Addressing the Local and Expansion I/O

The local I/O provided by the CPU provides a fixed set of I/O addresses. You can add I/O points to the S7-200 CPU by connecting expansion I/O modules to the right side of the CPU, forming an I/O chain.
The addresses of the points of the module are determined by the type of I/O and the position of the module in the chain, with respect to the preceding input or output module of the same type. For example,
an output module does not affect the addresses of the points on an input module, and vice versa.
Likewise, analog modules do not affect the addressing of digital modules, and vice versa.

Tip
Digital expansion modules always reserve process-image register space in increments of eight bits (one byte). If a module does not provide a physical point for each bit of each reserved byte, these
unused bits cannot be assigned to subsequent modules in the I/O chain. For input modules, the unused bits in reserved bytes are set to zero with each input update cycle.
Analog expansion modules are always allocated in increments of two points. If a module does not provide physical I/O for each of these points, these I/O points are lost and are not available for assignment to subsequent modules in the I/O chain.