Call Mr. Fairdeal

[boscoe99]

On another forum there was a question raised about varying voltage values being displayed on an MFD via an NMEA 2000 network, when the motor was running at a steady RPM. You offered a comment about the data being sent to the MFD via pgn sentences.

Do you happen to know the rate at which the sentences are sent? I can't find anything on the inner web about this.

It has got me to wondering. There are many sentences being sent continuously. Although electrons be quick, it will take some time to send a number of different messages. Let's say 100 messages. One of those messages is the voltage.

The R/R is also quick, given that it is also working with electrons. I am wondering if between the time that one voltage pgn is sent, say with a value of 14.3 volts, by the time the next voltage pgn is sent the value from the R/R might now be 14.8 volts. This might explain why the voltages seen on the MFD rise and fall.

What do you think?

[rodbolt17]

I tried to find some of that info as well.
the speed at which packets can be sent is going to be a matter of processor speed.
that is where the O scope would come in handy.
you could see the individual data bits, remember typically there are 8 bits to a byte.
then you could also see if it is even parity or odd.
binary octal or hexadecimal.
but why bother.
I would rather be refinishing a gun.

[tmann45]

Nmea 2000 is 250,000 bits/sec.
Should be enough info in this wiki for a swag:
https://en.m.wikipedia.org/wiki/NMEA_2000

[fairdeal]

while it seems unlikely (impossible?) that the bandwidth itself of the N2K protocol can explain the "jumpy" voltage readings

we have no idea of the "refresh rate" of the data the ECU broadcasts onto the network.

for example, the Garmin GPS 19 "external GPS antenna" normally broadcasts a NMEA 2000 position update at a rate of 10 per second

but can be user configured for a lower rate (can't find out how low it can be set - but seems may be many seconds)

Then, we likewise have no idea, for any particular model MFD, what the engineers have chosen for a display update rate.

coincidentally, I read last night that with Yamaha CL (square?) gauges
the tach displays "ECU reported" voltage while the speedo displays the actual network power.

Wonder if the voltage "jumps" on a CL tach?

[99yam40]

interesting info in that link

thought this was informative ,seems there was some discussion on this not long ago. Did not know about the maximum distance

The NMEA 2000 network, like the SAE J1939 network on which it is based, is organized around a bus topology, and requires a single 120Ω termination resistor at each end of the bus. (The resistors are in parallel, so a properly terminated bus should have a total resistance of 60Ω). The maximum distance for any device from the bus is six meters.

I would think that the voltage is not important enough to be sent a high refresh rate maybe, so they do not do it