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Chapter 4 I/O Processing > Summary - Pg. 104

104 Chapter 4 I = input O = output Module number X: X X X / X X Terminal number Rack number Figure 4.32: Allen-Bradley PLC-5 addressing. I = input Q = output XXX.X Bit number Byte number Figure 4.33: Siemens SIMATIC S5 addressing. With the Siemens SIMATIC S5, the inputs and outputs are arranged in groups of eight. Each such group is termed a byte, and each input or output within a group of eight is termed a bit. The inputs and outputs thus have their addresses in terms of the byte and bit numbers, effectively giving a module number followed by a terminal number, a full stop (.) separating the two numbers. Figure 4.33 shows the system. Thus I0.1 is an input at bit 1 in byte 0, and Q2.0 is an output at bit 0 in byte 2. The GEM-80 PLC assigns inputs and output addresses in terms of the module number and terminal number within that module. The letter A is used to designate inputs, and B outputs. Thus A3.02 is an input at terminal 02 in module 3, and B5.12 is an output at terminal 12 in module 5. In addition to using addresses to identify inputs and outputs, PLCs also use their addressing systems to identify internal, software-created devices, such as relays, timers, and counters. Summary The input/output units of PLCs are designed so that a range of input signals can be changed into 5 V digital signals and a range of output signals are available. For a PLC input unit with sourcing, it is the source of the current supply for the input device connected to it; with sinking, the input device provides the current to the input unit. For a PLC output unit with sourcing, it provides the current to the output device, and for sinking, the output device produces the current for the PLC output. Output units can be relay, transistor, or triac. For inputs, signal conditioning is generally used to convert analog signals to a current in the range 4 to 20 mA and, thus, by passing through a 250 O resistor, to a 1 to 5 V input signal. This might be achieved by a potential divider or perhaps an operational amplifier. An