D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual download pdf (Page 12)

Modbus for Field Technicians

Page 12

4. WHAT ABOUT SCALING IN MODBUS

Modbus does not provide a method for transporting large or Floating Point

numbers or a mechanism for scaling analog values.

A 16 bit word can only contain values in the range 0-65535. Only whole

numbers are permitted.

To work around this many server device manufacturers use multipliers and

document them in their manuals. For example, to report a temperature of

58.5 the device reports a value of 585, and makes a note in the manual that

the master should scale by 10.

This scaling is achieved by adopting a convention between the client and the

server.

What about large numbers > 65535

Modbus does not provide a mechanism but 3 important schemes are widely

used.

Long Integers – Two consecutive 16 bit words are interpreted as a 32 bit long

integer.

MK10 values – Two consecutive words are used. The 1

reports the number

of units and the 2

reports the number of 10,000’s.

Floating Point Numbers – Two consecutive words are used and a scheme.

(See section X)

These schemes are conventions and not all servers or clients support them.

The protocol does not identify these big numbers. Only the vendor docs do.

What we mean by this is – if you look at the byte stream in a Modbus

1 2 ... 7 8 9 10 11 12 13 14 15 16 17 ... 76 77

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D-Link DE-805TP - 10Mbps Ethernet Mini Hub User Manual Page 12