Mitsubishi MELSEC protocol (MELSEC Communication Protocol)

Protocol description
Communication line configuration
Station configuration
I/O tag configuration
Literature
Changes and modifications
Document revisions

Protocol description

The Mitsubishi MELSEC protocol is used for communication with Mitsubishi PLCs (MELSEC L-series and MELSEC Q-series series) equipped with Ethernet communication modules. The UDP variant using the UDP protocol (on the TCP/IP-UDP line) and the TCP variant using the TCP protocol (on the TCP/IP-TCP line) are implemented. Read and write commands are supported.

Following rame formats are supported (see Frame type protocol parameter): 1E, 3E, 4E. Both binary and ASCII message encoding are supported (see the Code type protocol parameter). ASCII encoded messages are approximately 2 times longer than binary encoded messages.

Reading and writing various types of objects (device codes) is supported. These objects are bit-oriented or word-oriented, and the interpretation of several (16 or 32) consecutive bit objects as a signed/unsigned number or two consecutive word objects as a number (unsigned, signed, a real 32-bit number) is supported. Access to a specific bit of a word object is also supported.

Communication line configuration

• Communication line category: TCP/IP-UDP (UDP variant) or  TCP/IP-TCP (TCP variant).
• TCP/IP-UDP parameters:
  • Host: IP address or of the network interface that is used for communication by the KOM process. A symbolic name that can be translated to an IP address can be entered too.
    Note: a symbolic name ALL or * can be entered - in which case all available interfaces are used.
  • Port: UDP port number that is used for communication by the KOM process.
  • Note: The parameters of the backup server (Host and Port) are not used in this protocol.
• TCP/IP-TCP parameters:
  • As the Host, Port, and Line Number parameters are not used (PLC address is configured on the station), arbitrary values can be specified.

Station configuration

• The communication station corresponds to the device with which the KOM process communicates.
• Communication protocol "Mitsubishi MELSEC".
• IP Address: IP address of the Mitsubishi device with which the KOM process communicates directly via the Ethernet network. This can be the target device or an intermediary if the target PLC is in another network. If it is an intermediary, the destination device is specified using the station protocol parameters Station No., Network No., Request destination module I/O No., Request destination module station No.
• Port: TCP/UDP port number of the Mitsubishi device

Station protocol parameters

Configuration dialog box - tab Parameter.
They influence some optional parameters of the protocol. The following station protocol parameters can be set:

Table 1

I/O tag configuration

Possible types of I/O tag values: Ai, Ao, Di, Do, Ci, Co, TiR, ToR.

I/O tag address:

The address of the I/O tag is in the form <device code><device number>{ .bit} { ,<value type>} {, items}
Note: the I/O tag is ignored if its address starts with %IGNORE

Device code

In Mitsubishi protocol terminology, individual objects whose values are read and written are called devices. The protocol allows access to multiple types of devices. Working with the following types of devices is supported:

Device number

For a specific type of device (device code), individual objects are identified by a number (device number). When using 3E and 4E frames, a 3-byte number is specified, when using 1E frame, a 4-byte number is specified. Numbering starts from 0, the maximum number for a specific device type depends on the specific PLC.

 Bit

Optional parameter - access to a specific bit. For device types (device code) that have word data, a specific bit (0-15) can be specified.
Note: writing is not yet implemented for the  I/O tag with a specified bit, only reading.

 Value type

Optional parameter - data interpretation.
For device types (device code) that have word data, the default interpretation is WORD (e.g. SD5 address).
For device types (device code) that have bit data, the BIT interpretation is preset (e.g. X5 address)

Note: when interpreting 32-bit numbers, the data is interpreted as little-endian (a lower word at lower address, higher at higher address).

 Items

The number of elements for structured I/O tags with configured Destination column. Every read element (1,2,3 .. items) will be written to one item of the destination column.
Note: All "items" elements are read at once. If e.g. 100 elements of device type (device code) which has data of word type (e.g. address D10, WORD, 100), are configured, it means reading of a block of 200 bytes.  If the number of objects is greater than the value of the Max Points, parameter, or the data size is greater than the Max Data Bytes, the reading will not be performed and an error message Too many elements in point < I/O tag name> will be added to the line log.

Configuration examples:

• X10 - Input 10, interpreted as a BIT (default)
• D0 - Data register 0, interpreted as a WORD (default)
• M12, WORD - Internal relays 12-27, interpreted as a WORD
• D4, REAL - Data registers 4 and 5, interpreted as REAL
• Y10, BIT, 20 - Inputs 10-29, interpreted as BIT (default), assigned to 20 items of the Destination column
• D4, REAL, 10 - Data registers 4-23, each couple interpreted as REAL, assigned to 10 items of the Destination column
• D4, WORD, 10 - Data register 4-13, interpreted as WORD, assigned to 10 items of the Destination column

Literature

• MELSEC Communication Protocol Reference Manual

Changes and modifications

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Document revisions

• Ver. 1.0 – March 10, 2021 - a document created