MC protocol (Mitsubishi’s own name is the MELSEC Communication protocol) reads a PLC’s memory by address, D100, M50, R200, over a wire, and it works essentially the same whether that wire is Ethernet or serial. This guide covers the serial path, RS-232 or RS-422/485, which is the one that matters on an older Q or A series station that never got an Ethernet module fitted, or on a panel where the only spare port left is a serial one. If your controller has Ethernet available, the Ethernet version of this guide is almost always the simpler setup, this one exists for the machines that don’t have that option.
Before you start You’ll need the PLC’s serial port wired to the gateway, a device address list (which D, M, R, or W addresses carry the values you want), the message format and frame type from the PLC’s own project, and its station, network, and PC numbers if it sits behind any routing at all. Spall reads. It never writes to the controller.
1. When serial is the only way in
A controller earns this guide over the Ethernet one for one of two reasons. Some older Q series and A series stations were built or configured before an Ethernet module was standard and never had one added. The only communication port on the CPU is a built-in RS-232 or RS-422 connector. Others have an Ethernet module physically present but already committed to something else: a HMI, an engineering laptop’s dedicated link, a SCADA system with its own connection budget. Adding a second Ethernet client isn’t an option without touching that existing setup. In both cases, the serial port is the one path available without changing anything already running.
2. Wire the serial port
RS-232 is point-to-point, one device, wired GND/TXD/RXD straight to the gateway’s serial port. RS-422/485 supports a longer run and, on some Mitsubishi links, multiple stations sharing a network number, wired A/B the same as a Modbus RTU bus. The gateway’s panel breaks out RS-232 and RS-485 terminals. Wire to whichever matches the PLC’s own port and cable, and check the CPU’s manual for its exact pinout. Mitsubishi’s serial connectors aren’t always wired the way a generic RS-232 cable assumes.
Power the gateway off while landing these terminals, and don’t assume a cable sold as “PLC programming cable” for a different Mitsubishi family will pin out the same way, some product lines route the same signals to different physical pins between one CPU generation and the next.
3. Get the message format and frame type from the project
MC protocol’s serial framing comes in five message formats, differing in how a request is wrapped, plain ENQ-prefixed, ENQ with a block number, STX/ETX-bounded, ENQ with a trailing CRLF, or a binary format that always uses 4C framing regardless of the other setting, and four frame types (1C, 2C, 3C, 4C) describing the station addressing inside the frame. Format and frame type aren’t something Spall guesses. They’re a real setting on the PLC’s serial communication configuration in GX Works, and they have to match exactly or every request goes unanswered. Note the PLC series too (Q, L, QnA, iQ-L, iQ-R, or an older A, AnA, AnU station), since some formats and defaults differ by family.
4. Station, network, and PC numbers
On a single PLC wired point-to-point, the defaults (station 0, network 0, PC number 255) usually work unchanged. They start to matter once the PLC sits behind any routing, a multi-drop RS-485 link with more than one station, or a CPU reached indirectly through a network module. StationNum identifies which device on the link you’re addressing, the same role a unit ID plays on Modbus. NetworkNum and PcNum matter only when the request has to cross a network module to reach its target. Most direct serial links leave both at their defaults. If your setup involves more than a single point-to-point cable, get these numbers from whoever configured the network module. Don’t guess at them.
5. Confirm the settings, then add the source in Spall
Serial has no port-answering test the way a networked protocol does. Wiring and settings are what to double-check instead. Confirm baud rate, data bits, stop bits, and parity all match the PLC’s own serial configuration. Get any one wrong and nothing answers.
In Spall, add the machine, choose Mitsubishi MC (Serial) as the source, and set the serial device, baud rate, and the format, frame, PLC series, and station/network/PC numbers from steps 2 through 4. Assign the Spall gateway and save.
6. Add a tag per device address
For every value you want, add a tag with its device address, D100, M50, R200, and the same set of device codes (D, W, M, X, Y, B, L, F, V, SM, SD) the Ethernet version of MC protocol reads. Set the data type: 16 or 32-bit integer, float, or bool for a flag packed into an M or X device, and a string length for text. This depends entirely on the address list from your PLC’s own program or I/O documentation. There’s no shortcut around knowing what the ladder program put where.
What Spall does with this data
Every MC serial tag lands in the Signal inbox, unclassified until someone confirms what it means, exactly the same as the Ethernet version of this protocol or any other source. Once classified as a run signal, part counter, or sensor reading, it feeds availability, downtime, production, and OEE the same way a native FOCAS or MTConnect feed does.
One thing worth repeating Spall issues MC protocol read requests only, over the wired serial link. It never writes a device or a coil back to the controller. Worst case if the link goes down is a gap in the chart. Nothing about the PLC’s own ladder logic is touched either way.
Quick recap
- Serial is the right path when there’s no Ethernet module fitted, or the one that exists is already committed elsewhere
- RS-232 is point-to-point, RS-422/485 supports a longer run and, on some links, multiple stations
- Get the exact message format (1 through 5) and frame type (1C-4C) from the PLC’s own serial settings in GX Works, never guessed
- Station, network, and PC numbers matter once the link isn’t a simple single-station point-to-point cable
- Baud rate, data bits, stop bits, and parity have to match exactly. Serial has no port test the way a networked protocol does
- Add the source with the serial settings, then a tag per device address, same address list work as the Ethernet path
If the PLC’s serial settings aren’t documented anywhere obvious, bring the project file or a screenshot of GX Works’ communication configuration to a pilot call. We’ll read it with you before anyone touches a cable.