O UTPUT C OIL

O UTPUT C OIL

An output coil instruction controls either a real output (connected to the PLC via output interfaces) or an internal output (control relay). This instruction uses an output coil address bit in the internal storage area as its reference address. The —( )— symbol may also represent an output coil instruction.

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Languages 9

During the execution of an output coil instruction, the processor evaluates all the input conditions in the ladder rung. If no continuity exists, the processor places a 0 in the output coil address bit, indicating an OFF condition to the output coil instruction (see Figure 9-19a). However, if the processor detects continuity in any path, the processor places a logic 1 in the output coil address bit referenced by the instruction (see Figure 9-19b). This logic 1 status indicates an ON condition to the output coil instruction. Therefore, if the output coil address references an output bit in the output table, the processor will turn ON the corresponding output. This will turn ON the field device connected to the terminal referenced by the output coil address. Remember that the processor turns ON the device only after it has completely solved (scanned) the ladder program and updated the output at the end of the scan.

0 OFF (Output OFF)

1 ON (Output ON)

Figure 9-19. (a) An output coil instruction with a logic 0 reference address and (b) an output coil instruction with a logic 1 reference address.

When an output coil is used as an internal output, its coil address maps an internal bit storage address, rather than an output table bit that maps a real field device. In this case, when the output coil is turned ON, the corresponding bit in the internal bit storage area becomes logic 1. These internal outputs are used when a program requires interlocking sequences or when a real output is not necessary.

Normally open and normally closed reference contacts for an output coil open and close according to the status of the output coil. Figure 9-20 illustrates an example of a simple ladder diagram with normally open and

Industrial Text & Video Company 1-800-752-8398

www.industrialtext.com

S ECTION PLC Programming C HAPTER 3 Programming

Languages 9

normally closed contacts driving an output rung. For output 20 to turn ON, two things must happen: (1) PB1 must be pushed to turn ON reference input

10 and (2) limit switch LS1 must not be activated to keep reference input 11 OFF. In this case, the processor examines input 10 for an ON condition and input 11 for an OFF condition; if both logic conditions are met, it energizes output 20. With output 20 ON, the normally open contact 20 will close, turning internal output 100 ON. Also, the normally closed contact 20 will open because the test for an OFF condition at output 20 is not true (reference

20 is ON); therefore, it will turn internal output 101 OFF. At the EOS, the pilot light (PL1) will be lit because the processor will send a 1 to the output module, which will latch the logic 1 signal until continuity in the rung (output 20) is disrupted. Note that outputs 100 and 101 do not control real output devices because they reference internal bits that are not mapped to the I/O table.

Figure 9-20. Normally open and normally closed contacts driving real and internal

output coils.