RSLogix500, PLC5, and earlier versions of the RSLogix5000 series use one letter and two numbers for the addressing of input or output instructions. The initial letter specifies whether the address is an input or output function. If it is the letter "O" the address is an output. If it is the letter "I" the address is an input instruction. An element delimiter : follows. The first number indicates the I/O rack and slot number, which is an octal identifier. A bit delimiter / is next. The second number indicates the bit address in the module itself, which can generally range from terminal number 0 to 15. Generally, the format is O:e/b for output, and I:e/b for input, where e is element number and b is bit or terminal number.