The digital I/O in SIEMENS PLCs is arranged into groups of 8 bits, calleda byte (see Appendix). A signal is identified by its bit number (0â7) andits byte number (0â127). Inputs are denoted I<byte>. <bit> and outputsby Q<byte>. <bit>. I9.4 is thus an input with bit address 4 inbyte 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. Thecards are available in 16-bit (2-byte) or 32-bit (4-byte) form.

 A systemcan be built with local racks connected via a parallel bus cable or asremote racks with a serial link. Local racks are faster and overcomesome of the scan problems associated with serially connected remoteracks in high-speed applications, but are, literally, local. They can be nomore than a few metres from the processor.The simplest form of addressing is fixed slot.Four bytes are assigned sequentially to each slot; 0â3 to the first slot,4â7 to the next slot and so on. Input 12.4 is thus input bit 4 on the firstbyte of the card in slot 3 of the first rack. If 16-bit (2-byte) cards are usedwith 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 byteaddresses. This is known as variable slot addressing.

The first byteaddress 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 modulein each rack. Although any legitimate combination can be set up, it is recommendedthat a logical order is used similar.Siemens use different notations in different countries with multilingualprogramming terminals. A common European standard is German,where E (for Eingang or input) is used for inputs (e.g. E4.7) and A (forAusgang) is used for outputs (e.g. A3.5).