One MODBus RTU Slave and Two Masters

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Thread Starter

Rashid

Hi

I need to communicate Deep Sea DSE-75xx controller with Two MODBus RTU Masters. One Master is DSE-850 for local monitoring and control while other Master can be a MODBus OPCServer.

Deep Sea is simple saying that it is not possible ..

Any help would be appreciated.

Regards
 
Hi,

Is the Deep Sea DSE-75XX also a Modbus RTU interface? if so using a FieldServer we can connected multiple Modbus Masters together, as we have done it the past

Regards,
Richard
 
One option might be to insert a gateway in between the 3 devices. the ETH-200 gateway from ICC supports Modbus/TCP (server) on its Ethernet port, and then also has 2X RS-485 ports and 1X RS-232 port, each of which can be independently configured with Modbus RTU master or slave drivers. So with one of the gateway's ports communicating to the DSE RTU slave, all other ports are available to allow simultaneous communication to other Modbus (RTU or TCP) masters/clients.

http://www.iccdesigns.com/products/gateway/eth-200.html

- Darrin
 
L

Lynn August Linse

I looked up the product - the DSE850 is a Windows app, so if it 'demands' direct serial control of the link to the DSE-75xx, the they are 'correct' in that you can't sneek another Windows APP like OPC into sharing the same hardware serial port.

However, several firms make devices with multiple serial ports which could allow 2 serial Masters to share the same serial slave.

Plus, you won't need that if the DSE850 can use Modbus/TCP (not serial). if so, there are many Modbus/TCP-to-RTU boxes which allow multiple Modbus/TCP masters (such as the DSE850 and OPC) to share the same serial slave at the same time. Like a 'print server', this box understands the Modbus transactions and gracefully interleaves the Modbus requests to the serial slave.

For example the Digi One SP or Digi Connect SP both would support this cheaply (without DIN rail or power-isolation).

There is no shortage of competing products out there - just do a google search for "Modbus Ethernet Bridge" and you'll get lots of links - look especially at the paid 'Ads' links as every one of those firms makes such a device! You'll have too many options.
 
I need to communicate Deep Sea DSE-75xx controller with Two MODBus RTU Masters. One Master is DSE-850 for local monitoring and control while other Master can be a MODBus OPCServer.

Thanks
 
What if I use a MODBUS-RTU to MODBUS-TCP convertor and use two masters to get data from convertor.

Currently MODBus-RTU over RS-485 is giving data from all 7 controllers using slave IDs.

Need to know, if converting RTU-TCP will allow me to use Slave-IDs? or I will have to use convertors for all controllers?

Thanks
 
L

Lynn August Linse

>What if I use a MODBUS-RTU to
>MODBUS-TCP convertor and use two masters
>to get data from convertor.

Yes, that will work if both tools can support Modbus/TCP - which is what I was suggesting above. Having worked in this space for 20 years, my jargon is probably too heavy at times.

The only risk you have is:

1) some tools mistakenly 'assume' Modbus/TCP means fancier services supported. For example, some low-end serial PLC cannot be firmware-flashed by Modbus/TCP to RTU converter because the 'flash tool' assumes they support fancy, advanced extensions which don't map to serial.

2) both hosts/masters will compete for bandwidth, so it is possible 1 'selfish' Master saterates the channel, taking 80-90% of the bandwidth while the more graceful one is squeezed out.

3) some Modbus/TCP tools assume Ethernet=Fast and start with very unrealistic timeouts like 100msec. Likely this setting can be changed, but keep this in mind if it first fails.
 
Thanks Lynn August Linse,

You have been very helpful.
Just one thing more :p

Can I get data of all 7 controllers from one MODBus TCP using Slave IDs (as I do in MODBus RTU-over RS-485)?

Thanks
 
L

Lynn August Linse

>Can I get data of all 7 controllers
>from one MODBus TCP using Slave IDs (as
>I do in MODBus RTU-over RS-485)?

Yes, you would set the 'Unit Id' within the Modbus/TCP request, and the 'Modbus Ethernet to Serial Bridge Device' will by default use the 'Unit Id' for the RTU slave address. Seems a bit like magic at first, but these devices have been on the market since 1996 when I helped a small German company create products for Ethernet and Token Ring (the Ethernet model was later private labeled for Modicon as the CEV).

If you need to increase performance, a bridge device with 2 or 4 RS-485 ports would allow you to define slaves 1&2 being on port 1, slaves 3&4 being on port 2 and so on. This allows more serial transactions in parallel. So the device is smart enough to select the correct port based on Unit Id/Slave Address which you configured.

I know of pipeline companies (as example) who do this for remote IP access via microwave or DSL. They use a 4 or 8 port bridge and run each device as an individual port. It also gives them more robustness as surge or water damage in a single device isn't going to ground/shunt the entire multidrop.
 
S

Shashi Kanth Sharma

Hi,

Can you tell if this setting worked for you?
Practically i had faced lots of trouble during commissioning of one of my projects where the historical data quality use to fluctuate when both the masters started to poll the data.

Regds,
Shashi
 
L

Lynn August Linse

> Practically i had faced lots of trouble during commissioning of one of my projects where the historical data
> quality use to fluctuate when both the masters started to poll the data.

I'm not sure what you mean by 'quality', but obviously, two masters competing means each has less bandwidth. So if a slave takes from 200 to 700 msec to answer, and both masters have a 1 second (1000 msec) time out, then you are going to have a lot of timeouts.

As a rule of thumb - if with 1 master you normally have a 1 second timeout, then with 3 masters you should consider a 3 second timeout!

This can be worse with Modbus/TCP, as some masters assume a 250msec timeout, which when you convert to Modbus/RTU at 9600 baud, you'll rarely get any good responses since a 255-byte message takes over 255msec just to serially shift, ignoring all other overhead. So you might need to change the default response timeouts in a tool which doesn't make it obvious how to change.

The other common problem: Most Modbus/TCP masters 'pipeline', so if you have 7 serial slaves on an RS-485 at 1 bridge on 1 IP, the master might send 7 message at a time on 7 sockets, treating each as an independent slave (one to each). If a second master is only sending 1 at a time in a graceful round-robin of the 7, then it will have much worse performance - in theory only about 1/8th the bandwidth because the master sending 7 requests through 7 sockets may get 7 requests answered for each 1 the other 'graceful' master gets answered.
 
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