Lab 04: Two-Location Control

Application Scenario

A warehouse runs a 200-foot conveyor from the loading dock to the shipping area. One operator works at each end, and the two cannot see each other. Each end has its own push-button station:

• Station A (loading dock): PB1 START, PB2 STOP
• Station B (shipping area): PB3 START, PB4 STOP

Either operator must be able to start the conveyor, and either operator must be able to stop it at any time. Multi-station control is required on almost every long conveyor, escalator, and production line.

The stack light from Lab 03 stays on this machine: G1 = RUNNING, R1 = STOPPED.

Learning Outcomes

After this lab you will be able to:

• Translate a written control requirement into AND/OR logic.
• Wire multiple START buttons in parallel and multiple STOP buttons in series.
• Place the seal-in contact correctly in a multi-station circuit.
• Convert a ladder diagram into a wiring table using the node method.
• Daisy-chain wires so that no terminal holds more than two wires.

Required Components

Functional Requirements

Your circuit must meet all of these requirements:

1. Pressing either PB1 or PB3 starts the conveyor, and it keeps running after the button is released.
2. Pressing either PB2 or PB4 stops the conveyor.
3. A STOP button must stop the conveyor even while a START button is being held at the other station.
4. G1 is ON only while the conveyor runs. R1 is ON only while it is stopped.
5. After a power outage, the conveyor does not restart by itself.

Write the requirement as one sentence before you draw: The conveyor runs if (START A or START B or it is already running) and STOP A is not pressed and STOP B is not pressed. In a ladder diagram, OR is built with devices in parallel, and AND is built with devices in series.

Ladder Diagram — Step-by-Step Drawing Guide

Draw on the Ladder Diagram Worksheet. L1 (+24 V) is the left rail and L2 (GND) is the right rail. Use pencil, because you will trace paths on the drawing later. This design uses 5 grid lines.

Step 1 — Rung 1 (grid line 1): the main control line

1. Starting from L1, draw PB2 (N.C.), the Station A STOP button. Label its pins 1 and 2.
2. To the right of PB2, draw PB4 (N.C.), the Station B STOP button. Label its pins 1 and 2.
3. To the right of PB4, draw PB1 (N.O.), the Station A START button. Label its pins 1 and 2.
4. At the right end, draw the CR1 coil. Label pin 2 on the left and pin 7 on the right. Connect pin 7 to L2.
5. Mark the node numbers with a dot and a label:
   • between PB2 and PB4: N2
   • between PB4 and PB1: N3
   • between PB1 and the CR1 coil: N4

Step 2 — Rung 2 (grid line 2): add the Station B START button

1. On grid line 2, directly under PB1, draw PB3 (N.O.), the Station B START button. Label its pins 1 and 2.
2. Draw a vertical line from the left end of PB3 up to node N3.
3. Draw a vertical line from the right end of PB3 up to node N4.

Step 3 — Rung 3 (grid line 3): add the seal-in contact

1. On grid line 3, directly under PB3, draw a CR1A N.O. contact. Label pin 1 on the left and pin 3 on the right.
2. Connect pin 1 up to node N3, and pin 3 up to node N4.

Step 4 — Rung 4 (grid line 4): RUNNING lamp

1. Starting from L1, draw a CR1B N.O. contact. Label pins 8 and 6.
2. Draw G1 at the right end and connect it to L2. Label pins 1 and 2.
3. Mark the node between the contact and G1 as N5.

Step 5 — Rung 5 (grid line 5): STOPPED lamp

1. Starting from L1, draw a CR1B N.C. contact (with a slash through it). Label pins 8 and 5.
2. Draw R1 at the right end and connect it to L2. Label pins 1 and 2.
3. Mark the node between the contact and R1 as N6.

Step 6 — Trace your drawing with a pencil

Answer each question by tracing the path on your drawing.

1. PB1 is pressed. Can you trace a path from L1 to the CR1 coil?
2. PB3 is pressed. Can you trace a path from L1 to the CR1 coil?
3. The conveyor is running and no button is pressed. Which path keeps the coil energized?
4. The conveyor is running and PB4 is pressed. Is there any path left from L1 to the coil? (There must not be.)
5. Repeat question 4 for PB2.

If any answer is wrong, fix the drawing before you continue.

Step 7 — Fill in the contact allocation table

From Ladder Diagram to Wiring Table

A ladder diagram shows how the circuit works. A wiring table shows which terminals to connect. Use the node method to convert one into the other.

Step 1 — List every terminal on each node

Look at each node dot on your drawing and write down every terminal that touches it. The N2 row is done for you.

Step 2 — Chain the terminals of each node together

A node with k terminals needs k − 1 wires. Connect them in a chain (daisy-chain): first to second, second to third, and so on. This keeps no more than two wires under any terminal screw. Most industrial terminal blocks are rated for two wires at most.

For example, if a node has terminals A, B, C, and D, the wires are A→B, B→C, and C→D. Do not run A→B, A→C, and A→D, because that puts three wires on terminal A.

Step 3 — Fill in the wiring table on the worksheet

Write one row per wire: Rung#, Wire#, From, To, and Purpose. In the Purpose column, include the node number in parentheses, for example: STOP A to STOP B (N2).

Step 4 — Get instructor approval

Show your ladder diagram, contact allocation table, node table, and wiring table to the instructor. Do not start wiring until the worksheet is stamped.

Key Wiring Points

These are the most important ideas in this lab. Many later labs, and many real machines, depend on them.

1. "Stops in series, starts in parallel." Every device that must be able to stop the machine goes in series on the main line. Every device that may start the machine goes in parallel with the other START buttons.
2. The STOP buttons come first. Place all STOP buttons between L1 and the START section, so that every path to the coil passes through every STOP button.
3. The seal-in contact belongs with the START buttons. It connects from the node after the last STOP button to the coil. If it connects any earlier, the STOP buttons after that point are bypassed once the machine is running.
4. Chain the wires on shared nodes. Keep no more than two wires under any terminal screw.
5. A remote station needs only three wires to the panel: one into its STOP button, one from its STOP button back to the panel, and one from its START button back to the panel.
6. Test every STOP button while the machine is running. A STOP button that blocks START but cannot stop a running machine is the most dangerous wiring mistake in this lab.

Wiring Procedure

1. Power OFF the trainer (or unplug the power cord).
2. Wire the circuit from your approved wiring table, one node at a time. Mark each wire as you install it.
3. Meter check, power still OFF:

   Reading the meter: once the circuit is wired, the meter can find a path through coils and lamps. A reading of tens to thousands of ohms is normal and counts as high. Only a reading near 0 Ω means a direct connection.

   • L1 to CR1:1 (seal-in common) should read near 0 Ω and become high while either PB2 or PB4 is pressed.
   • CR1:1 to CR1:2 should read high, and near 0 Ω while either PB1 or PB3 is pressed.
   • CR1:8 to CR1:5 should read near 0 Ω; CR1:8 to CR1:6 should read high.
4. Ask your lab partner to do a buddy check, then show the instructor.
5. Power ON and complete the test table below.

Test & Acceptance

Troubleshooting

Review Questions

1. Why are the START buttons connected in parallel, while the STOP buttons are connected in series?
2. Suppose PB4 were wired in parallel with PB2. What happens when the Station B operator presses only PB4? Why is this dangerous?
3. The Station B operator holds PB3 (START) while the Station A operator presses PB2 (STOP). The conveyor stops. What happens when PB2 is released while PB3 is still held? What does this mean for safe operation?
4. Station B is 200 feet from the control panel. How many wires must be run between Station B and the panel? Name the node of each wire.
5. A student connects the seal-in contact's common to the node between PB2 and PB4. Which STOP button will fail to stop the conveyor after it starts? Why?

Applying This Circuit Pattern

The circuit in this lab is not just for push buttons. It is a general pattern for any machine that must start on a request and stop on any one of several conditions:

• Series (stop) section: any N.C. contact that must be able to stop the machine.
• Parallel (start) section: any N.O. contact that may start the machine.
• Seal-in contact: in parallel with the start section.

Real devices that go in the series (stop) section include:

Real devices that go in the parallel (start) section include a start button in a remote control room, a PLC output, or a second operator panel.

Design question for the future: some conditions are needed only to start a machine, not to keep it running (for example, "lubrication pressure must be OK before starting"). Where would you put such a contact: on the main line, or in series with the START buttons only, before the seal-in branch joins? What difference would it make? You will use this idea in later labs.

Extension

A third station (Station C) is added at the midpoint of the conveyor. Without wiring it, sketch where its START and STOP buttons go on your ladder diagram. How many additional wires must be run to Station C?