question

Martyn Duerden avatar image

BMV RS232 interface

Hi all,


I have a fully operational new system but currently without monitoring.

Multiplus 2 48v, BMV 700, automation system


The bmv has a 4 pin serial port on the rear.


5v, Rx, Tx, Gnd.


I have the victron cable 9pin rs232 to the BMV


In the said cable there is circuitry in the 9 pin end, the other end is sealed. see photo.


Questions


1; Are the electronics in the lead converting VE bus signals to RS232?

2; Is the output from the BMV’s Rx, Tx, & Gnd standard RS232 protocol or proprietary and only RS232 after the electronics?

3; Does pin 1 on the BMV need 5v or is it supplying 5v?

4; Is the Gnd common to signal & the 5v line?

BMV Battery MonitorMultiplus-IIbmv48v battery
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2 Answers
WKirby avatar image

1) No, there is no VE.Bus here. It is VE.Direct. That cable is converting VE.Direct (3.3V TTL) to RS232. It is isolated (Si8422 I think).

2) The output from a BMV is standard 3.3V TTL, not RS232. That cable converts TTL (3.3V or 5V) to RS232 (probably a MAX232 on the other side of that PCB???).

3) Pin 1 is supplying 3.3V (I think it is 10mA max). This voltage is needed to supply the BMV's side of the digital isolator (Si8422).

4) GND is common on the BMV side of the interface cable. It is isolated therefore GND is not common on the RS232 side.

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Thanks for the reply.


Sorry, I did mean VE Direct!

The manual suggests its 5v not 3.3, are you saying VE direct operates as Rx,Tx & Ground? I had assumed it was CAN (Hi/Low) without Ground.


The issue at the moment is when plugged into an RS232 device there is no data stream, any thoughts?


Thanks

VE.Direct is TTL serial Rx, Tx and GND. It depends which manual you were reading. BMV's are 3.3V and MPPT's are 5V. The digital isolator is inherently a TTL Voltage level shifter too.

Now, on the 9pin side it's going to need to be powered in some way via the port. If you are expecting anything to come out of it between GND and Rx, then that won't happen on its own. The electronics need powering up on the RS232 side, probably by the DTR (pin 4) or RTS (pin 7) or something like that- difficult to say without seeing it in the flesh.

EDIT: I had another look at your picture, there is a track coming off pin 4 going to that giant capacitor?? That is most likely the pin powering the RS232 side of that converter.

That sounds like where we were


Pin 4 & 7 were powered up, in terms of grounding the RS232 side I had assumed it would be the housing, does this sound plausible?


Your edit...


So pin 4 & the shell or the rs232 gnd?


Will be back on it tomorrow!

RS232 GND (pin 5).

Sure, let's get on it tomorrow.

One other thing that strikes me, it will be expecting 3.3/5v on the 9 pin side too (as in not 10v for example)

No, it should be RS232 Voltage levels on the 9 pin side. It needs to interface with a computer (or USB serial thing) which will be at RS232 levels. I expect that there is a MAX232 or something like that on the other side of the PCB.

Hi both,

“When using an external power supply to power DTR (pin 4) or RTS (pin 7), the minimum voltage is 5V, and maximum is 12V. Note that it is normally not necessary to use an external power supply for this: normal serial ports will either automatically drive DTR or RTS high. And if not that, than you can control those pins to be high from within the software.”


From:

https://www.victronenergy.com/live/vedirect_protocol:faq


Have you already studied that page? It explains all about 3v3 and 5v and ttl (and not canbus) and more.

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WKirby avatar image

The BMV just sends a data stream, there is no configuration. You can see the data pulsing out the back of the BMV with just a multimeter over pin 1 (GND) and 3 (Tx).

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