D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 14)

Modbus for Field Technicians

Page 14

6. BYTE/WORD ORDER – AN AMBIGUOUS NIGHTMARE

It takes two bytes to make a 16 bit word. These bytes can be arranged in two

ways.

When floating point, long integer or MK10 value is transported there are 4

bytes in two words. The order in which the words are sent as well as the

order in which the bytes are packed into each word can change from device

to device.

How did this stupid situation come to be?

Some microprocessors arrange the bytes in a word in one order and other

microprocessors do it in the opposite order. Some programmers account for

this and take steps for the device to serve its bytes in the standard order but

some manufacturers had bad programmers who did not care and their device

put out data in the wrong order.

Most often you will learn of this issue the hard way – the most common

symptom – the values you see in the client are not what you expect.

The jargon word for the order in which bytes are packed into a word is

‘Endianess’.

Here is an example of how this works.

Each block represents one byte. The two bytes make a word. The value in

each block is in decimal.

1 2

This can be interpreted as

1x256 + 2 = 258 (High Order or Most Significant Byte 1

)

1 + 2x256 = 513

1 2 ... 9 10 11 12 13 14 15 16 17 18 19 ... 76 77

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 14