I thought that RS232 and RS485 were different protocols to MODBUS. However, some people have alluded to me that MODBUS and RS485 are the same thing. I am wondering if you can get different versions of MODBUS that are compatible with serial communications such as RS485...? Can anyone help?

 

Well, I'll start it - we'll hear at least 5 people say the same thing.

TIA/EIA-232-F and TIA/EIA-485-A (RS-232/485 to us mortals) are merely electricaly standards explaining how to get bits and bytes on a wire. The are NOT a protocol in any sense of the word. They impose NO meaning on those bits and bytes sent.

Modbus is a protocol and doesn't care if it's moving by RS-232, RS-485, Fiber Optics, BlueTooth, TCP or even scraps of paper by nerdy school kids cheating on Math examines. Modbus implies nothing about the electrical interface. The current www.modbus.org standards have some SUGGESTIONS, meaning if you use RS-232 do it this way. But Modbus is NOT like ProfiBus or CANBus or FF that include both a media spec and protocol spec.

I know some companies with bar-code scanner and other commodity products like to claim they speak "RS-485 protocol". It is their way of fooling users into thinking they speak an "industry standard protocol". But this is not true - they just use an RS-485 electrical interface to send their OWN non-standard protocol.

best regards
- LynnL, www.digi.com

 

RS232 and RS485 are not protocols, they are physical signal standards, i.e. voltage levels, bit rates, etc. MODBUS is a message protocol, i.e. how the commands and responses have to be formatted. Therefore a MODBUS message can be transmitted over either RS232 or RS485.

Don Zunti, P. Eng.
Delco Automation Inc.

 

RS232 and RS485 are electrical standards. Modbus is a protocol. Modbus doesn't care whether it is implemented on an RS232 or RS485 cable. There are any number of websites that can do a better job than I describing the details, but RS485 uses a differential signal, where each signal is carried over two wires, and an ON or OFF state is indicated by the potential difference between the two wires. In RS232, the ON or OFF state is indicated by the potential difference between a single signal wire and common.

 

Hello,

RS232 and RS485 are standards that pertain to serial communication. They are called "protocols" even though strictly speaking they are not. The standards specify electrical signals, maximun cable length, and in one case procedural specs. such as handshaking requirements. MODBUS on the other hand, is a specification of the exact form a frame of data must take in its implimentation. The frame can be transmitted using any of the two electrical standards above.

 

OK, you had better sit down as the # of responses on this one could get ugly.

Basically, RS-232 and RS-485 are serial communications standards, the later is for 2 wire differential line the former is not. MODBUS is a protocol or form of data communication specific to sending packets of data between two units, RS-232 and RS-485 are just the means of getting it there, MODBUS does not really care if it is 232 or 485 - you can take a 232-485 converter and put in the 232 line and now you have 485 data link or you go the other way around.

You should do some searching regarding the basics here for your own education.

Matt

 

RS-232 and RS-485 are not protocols, they are physical layer of protocols.

 

RS232, 485, 422 define the wiring, electrical signal levels, timing etc of serial transmission of bits and bytes. The contents of a message (a series of bytes), like request to read, request to write, response to a request, error detction etc is defined in a protocol like MODBUS.

 

Hello Andray,

RS232 and RS485 are not protocols - they are merely the media (physical layer) standard which protocols use to transmit and receive data. So you cannot compare MODBUS and RS232/RS485. Rather MODBUS requires RS232 or RS485 or for that matter some other media interface for actually transmitting data.

The MODBUS RTU standard basically has been specified for "Serial Communications" and "TCP/IP". MODBUS Serial does not really specify what media it has to run on - it can be RS232/422/485 etc. On the other hand MODBUS-TCP has been specified specifically to be used with TCP/IP as the network layer. It is interesting to note that even in the case of MODBUS TCP, the standard does not say what is the physical media to be used though, in most of the cases, it turns out to be Ethernet.

Hope this helps.

Ganesh Okade
Sunlux Technologies Ltd.
www.sunlux-india.com

 

RS-232 and RS-485 describe the physical layer of the communication media and not the actual protocol. Modbus would be the protocol that uses the media to communicate between two or more devices.

The following is the offical line from Modicon about Modbus RTU, Modbus ASCII and Modbus Plus.

*********Quote*****
Schneider Automation uses an open data communications network called Modbus as the basis for the exchange of information among products on the factory floor. In a typical Modbus network, Modbus messages are sent over an RS232 asynchronous serial communication link (EIA standard). This link inputs and outputs data alternately rather than concurrently. The two modes of communication within Modbus are 10-bit ASCII and 11-bit RTU.
Also, Schneider Automation uses an open data communications network called Modbus Plus as the basis for the exchange of information among products on the factory floor. In a typical Modbus Plus network, embedded Modbus messages are sent over an RS485 communication link on a peer-to-peer, token-exchange type network.
***********End Quote************

Quote is from http://www.modicon.com/opennetworking/modbus/index.htm

Modbus RTU/ASCII is always a Master Slave topology. This requires that there be only one master on the network at a time. I have not worked with the Modbus Plus or Modbus TCP/IP to be able to comment on how they communicate.

I do have to tell you that the company that I work for and others use the Modbus RTU on an RS-485 multi-drop network as well as an RS-232 asynchronous link and both work flawlessly.

Hope this helps.
Chad

 

RS-232 and RS-485 (as well as Ethernet) are physical layer protocols; they are ways of encoding digital information for transmission over their respective wire configurations.

RS-232 in its most basic form, simple uses a pair of transmit and recieve wires with additional wires designated for communication control.

RS-485 is similar but differs in its transceiver design (there is also 2 wire, which is common for industrial applications, and 4 wire, which you would see less often)

Capability to use either RS-232 or RS-485 is limited by the hardware. You either have such a port available, or you do not.

Modbus is a method of encoding data so that other Modbus compatible devices can interpret a Modbus packet. Like your telephone, your controller doesn't care what "language" you speak--it simply passes through the "information" to another transceiver. If the controller on the other end "speaks" Modbus, then it will understand the information and act according.

Most major automation vendors use a common data structure which they variously route over RS-232, RS-485, proprietary RS-485 and Ethernet. Modicon has modbus (232/485), Modbus Plus and Modicon TCP/IP--all using a fundamental Modbus data structure. Allen Bradley has (for their older stuff), PCCC over serial (DF-1), bluehose (DH+) and Ethernet (CSP Ethernet)

Its sounds confusing, but every network uses several protocols at several different function levels from physical layer at the bottom, to application layer at the top. Modbus is application, RS-485 is physical layer.

 

RS232 and RS485(422) are physical layers, and they are not the protocol. So the Modbus protocol can work on both Physical layers!

Best regards

 

MODBUS/MODBUS Plus
Schneider Automation uses an open data communications network called Modbus as the basis for the exchange of information among products on the factory floor. In a typical Modbus network, Modbus messages are sent over an RS232 asynchronous serial communication link (EIA standard). This link inputs and outputs data alternately rather than concurrently. The two modes of communication within Modbus are 10-bit ASCII and 11-bit RTU.
Also, Schneider Automation uses an open data communications network called Modbus Plus as the basis for the exchange of information among products on the factory floor. In a typical Modbus Plus network, embedded Modbus messages are sent over an RS485 communication link on a peer-to-peer, token-exchange type network.

The common language used by all Modicon controllers is the Modbus protocol. Modbus is the de facto industry standard, with support from over 500 industrial suppliers. On-line programming or data acquisition applications are easily supported directly from the serial port of any computer. Modbus can be used in either a simple point-to-point manner with a pair of devices or in a network architecture with up to 247 slave devices.

The above is copy and paste from
http://www.schneiderautomation.com/Default.htm

 

RS232 and RS485 define electrical standards for serial communication links. These links can carry any compatible serial comms, including, but not limited to, Modbus.

Modbus defines the communication protocol (like how to assemble a request, including its CRC) and what is allowed in the address structure of the "slaves". (Certain register range for integers, certain range for coils, etc.)

Modbus itself comes in two flavors, RTU and ASCII. In RTU, data is transmitted in binary form, in ASCII it's transmitted as, well, ASCII characters (the value 45 would be transmitted as the hex for the character "4" followed by the hex for the character "5" as opposed to the binary value 0010 1101).


> I thought that RS232 and RS485 were different protocols to MODBUS. However, some people have alluded to me that MODBUS and RS485 are the same thing. I am wondering if you can get different versions of MODBUS that are compatible with serial communications such as RS485...? Can anyone help?

( Complete thread: http://www.control.com/1026179178/index_html )

 

You are getting very confused, because MODBUS is a communications protocol implemented by several manufactures and RS-232 or RS-485 are serial interfaces no such protocols.

You should understand this thing: MODBUS queries goes over a serial interface such as RS-232 or RS-485 or other media as radio links or a telephone line.

MODBUS as protocol has a set of rules to define the manner in which two or more devices can share data and RS-232 or RS-485 are electrical interfaces they are defined in voltages levels for serving as the media for communications protocols.

Hope this helps.

Regards.

Raymundo

 

Modbus describes the "language" used the two units. It is register based system using defined function codes for a master to read from or write to a slave node.

RS232 or RS485 refers to the hardware used to affect the communications. The two transports are basically the same except RS4985 is a multi node system. RS232 is only point to point between no more than 2 nodes.

RS232 can be converted to RS485 easily by using a converter designed for that purpose.

There are many other communications system that use RS485 such as Allen Bradley, ION and others. It is not limited to Modbus.

I hope this helps

 

RS232 and RS485 are NOT protocols. On the simplest level they are interface definitions. Modbus is a protocol.

Check the B&B web site for execellent information on RS232/485 protocols at www.bb-elec.com.

Think of RS232 and RS485 as types of highways and Modbus as one of the types of vehicles that can run on these highways. Some highways are wider than others, some are faster and some go further. Similarily, some vehicles can carry more cargo and some go faster

 

RS232 and RS485 are media specifications (OSI layer 1) and are not content related, i.e. these media do not care if you speak Modbus, Morse code, or Spanglish. RS-232 provides a private, point-to-point conversvation media, like a pair of paper-cups and string. RS-485 is an audicienced, point to multi-point media,like a smoke signal. Modbus is content, one of many contents (OSI layer 4) that can be transported over an RS232 or RS485 media.

These "some people" who have "alluded" to you that "Modbus and RS485 are the same" are confused.

I'm not quite sure how to tell you where to get started myself but i am sure you need to draw a bold swiggly line between Modbus protocol and RS-485 or any other media.

jk

 

It´s true, RS 232 and RS 485 as the same thing, are a electrical especification of computer comunication, they have some diferents, RS232 is a especific point-to-ponit comunication and is efective only in a 15 mt length, RS485 permit multidroop comunication, you can have a master (query) and many slaves (aswering, and working up to 1500 mt., then RS232 or RS485 are a electrical resource to comunication.
Modbus is a package of instruction like read or write , registers, binary input or binary output from or to PLC or any equipment, then Modbus is a language and they work over the RS232 or RS485. depending on you want.
Speak whit only one device (RS232) or many devices (RS485).

 

RS232 and RS485 are physical protocols. They specify signal voltage levels, slew rate, number of start/stop/data bits per character, etc. Both the sending and receiving ends have to be set up the same, for example, 9600 baud, 8 data bits, one stop bit, odd parity.

MODBUS is a software protocol, specifying what the bytes in the data block mean depending on their position in the block. MODBUS can be transmitted in either RS232 or RS485. There are two forms of MODBUS, one that uses the actual ASCII characters, one that uses binary. For example, if you need to send the number "123", in the ASCII form, you would send three bytes, with each byte representing one digit: hex 31, hex 32, and hex 33. In RTU form of MODBUS, you would just send one byte, hex 7B.

Does that make sense?