Page 315 - Programmable Logic Controllers, Fifth Edition - Mobile version

JMP LBL JSR RET SBR TND MCR SUS JMP LBL JSR RET SBR TND MCR SUS

Program Control Ascii Control Ascii String Micro Program Control Ascii Control Ascii String Micro

Inputs Main program L1
L1 Main program
Inputs Trapped input conditions
A B C SUS
SW A
Suspend
SW Temporary end B Suspend ID 100
TND
C
Reminder of main program
Figure 13-27 SUS (suspend) diagnostic instruction.

• All ladder logic outputs are de-energized, but other
Figure 13-26 TND (temporary end) diagnostic instruction.
status files have the data present when the suspend
instruction was executed.
operation of this output instruction can be summarized as
follows: Most PLC system faults occur in the field wiring and
devices. The wiring between the field devices and the ter-
• The instruction operates only when its rung condi- minals of the I/O modules is a likely place for problems
tions are true and stops the processor from scanning to occur. Faulty wiring and mechanical connection prob-
any logic beyond the TND instruction. lems can interrupt or short the signals sent to and from
• When the processor encounters a true TND rung, the I/O modules.
it resets the watchdog timer (to 0), performs an I/O The sensors and actuators connected to the I/O of the
update, and begins running the ladder program at process can also fail. Mechanical switches can wear out
the first instruction in the main program. or be damaged during normal operation. Motors, heaters,
• If the TND rung is false, the processor continues lights, and sensors can also fail. Input and output field
the scan until the next TND instruction or the END devices must be compatible with the I/O module to ensure
statement. proper operation.
• By inserting the TND instruction at different loca- When an instruction does not seem to be working cor-
tions in the program you can test parts of the program rectly, the problem may be an addressing conflict caused
sequentially until the entire program has been tested. by the same address being used for two or more coil in-
• Once the troubleshooting process has been com- structions in the same program. As a result, multiple rung
pleted, any remaining TND instructions are re- conditions can control the same output coil, making trou-
moved from the program. bleshooting more difficult. In the case of duplicate out-
puts, the monitored rung may be true; but if a rung farther
The suspend (SUS) instruction, shown in Figure 13-27, down in the ladder diagram is false, the PLC will keep the
is used to trap and identify specific conditions for program output off. The program of Figure 13-28 illustrates what
debugging and system troubleshooting. The operation of
this output instruction can be summarized as follows: Inputs Ladder logic program Output

• When the rung is true, this instruction places the L1 I:1/1 O:2/1 L2
controller in the suspend or idle mode. I:1/1
• The suspend ID, in this case 100, must be selected PL
by the programmer and entered in the instruction. Same address O:2/1
• When the SUS instruction executes, the ID number I:1/2 (OFF)
100 is written in word 7 (S:7) of the status file. I:1/2
• If multiple suspend instructions are present, then O:2/1
this will indicate which SUS instruction was active.
• The suspend file (program or subroutine number
identifying where the executed SUS instruction re- Figure 13-28 Program with the same address used for
sides) is placed in word 8 (S:8) of the status file. two coils.

296 Chapter 13 PLC Installation Practices, Editing, and Troubleshooting

pet73842_ch13_281-304.indd 296 03/11/15 7:23 PM