CNC Side vs. MT Side in 2026: PLC as the Boundary Between Controller and Machine

In any modern CNC machine the dividing line between the controller and the tool is the PLC. This 2026 engineering note refreshes our 2024 article on the CNC side and the MT side — what the boundary looks like on a FANUC 30i-B, a Siemens SINUMERIK ONE, and a Mitsubishi M80 — and how OPC UA over MQTT and MTConnect have reshaped it since 2024.

AI Summary

• The CNC side is the numerical-control loop (interpolator, servo drives, spindle, tool table) — anything that interprets G-code and produces motion.
• The MT side is the machine-tool periphery: hydraulics, coolant, chip conveyor, ATC, guarding, work-changer — the discrete I/O that the cell must coordinate.
• The PLC still sits at the boundary, but on FANUC PMC, Siemens SINUMERIK PLC, and Mitsubishi M80 it now also runs as a virtual PLC inside the controller and shares OPC UA / MTConnect with the line SCADA.
• Since 2024: OPC UA over MQTT, TSN on EtherCAT/PROFINET, and Sinumerik ONE virtual CNC have rewritten the boundary from “hard” to “soft” but still deterministic.

In 2026 the boundary is the same engineering idea, but the wire has gone digital — the PLC now brokers data both to the field and to the cloud.

1. The CNC Side

The CNC side handles closed-loop motion: parsing the part program, interpolating axes, commanding servo drives, sequencing spindle gear changes, and managing the tool-offset table. On a FANUC Series 30i-B this lives in the CNC CPU and CNC screen software; on a Siemens SINUMERIK ONE it is the NCK plus the Sinumerik Operate HMI; on a Mitsubishi M80/M800 it is the NC controller block.

What is worth keeping in mind: the CNC side is the only loop on the machine with a guaranteed cycle time in the sub-millisecond range. Servo tuning, look-ahead, and jerk limiting all happen here. The PLC never owns the trajectory.

Vendor / Platform	CNC-side module	What it owns	Native boundary to PLC
FANUC Series 30i-B / 31i-B5	CNC + Servo (A06B / A02B drives)	Interpolation, tool table, spindle, 5-axis	PMC (built-in ladder over I/O Link / I/O Link i)
Siemens SINUMERIK ONE / 840D sl	NCU 1750 / NCK + Sinamics S120	Path control, kinematics, Adaptive Control	SINUMERIK PLC (S7-1500-based virtual PLC)
Mitsubishi M80 / M800	MDS / MDS2 drives	G-code, SSS (smooth tolerance), OMR-CC link	Built-in PLC (MELSEC iQ-R style, CC-Link IE Field)
Heidenhain TNC 640 / TNC7	CC, UEC / NCK	Conversational + DIN/ISO, 5-axis kinematics	PLCps (Python or PLC programming interface)

2. The MT Side

The MT (Machine Tool) side is everything that physically shapes the workpiece but does not run a path: tool changer, pallet changer, spindle speed selector, coolant through-spindle, chip conveyor, work-zone guards, hydraulic counter- balance, lubrication skids, and operator-panel pushbuttons. These are pure discrete (and a few analog) I/O — no interpolation, no look-ahead.

The MT side owns the “around the cut” choreography. Cutting only happens when every MT-side permissive is closed: guard interlocked, coolant pressure at minimum, lubrication OK, ATC seated, spindle oriented, axes referenced.

MT-side function	Typical I/O type	Why it is MT, not CNC
Automatic Tool Changer (ATC)	24 V DC DI / DO + prox sensors	Sequential discrete logic, no path interpolation
Pallet / Work-changer	Proximity + hydraulic solenoid DO	Driven by hydraulics and limit switches
Coolant / Through-Spindle Coolant	Analog pressure + flow sensor	Permissive into the cutting cycle
Chip Conveyor / Auger	Motor contactor + overload DO	Auxiliary, asynchronous to the part program
Guard Interlocks	Safety DI to safety relay / SI module	Hardwired or PROFIsafe, scannable independently

3. The PLC as the Boundary

The PLC’s job at the boundary is the same as it was in 2024: read sensor states and operator inputs from the MT side, sequence the discrete logic, and gate the CNC side through a small, fixed interface of control signals and status words. What changed is how big — and how software-defined — the PLC has become.

• FANUC PMC is a built-in ladder engine that shares the same backplane as the CNC. Modern PMC runs the same program whether I/O Link, I/O Link i, or FANUC A06B servo amplifiers are connected.
• Siemens SINUMERIK ONE runs a virtual PLC (S7-1500F-compatible) inside the NCU, with the option to out-source the same image to a real SIMATIC S7-1500 on PROFIsafe / PROFINET.
• Mitsubishi M80 / iQ-R PMC is essentially a MELSEC iQ-R CPU; the boundary interface is tag-based over CC-Link IE Field.

4. What Changed Since 2024

The boundary itself — PLC between CNC and MT — has not moved. But the way data crosses that boundary has been rewritten since 2024:

Aspect	2024 baseline	2026 reality
CNC platform	FANUC 30i-B, SINUMERIK 840D sl, Mitsubishi M80	SINUMERIK ONE (virtual NCK), FANUC 31i-MB5 with AI Thermal, Mitsubishi M800V with Smart Tool Management
PLC form factor	PMC on the CNC backplane, or external S7 / MELSEC	Virtual PLC on the NCU (S7-1500F-style); PMC on FANUC 30i/31i; some CNC vendors expose CODESYS runtime
Line-level data	MTConnect / OPC UA on the production cell	OPC UA over MQTT + Sparkplug B brokers; MTConnect 2.0 with PROFINET TSN
Cybersecurity	Password + VLAN on the cell network	IEC 62443-3-3 zone & conduit; signed firmware on CNC; OPC UA certificate per node
Edge AI	OEM-piloted anomaly detection	Production deployments for tool wear, chatter suppression, spindle-bearing FFT

5. Cross-Reference: Common CNC + PLC Stacks in 2026

Related on KOEED Blog

• Browse the full Engineering Notes series
• Fanuc CNC parts & servo amplifiers (A06B / A02B)
• Siemens SINUMERIK & SIMATIC S7-1500 stock
• Mitsubishi M80 / M800 / iQ-R modules
• PLC Error Code Database — Siemens / Mitsubishi / Omron
• AI Diagnostic Tool — photograph a nameplate to cross-reference FANUC or SINUMERIK P/Ns
• Create a Quote — BOM-based multi-line RFQ form

Frequently Asked Questions

Where exactly is the boundary between the CNC side and the MT side?

It is the PLC’s tag table. Inside the NC (interpolator, servos, tool table, spindle) is the CNC side. Anything bound to a discrete or analog I/O point through a PMC, SIMATIC S7, or MELSEC iQ-R module is the MT side.

Can the PLC disable a CNC axis by itself?

No. The PLC can request a feed-hold or servo-off handshake, but feed-hold and overtravel are enforced inside the NC. Motion permission must come through the NC’s normal status-word interface.

What is OPC UA over MQTT doing in a CNC cell?

OPC UA over MQTT (with Sparkplug B) is the standard way to push machine state — cycle counts, alarms, spindle load, tool offsets — from the PMC to the line SCADA and the cloud. Broker-based, survives WAN drops gracefully.

Is FANUC PMC the same as a Siemens SIMATIC S7-1500?

Both serve as the boundary between CNC and MT, but the execution models differ. FANUC PMC is a built-in ladder engine on the CNC backplane. Siemens runs a virtual S7-1500 PLC inside the SINUMERIK ONE NCU.

What is the most common CNC-side fault encountered in 2026?

Following error on a single axis, often triggered by a tool bind or an MT-side permissive dropping mid-cut. Read the NC alarm history first, then confirm the discrete sensor wiring before assuming a PMC logic fault.

Where can I source obsolete FANUC servo drives or SINUMERIK modules?

KOEED keeps active and refurbished stock of FANUC A06B servo amplifiers, SINUMERIK 6SN1xxx power modules, and Mitsubishi MDS / MDS2 spindle drives. Email your BOM to Moritta@KOEED.COM for a quote within 24 hours.