An interesting idiosyncrasy of Modbus communication is that the address values speciﬁed within Modbus data frames are relative rather than absolute.

Since each Modbus read or write function only operates on a limited range of register addresses, there is no need to specify the entire address in the data frame.

For example, Modbus function code 02 reads discrete input registers in the device with an absolute address range of 10001 to 19999 (i.e. all the addresses beginning with the digit “1”).

Therefore, it is not necessary for the “read” command function 02 to specify the ﬁrst digit of the register address.

Instead, the read command only needs to specify a four-digit “relative address” specifying how far up from the beginning of the address range (in this case, from 10001) to go.

Modbus Analogy

An analogy to aid your understanding of relative addressing is to envision a hotel building with multiple ﬂoors.

The ﬁrst digit of every room number is the same as the ﬂoor number, so that the ﬁrst ﬂoor only contains rooms numbered in the 100’s, the second ﬂoor only contains rooms numbered in the 200’s, etc.

With this very orderly system of room numbers, it becomes possible to specify a room’s location in more than one way.

For example, you could give instructions to go to room 314 (an absolute room number), or alternatively, you could specify that same room as “number 14 (a relative room number) on the third ﬂoor”. To a hotel employee who only works on the third ﬂoor, the shortened room number might be easier to remember.

In Modbus, relative addresses are just a little bit more complicated than this. Relative addresses actually span a range beginning at zero, while absolute addresses begin with “1” as the leastsigniﬁcant digit.

This means there is an additional oﬀset of 1 between a Modbus relative address and its corresponding absolute address.

Returning to the hotel analogy, imagine the very ﬁrst room on the third ﬂoor was room 301 (i.e. there was no room 300) and that the relative address represented the number of rooms past that ﬁrst room.

In this unintuitive scheme, room 314 could be speciﬁed as “the 13th room after the starting room on the third ﬂoor”. If this seems needlessly confusing, you are not alone. Welcome to Hotel Modbus.

A few examples are given here for illustration:

• Write multiple output coils in register 00008: Modbus write function 15; relative address 7

In each case, the pattern is the same: the relative address gets added to the ﬁrst address of that range in order to arrive at the absolute address within the Modbus device. Referencing the ﬁrst example shown above: 2439 (relative address) + 10001 (ﬁrst address of register range) = 12440 (absolute address).

Thankfully, the only time you are likely to contend with relative addressing is if you program a computer using some low-level language such as assembly or C++.

Most high-level industrial programming languages such as Function Block or Ladder Diagram make it easy for the end-user by allowing absolute addresses to be directly speciﬁed in the read and write commands.

In a typical PLC program, for example, you would read contact register 12440 by simply specifying the number 12440 within the address ﬁeld of a “read 02” instruction.

`© 2019-2021 by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License`