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 range1â€“7) by set-up switches. The racks other than rack 0 connect to theprocessor via a remote I/O serial communications cable.There are three different ways in which an Allen Bradley rack can beconfigured, but we shall discuss the simplest (and possibly the most logical)method.
Each rack contains 16 card positions which are grouped in pairscalled a â€˜slotâ€™. A rack thus contains eight slots, numbered 0â€“7. A slot cancontain one 16-way input card and one 16-way output card or two eightwaycards usually (but not necessarily) of the same type. For example, slot 1 contains a 16-way input card and 16-way outputcard, and slot 2 contains two eight-way output cards.
The addressing for inputs is
I:Rack Slot/Bitwith Bit being two digits. Allen Bradley use octal addressing for bits,so allowable numbers are 00â€“07 and 10â€“17. The address I:27/14is input 14 on slot 7 in rack 2. Outputs are addressed in a similarmanner:
O:Rack Slot/Bitso O:35/06 is output 6 in slot 5 of rack 2. Note that if 16-way cards areused, 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 therecan be no sharing of rack/slot/bit addressing. Figure 1.12 earlier showedthe addressing of several signals.