Page 108 - Programmable Logic Controllers, Fifth Edition - Mobile version

not directly control an output field device. The internal
A
B
C
Y
output operates just like any output that is controlled by
programmed logic; however, the output is used strictly
D E
for internal purposes.
The advantage of using internal outputs is that there
F
are many situations in which an output instruction is re-
quired in a program but no physical connection to a field
Boolean equation: Y = (ABC) + (ADE) + (FE) + (FDBC) device is needed. If there are no physical outputs wired
to a bit address, the address can be used as an internal
Figure 5-39 Original circuit.
storage point. Internal storage bits or points can be pro-
A B C Y grammed by the user to perform relay functions without
occupying a physical output. In this way internal outputs
A D E can minimize output module point requirements when-
ever practical.
Internal outputs are single-bit storage locations in
F E
memory and are addressed as such. SLC 500 control-
lers use bit file B3 for storage and addressing of internal
F D B C
output bits. The addressing for bit B3:1/3 illustrated in
Figure 5-41 consists of the file number followed by word
and bit numbers.
Figure 5-40 Reprogrammed circuit. An internal control relay can be used when a pro-

problem for user program circuits similar to that shown in gram requires more series contacts than the rung allows.
Figure 5-39. If programmed as shown, contact combination Figure 5-42 shows a circuit that allows for only 7 series
FDBC would be ignored. To obtain the required logic, the contacts when 12 are actually required for the pro-
circuit would be reprogrammed as shown in Figure 5-40. grammed logic. To solve this problem, the contacts are
split into two rungs. Rung 1 contains seven of the re-
5.7 Internal Relay Instructions quired contacts and is programmed to control internal
relay coil B3:1/3. The address of the first programmed
Most PLCs have an area of the memory allocated for contact on Rung 2 is B3:1/3 followed by the remain-
what are known as internal storage bits. These storage ing five contacts and the discrete output. When the logic
bits are also called internal outputs, internal coils, in- controlling the internal output is true, the referenced bit
ternal control relays, or simply internal bits. Internal B3:1/3 is turned on or set to 1. The advantage of an in-
outputs are on/off signals generated by programmed ternal storage bit in this manner is that it saves an output
logic. Unlike a discrete output, an internal output does bit from being used.

0
1
2
3
4
5
Output File
Input File
6
Bit File
Status File
7
Timer File
Counter File
Control Files
3 Integer Files
Data File B3 (bin) -- BINARY
O set 15 14 13 12 11 10 98 76 5432 10 File (B3)
B3:0 1 1 1 10 01 000000000 Address
Bit File
B3:1 0 000 10 0 000 10 00 10 Word (1) B3:1/3
B3:2 0 000 01 0 000000000 Bit (3)
B3:3 0 00000 0 000000000
B3:4 0 00000 0 000000000
B3:5 0 00000 0 000000000
B3:6 0 00000 0 000000000
Figure 5-41 SLC 500 controllers use bit file B3 for internal bit addressing.

Basics of PLC Programming Chapter 5 89

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