Allen Bradley PLC-5
The Allen Bradley PLC-5 can have up to eight racks in its 5/25 version. The rack containing the processor is automatically defined as rack 0,but the designer can allocate addresses of the other racks (in the range1“7) by set-up switches. The racks other than rack 0 connect to the processor via a remote I/O serial communications cable. There are three different ways in which an Allen Bradley rack can be configured, but we shall discuss the simplest (and possibly the most logical)method.
Each rack contains 16 card positions which are grouped in pairs called a ˜slot” A rack thus contains eight slots, numbered 0“7. A slot can contain one 16-way input card and one 16-way output card or two eight way cards usually (but not necessarily) of the same type. For example, slot 1 contains a 16-way input card and 16-way output card, and slot 2 contains two eight-way output cards.
The addressing for inputs is
I:Rack Slot/Bit with Bit being two digits. Allen Bradley use octal addressing for bits, so allowable numbers are 00“07 and 10“17. The address I:27/14is input 14 on slot 7 in rack 2. Outputs are addressed in a similar manner:
O:Rack Slot/Bits O:35/06 is output 6 in slot 5 of rack 2. Note that if 16-way cards are used, an input and an output can have the same rack/slot/bit address, being distinguished only by the I: or the O:. With eight-way cards there can be no sharing of rack/slot/bit addressing. Figure 1.12 earlier showed the addressing of several signals.