Model IC-M506 is the basic model, designated M506-01. It has a front-mounted microphone and NMEA-0183 only. This model does not display any AIS information even if connected to an external AIS receiver data output via either NMEA-0183 or NMEA-2000.
Model IC-M506 PLUS is available in two versions. Both include NMEA-0183 and NMEA-2000. The front-mounted microphone version is designated M506-11; the rear-mounted microphone version is designated M506-31. Neither version has an integral AIS receiver, but both versions will show AIS targets on their display if connected to an external AIS receiver via NMEA-2000. These models do not display AIS data if connected to an external AIS receiver via NMEA-0183. Models with NMEA-2000 have an additional component, the UX-232 subassembly.
Model IC-M506 AIS is available in two versions. Both include NMEA-0183, NMEA-2000, and an internal AIS receiver. The front-mounted microphone version is designated M506-21; the rear-mounted microphone version is designated M506-41. Neither model sends any AIS data via NMEA-0183; AIS data is only sent via NMEA-2000. Models with AIS have an additional component, the UX-231 subassembly. Since the AIS option is only available on radios with NMEA-2000, these models then have both the UX-231 and UX-232 subassemblies.
When buying an IC-M506 in any model, be certain to get a product that has the latest firmware revisions installed for both the radio and the AIS receiver. There have been several firmware updates since initial production of the IC-M506 radio that provide remedies for problems encountered in the original firmware. As far as I can tell, the firmware in the IC-M506 radio cannot be updated in the field by the user. The firmware updates are not generally available for download, and installation of new firmware requires a specialized computer application and two specialized cables to make the connection to the radio from the host terminal executing the specialized software. If you purchase an IC-M506 radio with older firmware, the radio may have to returned to a service center to have the firmware updated.
The most recent firmware update occurred November 7, 2018. This means that there is a very good probability that a radio sitting on a dealer shelf that was shipped before that date will have out-of-date firmware.
A knowledgeable user has reported that if two IC-M506 radios are installed on the same NMEA-2000 network, there is a serious malfunction in their operation which produces an alarm for loss of GPS signal. For details see this well-written and well-illustrated explanation of the problem and the failure of ICOM to provide a remedy:
Icom M506 NMEA 2000 problems
http://www.mvtanglewood.com/2016/04/ico ... ssues.html
Although mentioned above, particular notice should be made that the AIS receiver in the IC-M506 radio does not output any AIS data via NMEA-0183. This can be a significant problem for interfacing to chartplotters which need AIS data on their NMEA-0183 input in order to display the data.
Although there was a serious anomaly in the NMEA-2000 behaviors of the IC-M506 radio, and the radio omits any output of NMEA-0183 data for AIS information, the NMEA organization bestowed upon the IC-M506 radio an award for "Best VHF Marine Radio" in their judgement in three consecutive years, 2014 to 2016.
Here are press releases attesting to these awards:
2016: https://www.nmea.org/Assets/2016%20nmea ... s%20v4.pdf
2015: https://www.nmea.org/Assets/10.nmea_201 ... ds-rev.pdf
2014: https://www.nmea.org/Assets/2014-nmea-p ... inners.pdf
ICOM also made specific claims for the basis on which the IC-M506 radio was able to win the award, saying that it "...captured the [2014 NMEA Best Radio] title based on its NMEA 0183/2000 plug-and-play functionality and integrated AIS receiver..." and "...also recognized for its new intuitive user interface, which promotes fast access to radio functions and settings like Class D DSC." I could not find any statement from NMEA to affirm these criteria. Note that the ITU recommendation on digital selective calling sets specific requirements for the user interface on ALL Class-D DSC radios, and compliance with those criteria are mandatory for any DSC Class-D radio to be sold in the USA.
It seems a bit incongruous that the NMEA organization could award a "best" designator to a device that omitted AIS output on one of its NMEA interfaces. Also, the integration of an AIS receiver into a VHF Marine Band Class D DSC radio was, by 2014, already achieved earlier by other brands, notably Standard-Horizon, who pioneered the practice. The principal distinguishing feature of the IC-M506 would seem to be its NMEA-2000 interface, but that, too, was already available in other Class D DSC radios such as the Lowrance LINK-8 or SIMRAD RS35.
It should also be noted that the IC-M506 radio is a "NMEA-2000 certified product." NMEA explains what that means:
Only “NMEA 2000 certified” products meet the rigorous and mandatory software
requirements. The certification process tests equipment to ensure the products meet the
conditions established in the NMEA 2000 Standard. NMEA 2000 defines the message
formats for transmitting electronic information amongst products. NMEA 2000 also
defines certain messages that all products must implement and use correctly. These
required messages are what make it possible to add devices to the backbone and use them
with little or no configuration required. Only those manufacturers that have passed the
NMEA certification tests can use the NMEA 2000 logo and market their products as
NMEA 2000 Certified.
I do not know what the test "conditions established in the NMEA 2000 Standard" actually are because I have not purchased the standard--it would cost perhaps $3,000 to $5,000 for me to buy it. Further, the contents of the standard cannot be disclosed, so even if I bought the standard, read it carefully, and found the precise details of what tests must be conducted and met, I could not tell you about them.
The fact that the firmware in the IC-M506 radio has been revised several times and there continues to be a reliable report of a significant bug or anomaly in the NMEA-2000 behavior of the radio seems to render the notion of being a "NMEA-200 certified product" as having perhaps less significance than one would like to expect. I don't know this for a fact, but I would presume that one element of being a certified product would be that connection of the product to a NMEA-2000 network should not cause harm to other devices on the network, a test that this radio appears to fail.