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Unread December 3rd, 2019, 02:12 AM   #1
lijochrn
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M340 MODBUS TCP communicate with OPC

Hello Experts,

We are currently in the design phase of a system with 24 PLC's.
CPU is BMXP342020 [ CPU onboard Serial and Ethernet ports, both capable of MODBUS]

Each PLC's are stand-alone and have communication with 9 Modbus RTU slaves.
Also, each PLC will be connected to OPC at central SCADA room via MODBUS TCP.


Questions:
Regarding Serial MODBUS RTU
1. CPU onboard RJ45 Serial port can directly make communication with all 9 slaves via multidrop? Using READ VAR/WRITE VAR function?
2. How many READ VAR/WRITE function can be used simultaneously?

Regarding MODBUS TCP
1. Which module is required for connecting to OPC?
BMX NOC 0401 or BMX NOE 0100?
Can the above two modules be connected to OPC without any programming? We need to read/write data to each CPU all the time.

2. Does BMX NOC 0401/BMX NOE 0100 need programming in PLC for communication?

3. Does CPU onboard ethernet port require programming for communicating on MODBUS TCP?

Please guide.



We have used M580 CPU in 10 projects and all the time DCS [Client] can read/write data with M580[server] without any programming. But we used CPU onboard port or BME NOC 0301.2

So does that sounds similar to BMX NOC 0401/BMX NOE 0100
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Unread December 3rd, 2019, 01:29 PM   #2
danw
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Quote:
Originally Posted by lijochrn View Post
Serial port can directly make communication with all 9 slaves via multidrop?
1a. Assuming the serial port is RS-485, then yes, RS-485 is likely to support 9 slaves. Baud rate can be limited by total bus distance.

Modbus is always a sequential polling protocol, so one only gets around to polling slave #1 after having polled slaves 2-9. If a slave goes off-line or fails, the master's time-out will likely slow the entire throughput cycle, as the master waits for reply that will not be forthcoming.


Quote:
Originally Posted by lijochrn View Post
Using READ VAR/WRITE VAR function?
1b. It depends on what you mean by that.

The Modbus spec has defined a Function Code 23 (FC23), read/write Multiple Registers, that both reads and writes in one master message/transaction..

I know of only one a commercial implementation of FC23 by a vendor who uses it to upload and download configuration parameters for single loop controllers within their own software package, but it is intended for general use. I know of no commercial implementations otherwise (HMI's, field devices) on either the Master/client end or the slave/server end.
That doesn't mean FC23 is not out there, but good luck finding a commercial package that supports it.

Remember that both ends have to support any given Function code 23, so finding a master that supports FC23, but the slaves do not (or vice versa), does not allow you to use FC23.

What is almost universally supported are separate read functions from write functions. Widely implemented Read functions are FC01, FC02, FC03, FC04. Widely implemented write functions are FC05, FC06, FC15 (0Fh), FC16 (10h).

But read functions (messages) are implemented separately from write functions.

If you discover that your master hardware/firmware, slave hardware firmware network does support FC23, this forum like to hear about that.

Quote:
Originally Posted by lijochrn View Post
How many READ VAR/WRITE function can be used simultaneously?
2. None. Nothing in Modbus RTU is simultaneous. All actions in Modbus RTU are sequential, one at a time. See item 1a above.

The rest of your questions are very hardware/firmware specific for which I have no experience.

I would think that in a project involving 24 PLC's that the PLC vendor should be capable of providing guidance on these points.
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Unread December 4th, 2019, 05:40 AM   #3
tragically1969
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Answers in line below

Quote:
Originally Posted by lijochrn View Post
Hello Experts,

We are currently in the design phase of a system with 24 PLC's.
CPU is BMXP342020 [ CPU onboard Serial and Ethernet ports, both capable of MODBUS]

Each PLC's are stand-alone and have communication with 9 Modbus RTU slaves.
Also, each PLC will be connected to OPC at central SCADA room via MODBUS TCP.


Questions:
Regarding Serial MODBUS RTU
1. CPU onboard RJ45 Serial port can directly make communication with all 9 slaves via multidrop? Using READ VAR/WRITE VAR function? Yes
2. How many READ VAR/WRITE function can be used simultaneously? 1, they should be sequenced one after another after each rad is successful or has timed out

Regarding MODBUS TCP
1. Which module is required for connecting to OPC? Either
BMX NOC 0401 or BMX NOE 0100? The NOE will be fine, it can be Modbus TCP slave
Can the above two modules be connected to OPC without any programming? We need to read/write data to each CPU all the time. they are default slaves/master, all you need to do is assign the IP in the PLC application

2. Does BMX NOC 0401/BMX NOE 0100 need programming in PLC for communication? No, as above

3. Does CPU onboard ethernet port require programming for communicating on MODBUS TCP? No, as above

Please guide.



We have used M580 CPU in 10 projects and all the time DCS [Client] can read/write data with M580[server] without any programming. But we used CPU onboard port or BME NOC 0301.2

So does that sounds similar to BMX NOC 0401/BMX NOE 0100 Yes
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