The short answer
Modicon M340
The Modicon M340 is a rack-based programmable automation controller in the middle of the Schneider Electric range: larger than a compact controller, smaller than the M580, and found in a great many water and wastewater plants built or upgraded in the last fifteen years. A CPU in the first slot of a rack drives X80 I/O modules in the same rack and in extension racks; CPU variants add an Ethernet port, a CANopen port, and always a Modbus serial port; and communication modules add further Ethernet ports for Modbus TCP and EtherNet/IP. The application lives in the CPU and can be backed up on the removable memory card, which is the first thing to understand on a site visit, because a card that holds an old application will restore that old application on a power cycle if the CPU memory is lost. It is programmed in Control Expert, formerly Unity Pro, in the five IEC 61131-3 languages, and it has no hot standby option; sites that need controller redundancy use the M580.
Key points
- Rack-based CPU with X80 I/O; extension racks through bus extender modules; the same I/O modules as the M580.
- Every CPU has a Modbus serial port; the Ethernet and CANopen ports depend on the CPU variant.
- The memory card can hold a copy of the application; know what is on it before power cycling a CPU.
- Programmed in Control Expert or its predecessor Unity Pro, in ladder, function block, structured text, instruction list, and sequential function chart.
- No hot standby; a plant that needs controller redundancy is an M580 application.
Layout
| Element | Notes |
|---|---|
| Rack | Backplane racks of several slot sizes; the CPU occupies slot 0 of the main rack; power supply at the left |
| CPU | Variants differ by memory, by the presence of an Ethernet port, and by a CANopen port; all carry a Modbus serial port and a USB programming port |
| I/O | X80 discrete, analog, counting, and special modules; screw terminal blocks or pre-wired connector systems |
| Extension | Bus extender modules link additional racks over a short bus cable; the number of racks and the total distance are limited |
| Communication modules | Ethernet modules for Modbus TCP and EtherNet/IP, with embedded web diagnostics; serial modules for extra Modbus ports |
| Memory card | A removable card that can store the application and, on some CPUs, files for the embedded web server |
Where it is found
Treatment plants of small to medium size, larger lift stations with several drives, and packaged systems from integrators who standardized on the Schneider range. In a water plant it typically runs the process areas from a main rack in the control room with remote racks or Modbus TCP I/O in the field, talking to drives over Modbus serial or Ethernet and to SCADA over Modbus TCP. Sites built as Modicon Premium or Quantum installations are sometimes replaced with M340 for the smaller cases and M580 for the larger.
Communications
- Modbus serial on every CPU: RS-485 or RS-232 depending on the cable, master or slave, for drives, meters, and radios.
- Modbus TCP on Ethernet CPUs and modules: I/O scanning of remote devices, messaging between controllers, and SCADA access.
- EtherNet/IP on the network modules that support it; used for drives and I/O from that ecosystem.
- CANopen on the CPUs with that port, for drives and distributed I/O on a CANopen bus.
- The embedded web pages on Ethernet CPUs and modules show port status, I/O scanner status, and diagnostics without the programming software.
Programming
The application is built in Control Expert, formerly Unity Pro, with the hardware configured in the catalog, variables declared with data types, and logic in sections written in any of the five languages. Derived function blocks let a site build its own pump, valve, and alarm blocks and reuse them. Online changes are built and downloaded as changes; a full download stops the controller. A project file from a site must match the software version that created it, or be an archive that a later version can open, and the upload information option determines whether an application can be recovered from the controller without the project file.
On a site visit
- 1
CPU indicators
Run, error, I/O fault, and the communication indicators on the front; the meaning is on the CPU label and in the manual.
- 2
Memory card
Present, seated, and matching the running application; a card error indicator on the CPU.
- 3
Rack faults
A module that is not the one configured, or a slot that is empty when it should not be, sets the I/O fault indicator; the diagnostics screen in the software or the web page names it.
- 4
Communications
The Ethernet module web page for the I/O scanner and the Modbus TCP messaging status; the serial port indicators for activity.
- 5
Battery
Older CPUs and some modules hold retentive data with a battery; the low battery indicator and the replacement date on the panel record.
- 6
Backup
Upload the application with the correct software version and store it with the site documentation before making any change.
Limits and lifecycle
- No hot standby or controller redundancy; single CPU per rack system.
- Rack and extension count are fixed; a plant that outgrows them moves to Ethernet I/O drops or to the M580.
- Security features are basic compared with the M580: application password, section protection, and network isolation are the tools.
- Firmware and software versions must be matched; keep the firmware record with the asset inventory.
- Spares: a CPU of the same variant, a power supply, and one of each I/O module type on the shelf, with the application on a labeled card.
Frequently asked questions
- Can I mix M340 and M580 I/O?
- The X80 I/O module family is shared, and modules move between the two in the same rack types, subject to the compatibility lists for the specific module and firmware. A migration from M340 to M580 often keeps the I/O and replaces the CPU and rack.
- How do I get the program out of a controller with no project file?
- If the application was downloaded with upload information enabled, Control Expert can upload it with variable names and comments. If not, the upload gives logic without the documentation. Check the memory card, which may hold the application, and ask the integrator for the project archive.
- The controller runs but SCADA cannot connect over Modbus TCP.
- The Ethernet port address and mask, the switch port, and the Modbus TCP access control if it is enabled on the module. The embedded web page on the Ethernet port shows connections and errors; if the page loads, the network path is fine and the problem is the Modbus configuration or a client limit.
- Is the M340 still current?
- It remains in production and supported as of this writing, positioned below the M580. Check the current lifecycle status with the manufacturer before specifying it for a new plant, and plan spares for any installed base regardless.
Related topics
- Modicon M580Field notes on the Schneider Electric Modicon M580, the Ethernet-backplane controller at the top of the range: remote I/O drops on a ring, hot standby CPUs, built-in security features, the networks it defines, Quantum migration, and what to check on site.
- Control ExpertField notes on Control Expert, the Schneider Electric engineering software for the Modicon M340 and M580: project structure, the five languages and derived function blocks, versions and files, online changes, upload information, and security settings.
- Modbus TCPModbus over Ethernet: what the MBAP header changes, why the CRC disappears, how the unit identifier works, and what does not get easier just because it is on Ethernet.
- PLC ArchitectureHow a programmable logic controller is physically organized: the chassis, backplane, processor, I/O modules, remote racks, and what those choices mean for wiring, expansion, and failure.
- PLC Program BackupsWhat a complete controller backup contains, how often to take one, where to keep it, why the upload from the running controller is not the master, and the verification that turns a folder of files into a recovery plan.
- Legacy SystemsKeeping older controllers running and planning a migration: the families still found in water plants, the risks that grow each year, software to keep alive, batteries and memory, tested spares, the security problem, and a migration the plant can survive.
Direct contact
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