Siemens SIMATIC S5
The digital I/O in Siemens PLCs is arranged into groups of 8 bits, called a byte . A signal is identified by its bit number (0“7) and its byte number (127). Inputs are denoted I. and outputs by Q. . I9.4 is thus an input with bit address 4 in byte 9, and Q63.6 is an output with bit address 6 in byte 63.Like Allen Bradley, Siemens use card slots in one or more racks. The cards are available in 16-bit (2-byte) or 32-bit (4-byte) form.
 A system can be built with local racks connected via a parallel bus cable or as remote racks with a serial link. Local racks are faster and overcome some of the scan problems associated with serially connected remote racks in high-speed applications, but are, literally, local. They can be no more than a few meters from the processor. The simplest form of addressing is fixed slot. Four bytes are assigned sequentially to each slot; 3 to the first slot,4“7 to the next slot and so on. Input 12.4 is thus input bit 4 on the first byte of the card in slot 3 of the first rack. If 16-bit (2-byte) cards are used with fixed (4-byte) addressing, the upper 2 bytes in each slot are lost. In all bar the simplest system the user has the ability to assign byte addresses. This is known as variable slot addressing.
The first byte address and the range (2 byte for 16-bit cards or 4 byte for 32-bit cards) can be set independently for each slot by switches in the adaptor module in each rack. Although any legitimate combination can be set up, it is recommended that a logical order is used similar. Siemens use different notations in different countries with multilingual programming terminals. A common European standard is German, where E (for Eingang or input) is used for inputs (e.g. E4.7) and A (for Ausgang) is used for outputs (e.g. A3.5).