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Old 07-16-2022, 03:29 PM   #55
twistedtree
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Quote:
Originally Posted by wkearney99 View Post

I'd definitely welcome some examples of how Maretron's perspective did not work. If just to avoid getting caught in the same scenario.

The problem with Maretron’s approach is that there are devices where you cannot change the Device Instance, and then Maretron can’t distinguish PGNs from the two devices. ICOM VHFs and Furuno autopilots come to mind.

The issue also extends to Data Instances which are a bit different. For some PGNs, a Data Instance in part of the PGN and intended to make the PGN’s self-identifying when a device can send The same PGN for different things. Battery status is a good example. A single device with a single Device instance can monitor multiple batteries, making it necessary to identify which battery is being reported in each PGN sent by the device. Data instanced are used to tell you that it’s battery 1, or 2 or whatever. The spec is clear that data instances must be unique for the PGNs coming from a particular device, but it is equally clear that they are only required to be unique within the scope of that device. If you have two battery monitor devices, each capable of monitoring two batteries, the first device would send PGNs with data instances 1 and 2, and the second device would also send PGNs with data instances 1 and 2. And listening device is expected to select the data it wants based on the device that’s sending it, and the data instance for the battery reported by that device.

Maretron doesn’t do it that, and instead requires that all the data instances be globally unique across the network, not just unique across a single device. Lots of devices that follow the spec do not provide for changing the data instances because there is no need to. But that renders those devices incompatible with Maretron because they can’t tell the PGNs apart from one another.

I have 8 Victron devices, each of which reports battery status for one or two batteries. But Maretron can’t tell any of them apart because the data instances are all either 1 or 2 rather than being 1-16. Victron has been good enough to show ways to hack the system to change the device instances, but when you do it breakers their management which expects things to be according to the standard.

It’s quite the cluster.
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