Local I/O And Expansion I/O - Siemens Simatic S7-200 System Manual

Input/Output Control
8.1

Local I/O and Expansion I/O

The inputs and outputs are the system control points: the inputs monitor the signals from the
field devices (such as sensors and switches), and the outputs control pumps, motors, or
other devices in your process. You can have local I/O (provided by the CPU) or expansion
I/O (provided by an expansion I/O module):
The S7-200 CPU module provides a certain number of digital local I/O points. For more
information about the amount of local I/O provided by your CPU module, refer to the data
sheets in Appendix A.
The S7-200 CPU modules support the addition of both digital and analog expansion I/O.
For more information about the capabilities of the different expansion I/O modules, refer
to the data sheets in Appendix A.
Addressing the Local and Expansion I/O
The local I/O provided by the CPU module provides a fixed set of I/O addresses. You can
add I/O points to the 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.
Discrete or 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 output modules, the unused bits in the reserved bytes can be used like internal
memory bits (M bits). For input modules, the unused bits in reserved bytes are set to zero
with each input update cycle, and therefore cannot be used as internal memory bits.
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. Since there is no image
memory provided for analog I/O, there is no way to use these unused analog I/O points. All
analog I/O accesses are made immediately at the time of instruction execution.
Examples of Local and Expansion I/O
Figures 8-1, 8-2, and 8-3 provide examples that show how different hardware configurations
affect the I/O numbering. Notice that some of the configurations contain gaps in the
addressing that cannot be used by your program, while other I/O addresses can be used in
the same manner as the internal memory (M) bits.
8-2
S7-200 Programmable Controller System Manual
C79000-G7076-C230-02