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Old June 20th, 2019, 04:32 AM   #1
karthiknd7
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Serial Communication RS_232

How many device can be connected by using RS_232?

I know that there will be only one RS-232 transmitting device and i would like to know how many receivers can be connected to one transmitting device.
Kindly suggest me a splitter or HUB or Switch if is it available.

************************************************** ******
There is an existing plc which has only one RS-232 port. That port is already connected to an HMI.

Can I use RS232 splitter in PLC, so that one cable goes to HMI and another cable goes to SCADA to get the data from PLC... Is it possible to do so

Here, i am not going to send anything from SCADA to PLC. Just half duplex getting data from PLC like status, performance and alarm.

Thanks in advance
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Old June 20th, 2019, 04:47 AM   #2
parky
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The problem is that both the HMI & the Scada will transmit data to request information, I know some time ago I used an intelligent splitter on an S5 PLC and a HMI so that I could program while still connected to the HMI, I'm sure there are such units still out there. Some HMI's like Beijers have a pass through mode and two or three ports.
Found this at Amazon but no idea if it will work
https://www.amazon.co.uk/Splitter-Bo...%2C194&sr=8-19

Last edited by parky; June 20th, 2019 at 04:58 AM.
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Old June 20th, 2019, 05:17 AM   #3
gclshortt
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Serial RS232 is the cable and signal that is being used. You need to look at the protocol in order to determine the number of receivers (Slaves) you can have for the transmitting device (Master).
Example:
Serial Modbus RTU = 127 slaves
Serial Omron RS232 = 32 slaves

What equipment are you using? What protocol?

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Old June 20th, 2019, 05:44 AM   #4
JesperMP
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As other have posted, there are protocols that provide master/slave functionality on serial networks.

But without any further information, I am guessing it is NOT possible for karthiknd7.
The PLC and HMI must be setup for said protocol.
Serial master/slave is relatively rare. An existing setup with only 1 PLC and 1 HMI is highly unlikely to be setup for a protocol with master/slave.
It is not enough to insert a "splitter" somewhere between the existing PLC and existng HMI.
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Old June 20th, 2019, 09:15 AM   #5
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Agreed. There would be very little, if any reason to use a half duplex protocol when connecting an HMI to a PLC directly. To add another connection the the PLC's port it would have to be setup as a half duplex port (at the protocol level) and even then it would need to be able to respond to and/or initiate communications to the other device.
One thing to understand and/or remember, RS232 isn't a protocol, it's a communications platform used by communications protocols.


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Originally Posted by JesperMP View Post
As other have posted, there are protocols that provide master/slave functionality on serial networks.

But without any further information, I am guessing it is NOT possible for karthiknd7.
The PLC and HMI must be setup for said protocol.
Serial master/slave is relatively rare. An existing setup with only 1 PLC and 1 HMI is highly unlikely to be setup for a protocol with master/slave.
It is not enough to insert a "splitter" somewhere between the existing PLC and existng HMI.
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Old June 23rd, 2019, 08:52 PM   #6
karthiknd7
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Thank you for all your time and effort.

There is a site where most of the machines which have RS232 communication. Some machines dont have PLC. It runs in PCB Controller, which is connected to a DOS computer through RS232 which is used by the operator to control the machine.
My task is to take OEE of the Machine. Now i have to connect this machine with Ignition Scada to get OEE data. The machine and Dos computer connected through RS232.
What i am thinking is, using a RS232 splitter connect one end to the DOS computer and another end to Moxa NPort 5150A [Serial/Ethernet] from MOXA using Modbus TCP/IP willl connect to Ignition Scada.... Does this method work?
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Old June 23rd, 2019, 09:03 PM   #7
GaryS
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RS232 is point to point one computer to one device
to get multi drop you need to go to RS485 I believe the limit with RS485 is 64 nodes.
while a windows computer supports 32 COM ports they cam be hardware, USN or Bluetooth even ethernet with a converter
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Old June 23rd, 2019, 11:57 PM   #8
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Only listening is easy but then you have to decode it and it's one way.
Better to open the DOS computer and see what it tells you. If it is the master and holds your OEE data then re-program it for a new serial slave.
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Old June 24th, 2019, 12:23 AM   #9
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You have no idea what protocol is used between pcb and DOS pc ?
And PCB based controller with DOS based HMI sounds like it us something proprietary.
For Ignition you need an OPC server that uses the protocol of the PCB.
I think you are out of luck.

Your only chance is contacting the original manufacturer.
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Old June 24th, 2019, 05:56 AM   #10
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Some HMIs are loop-through capable, like the old OP-15 from siemens. That way you can control your PLC and keep your HMI working without any mux-like device.
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Old June 24th, 2019, 05:58 AM   #11
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If the data is in the DOS PC . . . Why not add another serial port to it?
(Or, if it has one, use the second serial port?)

Poet.
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Old June 24th, 2019, 06:17 AM   #12
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RS232 is a 1:1 communication.

Although if you change to a 1:n network like RS242 or RS485, your idea that both HMI and SCADA request data from the same PLC would not work either, since in those serial networks there can only be one Master, not two.

it would be best if you could add a second RS232 port in your PLC
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Old June 24th, 2019, 11:14 AM   #13
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All of this is a moot point without knowing what the communications protocol is. It will have to be a protocol that will operate in a half duplex mode and it will have to be already programmed to operate in half duplex unless you are planning on going in and reprogramming the existing equipment. If it is already setup for half duplex the master will more than likely need to know that another remote has been added to the system. The details of this can be extensive but without knowing what the devices are speaking I.E. the communications protocol, the best you can do is guess. In other words, the ability to physically make a connection via RS232 is only one small piece of the puzzle.
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Old June 25th, 2019, 08:20 AM   #14
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RS232 is 1:1 although with a simple relais you can change the receiving unit. also the other way, but that can only be done with chip select .
we had long time ago a network running with several PC all with rs232 and some very small relias to switch the sender.
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