Siemens SIMATIC S7-200 System Manual page 47

Specifying a Constant Value for S7-200 Instructions
You can use a constant value in many of the S7-200 instructions. Constants can be bytes, words,
or double words. The S7-200 stores all constants as binary numbers, which can then be
represented in decimal, hexadecimal, ASCII, or real number (floating point) formats. See
Table 4-2.
Table 4-2 Representation of Constant Values
Representation
Decimal
Hexadecimal
Binary
ASCII
Real
String
Tip
The S7-200 CPU does not support "data typing" or data checking (such as specifying that the
constant is stored as an integer, a signed integer, or a double integer). For example, an Add
instruction can use the value in VW100 as a signed integer value, while an Exclusive Or
instruction can use the same value in VW100 as an unsigned binary value.
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
Process-image register space for digital I/O is always reserved 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 are set to zero with each input update cycle.
Analog I/O points 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.
Format
[decimal value]
16#[hexadecimal value]
2#[binary number]
'[ASCII text]'
ANSI/IEEE 754- -1985
"[stringtext]"
PLC Concepts
Sample
20047
16#4E4F
2#1010_0101_1010_0101
'ABCD'
+1.175495E- -38 (positive) - -1.175495E- -38 (negative)
"ABCDE"
Chapter 4
33