I have four Floboss 503 (flow computer). Three of them arranged in star topology and one last Floboss connected to one of star line branch. The output line of star connection is connected to one Lantronix Serial Converter. From there it is connected to two DASMBTCP as modbus master using ethernet.

The problem is only three of them is able to communicate with master. Even those three Floboss is often resulting an error in communication. I am forced to give the scan rate every 10 second for those three Floboss.

Anyone please give me advice

My question is :
How many modbus master allowed to send the request to slave?

Why the communication is always slow when I tried to read all slaves at the same time? But it is not when I only read one slaves?

I am using one serial converter for four slaves. Should I use one serial converter for one slaves?

Thanks

 

How long are your 'star branches'? If they are only a few feet, what you have may work. But if each 'star' is like 20-30 meters long, then that is your problem. Per the pure RS-485 spec, you are allowed 'zero inches' for star runs. Obviously, even with a nice bus you will have some inches of PCB trace etc. But the shorter you make it the better.

The Lantraonix will support up to 8 masters at once, and it tries its best to manage the polls "like printer jobs". Unless they changed the code, you get in trouble if you poll faster than the serial line can clear since they just use TCP/IP buffering to build up a queue (I know who wrote that code ... quite well :-] ) Thus the unit has no idea how large the TCP backlog is becoming.

Obviously, if speed is an issue, using 1 TCP/RTU unit per flow computer allows max throughput, while putting four on 1 TCp/RTU box promises four-times worse performance. That said, I don't know your baud rate (speed) or how large your requests are, but the average RS-485 multi-drop should be able to handle AT LEAST 4 requests per second, sometimes more.

But the problem is still likely your wiring. If you do need 'long stars', nothing stops you from folding a bus. So you need to run 2 cables to each 'star point' and in effect send out on one, then have the return come back but go to the next leg of your star. So in your case your run the bus to unit #1, then 'fold back' from #1 to the center and out to #2, then fold that back to the center and on to #3 and #4. You now have a bus, which normally can be up to about 330m/1000ft if you have good RS-485 cable.

 

the majority of slave devices use a single master protocol. Modbus RTU Serial is a single master protocol. Modbus Etherent allows for multiple masters to connect to the same RTU. When connecting to a serial device through a Modbus Ethernet to Serial gateway the Ethernet side will typically allow more then one connection but it will have to manage the requests so that there is only one request at a time on the serial side of the gateway. Depending on the poll rate, the speed of the serial side connection, and the amount of data being requested communications could be very slow and you could even see timeouts.