Getting started with the PLC-200

Objectives

1. Establish communication between STEP 7-Micro/WIN and the S7-200 CPU 224.
2. Write, download, run and monitor a first ladder program.
3. Program a normally closed stop button, and verify that a broken wire stops the machine exactly as pressing STOP does.

Background

A contactor started by a push button must stay closed after the button is released. In a hard-wired starter the contactor's own auxiliary (seal-in) contact does this. In a PLC it is one network: the output coil feeds back through its own contact, in parallel with START. Two details decide whether that network is safe: the STOP device is normally closed, and a fault must remove the output, never force it on.

Bench set-up

No patch lead. The CPU's output status LEDs show the result.

Preparation

1. The CPU 224 has 14 digital inputs and 10 digital outputs, numbered from I0.0 and Q0.0 in bytes of eight bits. Give the address of its last input and of its last output.
2. Predict the column “Q0.0 expected” of the test log below, steps 1 to 7, and write it as one binary word of 7 digits, step 1 first. In step 7 the “wrong” program has a normally-closed contact on a STOP device wired normally open.
3. A scan reads the inputs, executes the program, then writes the outputs. With a scan time of 5 ms, and ignoring the input filter, what is the longest time between STOP_NC opening and Q0.0 switching off?

Procedure

1. Following manual Exercise 1-2, connect the USB/PPI cable, set the PG/PC interface and read the CPU type. Record the CPU type and firmware version.

2. Create a project and enter the symbol table above, then write the self-holding network: START sets RUN, RUN holds itself through its own contact, and STOP_NC opening releases it.
3. Download with the CPU in STOP, switch to RUN and set I0.1 ON (healthy). Operate START and STOP and fill in the test log while you watch the program status (manual Exercise 2-3).
4. Build a status chart with I0.0, I0.1 and Q0.0, and use it to force I0.0 ON. What does the program do? Why must forcing be removed before a machine is handed over?

5. Fault injection. With the motor running, set I0.1 OFF: this is what a broken wire on a normally closed STOP looks like to the PLC. Record the result. Then rewrite the network the “wrong” way, with a normally-closed contact on a normally-open STOP input, repeat the test and record it again.

Test log

Step	Action	Q0.0 expected	Q0.0 observed
1	I0.1 ON, press START
2	release START
3	press START again
4	set I0.1 OFF (STOP pressed)
5	I0.1 ON again, without pressing START
6	broken STOP wire (I0.1 OFF) while running: correct program
7	broken STOP wire while running: “wrong” program

Questions

1. In step 5, why must the motor not restart by itself when STOP is released? Which rule of good practice is this?
2. Compare steps 6 and 7. Explain in one paragraph why industrial stop and safety devices are wired normally closed.
3. The CPU is switched from RUN to STOP and back to RUN while the motor runs. What happens to Q0.0, and is that acceptable for a machine?