Copyright Goodheart-Willcox Co., Inc. 102 Programmable Logic Controllers: Hardware and Programming • Bit (B3 and B10)—Bit files B3 and B10 are used for internal control relay contacts. Each file holds 256 words (B3:0 to B3:255 and B10:0 to B10:255). These registers are used by advanced PLC instructions such as bit shift and sequencer instructions. All Allen-Bradley SLC 500 series PLCs have 16-bit registers. For example, B3:0 has the bits B3:0/0 to B3:0/15. B3:0 represents a 16-bit word, while B3:0/0 or B3/0 represents the least significant bit in the B3:0 word. These bits can be used as internal coils or contacts in a PLC ladder logic diagram. The SLC 500 series PLCs have 256 bits addressed as B3/0 through B3/255. These bits are used for internal PLC coil or contact addresses. A programmer can also use bits addressed as B10:0 through B10:255. Therefore, 512 bit addresses labeled B3:0 through B3:255 and B10:0 through B10:255 are available in the SLC 500 series PLC systems. Note that the B3 data file has separate data file icons. 6.6 Programming Ladder Logic Diagrams with Latch and Unlatch Instructions Frequently, industrial control systems dictate that an output device must turn on when a pushbutton is pressed. The same output device must turn off when another pushbutton is pressed. This is possible if PLC latch and unlatch instructions are used. Figure 6-23 displays a relay logic diagram with two latch and unlatch instructions. Switch one and pushbutton one control the green light. Switch two and pushbutton two control the red light. Figure 6-24 displays the input/output connections of an Allen-Bradley fixed SLC 500 PLC device for the relay logic diagram in Figure 6-23. Table 6-4 illustrates the I/O port assignments. Latch #1 Latch #2 Pushbutton #1 Unlatch #1 L1 L2 Latch 1 G Switch #2 Switch #1 Pushbutton #2 Unlatch #2 Green light Latch 2 R Red light Goodheart-Willcox Publisher Figure 6-23. Latch and unlatch instructions.