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Wednesday, July 18, 2018
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Multi Baud Rate on the Same RS485 MODBUS RTU Bus
is it possible to have different baud rate for each slave on RS485 MODBUS RTU bus?

Is it possible to have different baud rate (and parity) for each slave on RS485 MODBUS RTU bus ?

for example:
1 slave ID=25 Bd=9600
3 slaves ID= 32,33,34 Bd=38400 parity=Even
1 slave ID=125 Bd=38400 parity=none

As slave must check parity error and frame error (CRC), a slave can not respond to an order sent to another slave with another baudrate, can it?

I find nothing in the specification that talks about this.

thank you in advance for your light.

You need a separate serial port for each unique set of serial settings.

In your case, 3 ports, unless you can change the parity on ID 125 to 'even' and run ID 125 with the ID 32, 33, and 34 where the baud rate and parity are common to all four IDs.

Then you'd need only two serial ports.

Hi

Thank you for answer.

Is there a specification to clarify this point?

I make a test; it works well! (mix 9600 and 38400 bd slaves) and it seems logical to me because slave must stay quiet if he can not understand (parity error, frame error or timing t1.5 t3.5 error)

Today I have 1 old device to connect to a new bus with several modern devices (38400Bd needed). The old is a 9600Bd one and nobody knows how reconfigure it.

thanks

Wow, different slaves at different serial settings work on the same RS-485 network/bus.

I can only speak experience and my experience is that slaves are fussy and refuse to communicate when the serial settings don't match those used by the master.

I have never tested a slave for functionality at a baud rate or parity setting other than that expected, but dozens of times I've encountered 'no comm' only to discover that the master and slave serial settings did not match and that when the settings were changed to match, comm was established.

If your test shows that your slaves with different serial settings work on the same network/bus at different settings that's great, run with it.

I'm not sure whose master you use that automatically witches between serial settings between polls to the various slaves, but I noted that the futuristic date, Oct 21, 2015 from the "Back to the Future" movie (1989) recently transpired with all the attendant fuss over the dated movie versus reality. Although there are no Mr. Fusion engines commercially available, apparently your Magic Mark XXXVIII Modbus Master handles serial change settings, or not; at least the bus works.

If the network works OK, what's the concern?

>If your test shows that your slaves with different serial
>settings work on the same network/bus at different settings
>that's great, run with it.

My slaves don't work at different settings. But master can works with 1 slave at 9600bd OR works with an other slave at 38400Bd. Master and slave are always with the same settings.

>If the network works OK, what's the concern?

I make my first master MODBUS electronic card.
Naturally I wrote code to set the baud rate and parity for each slave, it works but I find that it surprises a lot of people...I want to know what the specifications says on this point.

thanks

Denaize,

I ran into a situation one time where several of my Modbus slave devices had hardwired serial settings that could not be changed, and I needed to read them from a single Modbus master --- which sounds a lot like the problem you are facing.

My solution was to use a Digi One IAP protocol converter, but in this case I didn't really convert protocols, but rather baud rate only. The conversion went like this: Modbus/RTU/9600 - Modbus/RTU/19200. Extra hardware, yes --- but a much cleaner solution than hoping the slave devices don't complain loudly on the bus about framing and/or parity errors.

-Jim Mimlitz, SCADAmetrics

Hi

> My solution was to use a Digi One IAP protocol converter, but in this case
> I didn't really convert protocols, but rather baud rate only.

You use 1 Digi One IAP for each slave? with conversion to the single et universal master setting? it makes a lot of material...

As I find no prohibition in the MODBUS standard I will allow specific settings for each slaves on my master device.

Thanks

By scadametrics on 14 December, 2015 - 12:04 pm

> You use 1 Digi One IAP for each slave? with conversion to the single
> et universal master setting? it makes a lot of material...

In my situation, I had one Digi One IAP to perform the baud rate conversion for one problem device (problem meaning a device that had a baud rate incompatible with the other devices on the RS485 network).

All other devices were running at 9600bps and directly connected to the RS485 master bus, and the problem device could only run at 19200bps. If you need N different serial settings on your RS485 network, then you will need N-1 Digi One IAP's to bridge the various devices.

For me, it was a nice tool to solve a (hopefully) unusual problem.

We have a solution for this application. You can use our I-7514U. It has 1 master RS-485 port and 4 slave RS-485 ports which can be configured for different baud rates and communication settings on each. To the master device, it will seem as if it is talking to all of your devices on the same bus. The 7514 module will convert the responses to match the master's port configuration.

http://www.icpdas-usa.com/i_7514u.html

Interesting device, ICP DAS. Thanks for bringing it to the forum's attention!

I have tried the I-7514U from ICP DAS Co and it only works at one speed across everything connected to it. It is just a hub. The manufacturers have confirmed this to me as below.

“Hi Sir,
I-7514 can only be a 485hub.
It just supposeed communication with same baudrate.”
--
Best Regards,
ICP DAS CO., LTD. --Hayward Huang
Web: http://www.icpdas.com

You would think it should be possible for a smart master to switch serial settings to match slaves but I don't know of one.

Does anyone still use serial except for old hardware?

Yes, serial ports are still found on many of the four wire field instruments.

Almost all the vendors of Coriolis meters have an RS-485 serial option or a model that is Modbus RTU/RS-485 only.

Most magmeters have a serial port option for Modbus.

Because magnetostrictive is a higher accuracy class, it typically has a serial port option.

Most stand-alone temperature controllers have a serial Modbus option, the tiny 1/32 and 1/16 DIN don't have enough space for an RJ45 footprint.

A number of industrial flame safety controllers still have optional serial RS-485.

Custody transfer flowmeters for oil and gas seem to have both serial and Ethernet ports.

A number of ISM wireless gateways have a serial Modbus RS-485 port.

Siemens' 4 wire solids flow controllers and level controllers have 485 ports, no Ethernet.

Many of our overseas projects use Foundation Fieldbus for the reasons you give.