Page 137 - Programmable Logic Controllers, Fifth Edition - Mobile version

Pump
Low
L1 Inputs Program Output L2 running level High
level
ON
I:1/0 O:2/5 G R Y
I:1/0 Latch L PL
rung O:2/5
OFF ON/OFF High sensor switch
I:1/1 O:2/5
I:1/1 U
Unlatch
rung
15 14 13 12 11 10 98 76 54 32 10
O:2/
MAN/AUTO Low sensor switch
Status bit
Figure 6-55 Output latch and output unlatch operation.

• The status bit will remain set to 1 when the pushbut- Pump motor
ton is released and logical continuity of the latch Figure 6-56 Process used to control the level of water in a
rung is lost. storage tank.
• When the off pushbutton (I:1/1) is momentarily ac-
tuated, the unlatch rung becomes true and the latch Status Indicating Lights—Water pump running light
status bit (O:2/5) is reset back to 0 and so the light (green)
is switched off. - Low water level status light (red)
• The status bit will remain reset to 0 when the push- - High water level status light (yellow)
button is released and logical continuity of the latch Figure 6-57 shows a program that can be used to imple-
rung is lost. ment control of the water level in the storage tank. The
Output latch is an output instruction with a bit-level latch and unlatch instructions form part of the program.
address. When the instruction is true, it sets a bit in the The operation of the program can be summarized as
output image file. It is a retentive instruction because the follows:
bit remains set when the latch instruction goes false. In • An internal storage bit is used for the latch and ad-
most applications it is used with an unlatch instruction. dress rather than an actual discrete output address.
The output unlatch instruction is also an output instruc- Both have the same addresses.
tion with a bit-level address. When the instruction is true, • The rung 1 Examine-on instruction addressed
it resets a bit in the output image file. It, too, is a retentive to the off/on switch prevents the pump motor
instruction because the bit remains reset when the instruc- from starting under any condition when in the off
tion goes false. (open) state.
The process shown in Figure 6-56 is to be used to con- • In the MAN mode, the rung 1 Examine-on instruc-
trol the level of water in a storage tank by turning a dis- tion addressed to the low sensor switch allows the
charge pump on or off. The modes of operation are to be pump motor to operate only when the low level sen-
programmed as follows:
sor switch is closed.
OFF Position—The water pump will stop if it is run- • In the AUTO mode, whenever the high sensor
ning and will not start if it is stopped. switch is momentarily closed the Examine-on in-
Manual Mode—The pump will start if the water in struction of rung 1 addressed to it will energize the
the tank is at any level except low. latch coil. The pump will begin running and con-
Automatic Mode—If the level of water in the tank tinue to operate until the unlatch coil is energized by
reaches a high point, the water pump will start so that the rung 3 Examine-off instruction addressed to the
water can be removed from the tank, thus lowering low sensor switch.
the level. • The pump running status light is controlled by the
- When the water level reaches a low point, the pump rung 4 Examine-on instruction addressed to the
will stop. motor output.

118 Chapter 6 Developing Fundamental PLC Wiring Diagrams and Ladder Logic Programs

pet73842_ch06_098-130.indd 118 05/11/15 4:20 PM