors and a small piece of Veroboard onto which the voltage regulator and some electrostatic smoothing capacitors were attached. A small red LED was also included for diagnostic purposes. This lit when power was being provided to the GNSS receiver. Once assembled it was tested on the bench using an oscilloscope to monitor the resulting NMEA data. This data appeared as a square wave of the correct frequency so I had a high confidence that all was working well.
The pictures in the article show the completed interface unit. I’m happy to answer any questions if someone has a need for something similar.
Footnote: Everything worked great on the bench, however, on migrating the unit to the truck I realised that Icom and rather annoyingly used a 3-pole, 2.5mm connector for the GNSS receiver. This meant the whole project was delayed whilst I sourced a connector which as it was at a weekend – Maplin Electronics. Making the cable really was a short job but it took another 4 to 5 hours to get everything working reliably as there was a weird mechanical failure on the centre tip of the 2.5mm connector resulting in an intermittent connection. I really don’t like Maplin these days and this experience only served to reinforce my view that their products are of the lowest quality.
Andrew
G0RVM
