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Chapter 9. Configuring the I/O Driver with the Power Tool 139

Reading and Writing Digital Values

The SI7 driver requires you to use the BYTE data type when accessing digital values.

When reading a digital datablock, you can access the bit offset 0 to 7 within a BYTE

using a Digital Input or Digital Output block. Using this technique assumes the driver

is accessing an unsigned 8-bit value.

NOTE: S7 hardware uses a different method for organizing memory compared to S5

hardware. Refer to the section Understanding S7 Hardware Memory Organization

for more information.

Understanding S7 Hardware Memory Organization

S7 hardware uses a different memory organization from other Siemens hardware,

such as S5 hardware. In S5 hardware, words are single items with bit 15 first and bit

0 last as shown in the following figure:

With S7 hardware, words consist of two bytes, as shown in the following figure.

Alarms Alarms

ABCDEFGHIJ ABCDEFGHIJ

Word 0

Word 1

Word 2

bit 15 bit 0

1 2 ... 144 145 146 147 148 149 150 151 152 153 154 ... 267 268

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