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How to share RS232 paralley?

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bluehole

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Hi all,
I am in a problem, Hope some one have a solution. any comment and complement is welcome.

Situation:
1- I have 12 microcontroller boards
2- each board has RS232 interface
3- I have a a PC with one Serial port
4- Software in PC will send a command to the serial port
5- The Microcontroller boards have their configuration and can receive the command and parse it.
6- if the command is for any perticular board then it will execute that command.

My Question is Can I connect all the boards parallely and just take the TXd RXd and GND to all the 12 boards?
I doubt this cant be done because of impedance mismatch it may burn up the whole setup.

So what are the ways to achieve this. Please feed me with some inputs. if any body have tested schematic please also provide the same.

Thanks.
 

mohansaini

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You can use mux cd4051 between these RS232 ports. yes if you are going to connect all the TX RX and GND it may be the case of damaging the whole set up. even can damage your controller as well if there is no isolation between the two.
 

bluehole

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You can use mux cd4051 between these RS232 ports. yes if you are going to connect all the TX RX and GND it may be the case of damaging the whole set up. even can damage your controller as well if there is no isolation between the two.

thanks for a prompt reply.

Well do you have any schematic for your scheme? Please attach.

Any supporting documents or links how it can be done, will be helpfull.
 
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ud23

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https://www.edaboard.com/threads/253190/

see post number 13 by mr. brain if you using pic it might help you and if you want do that with software you have to use RS485 mod bus protocol with one master which is PC and other 12 slaves.
 

tpetar

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Of course this is possible, you need serial port sharer device something like this https://www.acceed.com/pdf_catalog/rayon/S400_Catalog.pdf.


Low-Cost RS232 Multiplexer
control four devices with a single RS232 port

https://www.vimms.org/multiplex/docu.html

low-cost-rs232-multiplexer.thumbnail.jpg
 
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bluehole

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Thanks ud23,

for clarification I am uploading a scetch ( which is not very good) which will help understand the scheme/situation i am planning.

 

tpetar

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bluehole

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Of course this is possible, you need serial port sharer device something like this https://www.acceed.com/pdf_catalog/rayon/S400_Catalog.pdf.


Low-Cost RS232 Multiplexer
control four devices with a single RS232 port

https://www.vimms.org/multiplex/docu.html

low-cost-rs232-multiplexer.thumbnail.jpg

But I need some kind of broadcast from PC and all the slave will receive it, currently I am not planning to implemet feedback from slaves.
But in future I need that feature,

Just a wild Guess.... Can I use a Bidirectional buffer(74245) intermittently to act as a repeter???
 

bluehole

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RS485 bus.

So You suggest I have to use RS232 -> RS485 converter on all boards as-well-as on the PC/PLC side??? that is I need 12 RS232 -> RS485 converter for 12 boards
and one for master(PC) = 12 + 1 = 13 ....

Am I right???
 

bluehole

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Multi-drop RS232 or logic level serial interface is at least a non-standard option that should be considered. I know some professional products that are using it.

https://www.edaboard.com/threads/144963/

Your answer in that thread is very nice. Well My PIC 18f4550 board has a max232 chip and a DB9 connector. I have 12 such boards.

I can program each PIC board with a station ID. My Aim is that I will connect all the PIC boards to a PC, and send a command string. "stnID,action"

so that all the PIC boards will get it and check if the "stnID" belongs to itself, if so then take the action, else forget the command.

A kind of broadcasting the command and receiving it by PIC boards.

Is it any way possible???
 

andres90125

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you have two options to do this, one is the one you suggested using RS485.
Another is to use rs232 9-bit protocol, which is exactly designed for this purpose, when the ninth bit is active is writing an Address (corresponds to one of your teams listed), then the information will come to this. otherwise it is not possible to RS232 multipoint.
Of course this option may only be a master, the other will inevitably slaves but communication may be full duplex.
The RS485 option you can put someone in it is proved necessary to adopt the position of master, but you will be half duplex communication
and last but most expensive course you can use RS422
 

FvM

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As explained in the old thread, you either need RS232 drivers with tristate function or a kind of wired "or" hardware, e.g. the said diode circuit. Wired "or" involves a risk of master TX to slave TX crosstalk if the total cable length exceeds 10 or 20 meters and usual cables with common screen are used. Crosstalk can be filtered in master receive operation, if you don't require full duplex communication. Special driver versions like MAX3322 are designed for multidrop operation.

P.S.: 9-Bit-protocol isn't but one of several available addressing options. It has some importance, because it's implemented in 8051 CPU hardware. Most protocols are using an address field within a 8-Bit protocol. It's preferred, because 9 Bit UART operation isn't well supported by all processors or computer operation systems. You can refer to MODBUS as a popular protocol that achieves addressing based either on an 8 binary or ASCII characters.
 
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bluehole

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It is nice that you all people tried to help me. So thanks a lot.

From above discussion I learned to go for RS485. 9-bit thing will be a tough task I guess.

Well If we eleminate the communication from slaves to master ( i.e from Micro controller boards to PC) then can I connect all the RX pin of micro controller boards to the TX pin of PC???

refere to image bellow



because i am on a low budget.
 

FvM

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That's the usual way in a multi drop configuration. You should check the input impedance of the RS232 receivers if they allow for 12 devices to be connected in parallel.
 

andres90125

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two things. FVM, is correct in that you see what happens to the impedance. and 9-bit protocol is from 8051, it now also taken several ARM7 (various manufacturers) and AVR ... etc. If you're using 232 pure, do not forget that if you hang 12 listeners just a transmitter, you can have problems. then I think it will be necessary to pass 485 to avoid this inconvenience.
The other is that the 9-bit RS232 protocol, not standard PC systems. But, there are specific RS232 PCI cards that will meet your needs and you can do.
finally. if the amount of data to be transmitted is important, etc. .. why do not you Ehernet, documentation and availability of low-cost integrated circuits makes it entirely possible
 

bluehole

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Thanks FvM and andres90125. So If i Remove ane communication channel ( TX of Slaves) then atleast I can achieve something.
As Fvm rightly said about Impedance, Can anybody suggest any way to overcome that issue????
 

andres90125

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I think in the electrical layer Rs232 not get it with such a number of listeners hung on your transmitter. and if you get will be by accident, So I guess it would be best to use a scheme that was suggested above ("tpetar") using a converter for 485, so you cominicarte circuit honoring the abundance if the amount you're saying
 

FvM

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I think in the electrical layer Rs232 not get it with such a number of listeners hung on your transmitter. and if you get will be by accident.
You can design multi-drop "RS232-like" systems that achieve high fanout by design, if necessary. If you are interested in the details, you should review e.g. MAX3322 datasheet. I mentioned this technique because RS232 multi-drop was asked explicitely in the original post.

RS-485 is the industry standard for serial multi-drop and multi-master networks and would be the first choice in most cases. Nevertheless, multi-drop RS232 has become popular for a number of applications, particularly with a single desktop PC master controlling multiple devices, not needing special interfaces execpt for a suitable daisy chain cable. You find it e.g. in lab automation.
 

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