Automating Manufacturing Systems - Process Control and ...

plc memory - 14.6
1st
00
bit - individual bits in accessed - this is like addressing a single output as a data bit.
I:000/02 - the third input bit from input card I:000
C5:4/DN - the DN bit of a counter
B3/4 - the fourth bit in bit memory
NOTE: Some bit addresses, especially inputs and outputs are addressed using octal.
This often leads to careless errors and mistakes. For example if you want the 11th
output bit, or bit 10, you would need to use 12 in octal to address it properly.
2nd
01
3rd
02
4th
03
5th
04
6th
05
7th
06
8th
07
9th
10
10th
11
11th
12
12th
13
13th
14
14th
15
15th
16
16th
17
Figure 14.5
Bit Level Addressing
Entire words can be addressed as shown in Figure 14.6. These values will normally
be assumed to be 2s compliment, but some functions may assume otherwise. The
first example shows a simple integer memory value. The next example gets up to inputs
(from card 0 in rack zero) as a single word. The last two examples are more complex and
they access the accumulator and preset values for a timer. Here a ’.’ is used as the ’/’ was
used for bit memory to indicate it is an integer. The first two examples don’t need the ’.’
because they are both integer value types. Other types of word addressing are possible,
including floating point numbers.
integer word - 16 bits can be manipulated as an integer.
N7:8 - the 9th value from integer memory
I:000 - an integer with all input values from an input card
T4:7.ACC - the accumulator value for a timer
T4:7.PRE - the preset value for a timer
Figure 14.6
Integer Word Addressing
Data values do not always need to be stored in memory, they can be define literally.
Figure 14.7 shows an example of two different data values. The first is an integer, the
second is a real number. Hexadecimal numbers can be indicated by following the number
with H, a leading zero is also needed when the first digit is A, B, C, D, E or F. A binary
number is indicated by adding a B to the end of the number.