🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Robotics / Mechatronics › Industrial Robots › Lesson 7
MFG-MECH15-06 · Lesson 7 · 14 clock hours · lab

Robot I/O Integration with PLCs and Sensors

Objective. Students wire and configure digital I/O connections between the FANUC robot controller and a PLC, map signals in both devices, and demonstrate a coordinated start/stop sequence controlled by the PLC.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed engineer or certified journey-level tradesperson or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Robot I/O Integration with PLCs and Sensors

Why I/O Integration Matters

A robot working alone is useful. A robot that communicates with a PLC and sensors is a manufacturing cell. In industrial practice, the PLC acts as the cell supervisor: it sequences conveyors, clamps, vision systems, and the robot itself through hard-wired digital signals. Understanding how those signals are wired, mapped, and timed is a core technician skill—and a tested area in both FANUC CERT and PMMI Mechatronics credentials.

Safety First: LOTO and Electrical Precautions

Before touching any terminal strip, apply lockout/tagout (LOTO) per OSHA 29 CFR 1910.147. De-energize the robot controller, the PLC panel, and any 24 VDC I/O supply. Attach your personal lock; verify absence of voltage at the terminal strip with a calibrated multimeter before touching conductors. Wear appropriate PPE (safety glasses, insulated gloves rated for the voltage level). Review NFPA 70E shock-boundary requirements: even 24 VDC systems can source enough current to damage components or cause burns if shorted. Do not release LOTO until wiring is complete, inspected, and terminal covers are reinstalled. During the live-test phase, keep hands clear of robot tooling and respect the safeguarded cell boundary per ANSI/RIA R15.06.

FANUC Controller I/O Architecture

The FANUC R-30iB controller provides dedicated I/O terminal strips—typically organized as digital inputs (DI) and digital outputs (DO) in banks of 8 or more points. Each terminal is numbered and assigned a voltage reference (0 V common or +24 VDC source, depending on configuration). The controller can be wired for sourcing (PNP) or sinking (NPN) I/O depending on the connected device. Check the cell wiring diagram before crimping any wire: connecting a sinking device to a sourcing circuit will damage the output transistor.

Key point: DI terminals on the robot receive signals from the PLC's output module; DO terminals on the robot drive inputs on the PLC's input module. Current must flow in the correct direction or the optocoupler inside the I/O circuit will not conduct.

Signal Mapping on the Teach Pendant

After wiring, power the cell and navigate the teach pendant to MENU → I/O → Digital. This screen lists every DI and DO point with its current hardware state (ON/OFF), its physical rack/slot/bit address, and an editable label field. Assign meaningful labels immediately—this is not optional in a production cell. Suggested convention for this lab:

To verify each signal without running a program, use the PLC programming software's Force Output function (or the PLC's online monitor) to turn a PLC output ON while watching the corresponding DI on the pendant toggle from OFF to ON. Confirm with your multimeter: measure the voltage at the robot DI terminal relative to the I/O common; a valid logic-high should read within the controller's specified input voltage range (often 18–26 VDC for a 24 VDC system). DO NOT use the pendant's forced-output function on robot DOs unless the robot is in a safe, caged state—the signal could command an external device to move.

The Handshake Sequence

A handshake is an agreed-upon sequence of signals that lets two controllers coordinate actions without collision or lost cycles. The sequence in this lab works as follows:

1. PLC verifies PART_PRESENT (DI[3] = ON) and ROBOT_BUSY (DO[1]) = OFF.

2. PLC pulses PLC_START (DI[1]) ON for 200 ms then OFF.

3. Robot program detects DI[1] rising edge, sets DO[1] (ROBOT_BUSY) ON, begins pick-and-place motion.

4. At end of motion, robot clears DO[1] OFF and pulses DO[2] (CYCLE_COMPLETE) ON for 200 ms.

5. PLC detects CYCLE_COMPLETE, increments its part counter, and waits for the next PART_PRESENT before issuing another start.

This interlocking prevents the PLC from commanding a new cycle while the robot is still moving—a critical safety function. Document each signal's state and the approximate time between edges in your lab log; this becomes your timing diagram and is the primary deliverable for instructor sign-off.

Common Wiring Faults and Troubleshooting

| Symptom | Likely Cause | Check |

|---|---|---|

| DI always reads ON | Wired to common instead of signal | Verify terminal numbers against diagram |

| DI never responds | Wrong polarity / NPN vs. PNP mismatch | Measure voltage at terminal; swap common wire |

| DO turns ON but PLC input stays OFF | Load current too low for PLC input threshold | Check PLC input minimum current spec |

| Controller fault on power-up | Short on I/O supply | Remove all field wiring, power up, add one wire at a time |

Always re-apply LOTO before correcting a wiring fault. Never probe a live terminal strip with a standard screwdriver—use an insulated, appropriately rated probe.

Connecting to Credentials

FANUC CERT technician exams (FCR-T1, FCR-T2) include I/O configuration using the pendant I/O menu and wiring knowledge. PMMI's Motor and Motor Controls and Programmable Logic Controllers exam modules both address PLC I/O addressing and signal verification. Building confidence with these procedures now directly prepares you for both credential assessments.

Key terms10

Digital Input (DI)A signal terminal on the FANUC controller that receives an ON/OFF voltage from an external device such as a PLC output or sensor; the robot program reads this state to make decisions.
Digital Output (DO)A signal terminal on the FANUC controller that the robot program drives ON or OFF to command an external device, such as a PLC input, relay, or indicator.
Sourcing (PNP) I/OAn I/O configuration in which the output device supplies positive voltage (+24 VDC) to the input device's signal terminal; current flows from the output to the input and back through the common.
Sinking (NPN) I/OAn I/O configuration in which the input device provides the path to common (0 V); the output pulls the signal line low. Mixing sourcing and sinking without proper wiring will damage I/O circuits.
HandshakeA coordinated exchange of signals between two controllers (e.g., PLC and robot) that ensures each device acts only when the other is in a known, safe state; prevents simultaneous conflicting commands.
LOTO (Lockout/Tagout)The energy-control procedure required by OSHA 29 CFR 1910.147: isolate, lock, and verify absence of all hazardous energy before working on equipment.
Signal MappingThe process of assigning a physical hardware address (rack/slot/bit) and a descriptive label to each I/O point in the robot controller or PLC so programs use readable names instead of raw numbers.
Force OutputA diagnostic feature in PLC programming software that temporarily drives a PLC output ON or OFF regardless of program logic, used to verify field wiring without running a production program.
ANSI/RIA R15.06The U.S. industrial robot safety standard (aligned with ISO 10218) that defines safeguarding requirements, safe-speed limits, and collaborative-operation rules for robot cells.
OptocouplerAn electrical isolation component inside I/O modules that transfers a signal using light, preventing high-voltage transients on field wiring from damaging the controller's logic circuits.

Worked examples4

1. The PLC output module supplies +24 VDC sourcing signals. The robot DI terminal datasheet states the input turns ON at a minimum of 18 VDC and draws 7 mA at 24 VDC. You measure 21.8 VDC at the DI terminal with the PLC output forced ON. Does the input meet the turn-on threshold, and what is the power dissipated in the input circuit at the measured voltage if the current is proportionally 6.36 mA?
  1. Step 1 – Check threshold: The minimum ON voltage is 18 VDC. The measured voltage is 21.8 VDC. 21.8 > 18, so the threshold is met and the input will read ON.
  2. Step 2 – Calculate power using P = V × I. Convert mA to A: 6.36 mA = 0.00636 A.
  3. Step 3 – P = 21.8 V × 0.00636 A = 0.1386 W, rounded to 0.139 W.
Answer: The 21.8 VDC signal exceeds the 18 VDC minimum threshold, so the DI will read ON. Power dissipated in the input circuit is approximately 0.139 W, well within typical I/O module ratings.
2. Your lab wiring diagram shows a PLC output wired through 22 AWG wire (resistance approximately 0.053 Ω per foot) to a robot DI terminal 6 feet away (12 feet of wire total for the round trip). The PLC output sources 24 VDC and the wire carries 7 mA. What is the voltage drop across the wire, and what voltage actually appears at the DI terminal?
  1. Step 1 – Calculate total wire resistance: 12 ft × 0.053 Ω/ft = 0.636 Ω.
  2. Step 2 – Apply Ohm's law: V_drop = I × R = 0.007 A × 0.636 Ω = 0.00445 V (approximately 4.5 mV).
  3. Step 3 – Subtract drop from source: V_terminal = 24 V − 0.00445 V ≈ 23.996 V.
Answer: The voltage drop is approximately 4.5 mV, negligible at this current level. The DI terminal sees essentially 24 VDC. This confirms that 22 AWG wire is more than adequate for low-current I/O runs of this length.
3. During the handshake test you record the following events in your lab log: PLC_START rising edge at T = 0.000 s; ROBOT_BUSY rising edge at T = 0.045 s; ROBOT_BUSY falling edge at T = 3.210 s; CYCLE_COMPLETE rising edge at T = 3.215 s; CYCLE_COMPLETE falling edge at T = 3.415 s. What is the robot program's response latency from the start pulse to the ROBOT_BUSY signal, and how long did the CYCLE_COMPLETE pulse last?
  1. Step 1 – Response latency = time of ROBOT_BUSY rising edge − time of PLC_START rising edge = 0.045 s − 0.000 s = 0.045 s (45 ms).
  2. Step 2 – CYCLE_COMPLETE pulse duration = falling edge − rising edge = 3.415 s − 3.215 s = 0.200 s (200 ms).
Answer: The robot program responded to the start signal in 45 ms (scan-cycle and program-scheduling latency). The CYCLE_COMPLETE pulse lasted exactly 200 ms, matching the specified 200 ms pulse in the pre-loaded PLC program.
4. A robot DO is rated to source a maximum of 100 mA at 24 VDC. The PLC input module you are connecting to requires a minimum of 5 mA to register a logic-high. You are adding a small LED indicator in series with the PLC input to show when the signal is active. The LED forward voltage is 2.1 V and you want 10 mA through the circuit. What series resistor value should you use, and does the total current stay within the DO rating?
  1. Step 1 – Available voltage across resistor: V_R = V_source − V_LED − V_input_drop. PLC input modules typically have a small voltage drop; assume negligible for this calculation, so V_R = 24 V − 2.1 V = 21.9 V.
  2. Step 2 – Apply Ohm's law: R = V_R / I = 21.9 V / 0.010 A = 2,190 Ω. Choose the nearest standard value: 2,200 Ω (2.2 kΩ).
  3. Step 3 – Verify current with 2.2 kΩ: I = 21.9 V / 2,200 Ω = 0.00995 A ≈ 9.95 mA. This exceeds the 5 mA minimum PLC input threshold.
  4. Step 4 – Check DO rating: 9.95 mA is well below the 100 mA maximum DO current. The circuit is safe.
Answer: Use a 2.2 kΩ resistor in series. The circuit draws approximately 9.95 mA, which satisfies the PLC input's 5 mA minimum and stays far below the robot DO's 100 mA maximum rating.

The math in this lesson2

Learners compute handshake intervals in seconds from timestamped I/O events.
N-Q.1A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: I/O wiring exercise: following the cell wiring diagram, students connect three PLC output signals to robot DI terminals and two robot DO signals to PLC input terminals, verifying polarity and source/sink configuration before applying power (LOTO applied during wiring, released for testing). · Signal mapping and configuration on the pendant: students navigate to the I/O menu, assign descriptive labels to each mapped digital input and output (e.g., DI[1]=PLC_START, DO[1]=ROBOT_BUSY), and verify each signal by toggling from the PLC output-force screen while observing the pendant I/O status. · Coordinated sequence test: PLC program (pre-loaded by instructor) sends a start pulse; students confirm the robot program receives the signal, executes one pick-and-place cycle, asserts the ROBOT_BUSY output during motion, clears it at cycle end, and sends a CYCLE_COMPLETE signal to the PLC; students document the handshake timing in their lab log.
Materials: FANUC robot cell, FANUC R-30iB controller I/O terminal strip, PLC (Allen-Bradley or Siemens training unit), wiring diagram, DIN rail, terminal screwdrivers, multimeter, teach pendant, PLC programming software on laptop, lab log
Check: All three wired signals toggle correctly (multimeter and pendant I/O screen verified). Coordinated sequence runs for three consecutive cycles without fault. Lab log documents signal labels, verified states, and handshake timing. Pass = wiring sign-off by instructor plus three clean cycles.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Safety and Regulatory Compliance
1. Before connecting wires to the FANUC R-30iB I/O terminal strip, what must a technician do first according to OSHA 29 CFR 1910.147?
FANUC Robot Configuration and Programming
2. On the FANUC teach pendant I/O menu, a technician assigns the label PLC_START to DI[1]. What is the primary purpose of this labeling step?
Industrial Wiring and I/O Hardware
3. A PLC output module is wired as sourcing (PNP). Which wiring arrangement is correct for the connected robot DI terminal?
Troubleshooting and Diagnostics
4. During live testing, a technician uses the PLC software's Force Output function to toggle a PLC output ON. The pendant DI screen does not change state. After re-checking LOTO is released for testing, what is the FIRST diagnostic step?
Robot Cell Integration and Safety
5. In the lab's coordinated handshake sequence, ROBOT_BUSY (DO[1]) turns ON when motion begins and OFF when motion ends. Why is this signal necessary from a safety and process perspective?
Programmable Logic Controllers
6. Which IEC standard defines the programming languages—including ladder diagram and function block diagram—used in the PLC that coordinates the robot cell in this lab?
Measurement and Verification
7. A technician measures 21.8 VDC at a robot DI terminal when the PLC output is forced ON, and 0.1 VDC when it is forced OFF. The robot DI threshold is ON ≥ 18 VDC, OFF ≤ 5 VDC. What is the correct interpretation?
FANUC Robot Programming and Troubleshooting
8. A student's lab log shows that CYCLE_COMPLETE (DO[2]) remained ON continuously for 45 seconds instead of the specified 200 ms pulse. Which robot program element most likely needs to be corrected?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →