Melsec Driver

dc3-driver-melsec onboards Mitsubishi PLCs into IoT DC3 over the MC protocol: it acts as an MC client, actively connects to the PLC, periodically reads by the device address configured on each Point, and supports commands that write values back to devices. After reading this page you can fill in the driver / point attributes, onboard a Mitsubishi PLC, and know where to look when it won't connect.

Protocol background

The MC protocol (MELSEC Communication) is the native communication protocol of Mitsubishi Electric PLCs, widely used across the A, QnA, Q/L, and iQ-R series. On the plant floor a Mitsubishi PLC exposes an MC server port through its Ethernet module or built-in CPU port; an upstream system connects as an MC client and reads/writes data units in PLC memory by their device address (e.g. D100, M0) — data registers hold process parameters, internal relays hold logic state, input/output relays map to field IO.

In the four-layer IoT architecture the MC protocol sits on the wired-industrial side of the network layer: it is the "last mile" language contract between a PLC and upstream systems, governing how bytes are laid out, how devices are addressed, and the request-response timing of each exchange. Like Siemens S7 and Omron FINS, it is a vendor-proprietary master/slave protocol — the PLC never reports unprompted; the master must ask. To see where it sits in the protocol landscape and why PLC vendors' protocols are mutually incompatible, read IoT network layer: industrial buses and protocols.

The driver is built on the McPLC of the iot-communication protocol library, automatically selecting the matching word width and codec by the point's data type when reading or writing.

• Driver name / code: Mitsubishi Melsec Driver / MelsecDriver
• Type: DRIVER_CLIENT (actively connects to the PLC)

A few MC concepts first

Device (also called memory address): a data unit in a Mitsubishi PLC partitioned by purpose — e.g. D (data register, the most common), M (internal relay), X (input relay), W (link register). Device address: a complete address string made of an area prefix plus a number, e.g. D100, M0, X10, W200 — the area and number are written together, not split into two fields. PLC Series: the MC frame format differs slightly between series, so choose A / QnA / Q_L / IQ_R to match the actual PLC.

Attribute configuration

Melsec driver configuration has two layers: driver attributes describe "which PLC to connect to" (device-level, one set per device), and point attributes describe "which device to read" (point-level, one set per point). Both layers are declared in the driver's application.yml; you fill the values in the console at onboarding time.

Driver attributes (device-level driver-attribute)

host / port decide which PLC's MC server port the TCP connection targets; series decides which framing the MC frame uses and must match the real PLC series. When onboarding a Melsec PLC, fill these Attributes on the Device:

Attribute	code	Type	Default	Description
Host	host	STRING	192.168.0.20	PLC host address (Ip)
Port	port	INT	6000	MC service port
PLC Series	series	STRING	QnA	PLC series, A/QnA/Q_L/IQ_R

Point attributes (point-attribute)

address specifies the device to read/write (whole-string notation, area + number written together); length is only used for the string type — the byte length of the string to read. Fill these on each Point:

Attribute	code	Type	Default	Description
Device Address	address	STRING	D100	Device address (D100, M0, X10, W200, etc.)
String Length	length	INT	0	String read length (0 for non-string types)

The data type decides how many words to read and how to decode them

The driver picks the read/write width and codec automatically from the point's data type (Point's pointTypeFlag): BOOLEAN reads/writes a bit, BYTE reads/writes 8 bits, SHORT reads/writes 16 bits (int16), INT/FLOAT read/write 32 bits, LONG/DOUBLE read/write 64 bits, STRING reads/writes a string. Only the STRING type uses length (the number of bytes to read; the driver falls back to 64 when it is 0 or empty); for non-string points keep length=0 and the driver ignores it.

Write command: reuses the point address, no separate attribute

This driver supports writing values to a point (numeric, boolean, and string alike), but there is no separate command-attribute — a write command reuses the address already configured on the point, the target device is the point's address, and the word width of the written value is determined by the data type of the value being issued. So a writable point needs no extra configuration; just mark the point as writable in the Profile.

Acquisition and health scheduling

These crons come from the schedule / health sections of application.yml and govern the acquisition cadence and online detection:

• Acquisition cycle: default cron 0/30 * * * * ? (one read round every 30 seconds).
• Custom task: default cron 0/5 * * * * ? (the Melsec driver currently has no custom task; schedule() is an empty implementation and the schedule slot is reserved).
• Health / online: device health check default cron 0/15 * * * * ?, lease timeout 45 seconds — the driver judges online status by whether the TCP connection is alive; on a read/write exception it proactively disconnects and evicts that connection from the cache, and the next acquisition round reconnects automatically. See Device for the online-status mechanism.

Troubleshooting

Write address as the full device address — don't split area and number

Fill address with Mitsubishi's whole-string notation, e.g. D100, M0, X10, W200 — the area prefix and the number go in the same string. This differs from protocols configured as "area + numeric offset" in two fields (such as FINS); do not split the area and number into two entries, or McPLC parses an invalid address and the read/write fails.

series must match the real PLC series

series accepts only A / QnA / Q_L / IQ_R, which determines the MC frame format. A wrong or unrecognized value makes the driver log Unknown series ... fallback to QnA and fall back to QnA — which may read incorrect data or error outright on other-series PLCs. Confirm the series against the actual model before onboarding.

When it won't connect / read-write fails, check the PLC-side MC service first

The driver opens a TCP connection directly to host:port; a failed connect throws Driver connection failed. Common causes: the PLC Ethernet module has no MC service enabled, the port differs from port (many models do not default to 6000), PLC-side IP filtering or an exhausted connection limit, or a firewall block. Confirm the port is reachable with telnet host port, then verify the MC service port and protocol (TCP) in the PLC project.

If strings or byte order read back wrong, check the type and length

A garbled or truncated string usually means length does not match the PLC-side string byte count (with 0 left in, the driver reads 64). An off numeric value usually means the point's pointTypeFlag does not match the word width actually stored in the device — configuring a 32-bit float device as SHORT only reads the low 16 bits.

One driver instance can serve multiple PLCs, with connections cached per device

A single Melsec driver process can serve multiple devices, each maintaining its own McPLC connection (cached by device ID in connectMap) and serializing read/write with its own ReentrantLock. Multiple PLCs are distinguished by their respective host. When a device is updated or deleted, the driver receives a metadata event and closes and evicts the corresponding connection.

How it lands in IoT DC3

Implementation status: available

Both the read and write paths of the Melsec driver are fully implemented — read() / write() call real MC reads/writes through iot-communication's McPLC (readInt16 / writeInt16 / readString, etc.), selecting the correct word width by data type. This is an available driver, and its behavior matches the application.yml declaration.

• dc3.driver.code: MelsecDriver — the driver's stable routing identifier inside the platform; device-to-driver binding and message dispatch address it by this code, so don't change it casually.
• Read capability: ✓ supported. Reads periodically on the acquisition cycle, covering all of BOOLEAN/BYTE/ SHORT/INT/LONG/FLOAT/DOUBLE/STRING, consistent with MELSEC's "read ✓" in the driver capability matrix.
• Write capability: ✓ supported. On a write command it writes the matching word width by the value's data type, consistent with the matrix's "write ✓".
• Subscribe capability: — not supported. MC is a master/slave polling model; this driver polls on the acquisition cycle and offers no PLC-pushed subscription, consistent with the matrix's "subscribe —".

Minimal onboarding example: onboard a QnA-series Mitsubishi PLC at IP 192.168.0.30:6000 and acquire a 16-bit integer from D100:

1. Create a Device using Mitsubishi Melsec Driver, with driver attributes host=192.168.0.30, port=6000, series=QnA.
2. Add a Point (pointTypeFlag=SHORT, READ_ONLY) to the Profile bound to the device, with point attributes address=D100, length=0.
3. Start the driver, and within 30 seconds the acquired value of D100 appears in PointValue.

For the full onboarding flow (modeling, binding, dispatch) see Device onboarding.

Further reading

• Driver overview — entry point to all drivers and their categories
• Driver capability matrix — a quick read/write/subscribe lookup per driver
• Device onboarding — a complete onboarding walkthrough
• Industrial buses and protocols — where the MC protocol sits in the network-layer protocol landscape
• FINS Driver — a TCP industrial protocol for Omron PLCs, addressed by "area + offset" in two fields