If Your Modbus Host Software Requires an Address ONLY
Some host software does not allow you to specify the Modbus data type and address. Instead,
you specify an address only. This method requires another step to determine the address, but
it is not difficult. Basically, Modbus separates the data types by address ranges as well. So this
means an address alone can actually describe the type of data and location. This is often
referred to as “adding the offset”. One important thing to remember here is that two different
addressing modes may be available in your host software package. These are:
• 484 Mode
• 584/984 Mode
We recommend that you use the 584/984 addressing mode if your host software allows you
to choose. This is because the 584/984 mode allows access to a higher number of memory
locations within each data type. If your software only supports 484 mode, then there may be
some PLC memory locations that will be unavailable. The actual equation used to calculate
the address depends on the type of PLC data you are using. The PLC memory types are split
into two categories for this purpose.
• Discrete – X, GX, SP, Y, R, S, T, CT (contacts), C (contacts)
• Word – V, Timer current value, Counter current value
In either case, you basically convert the PLC octal address to decimal and add the appropriate
Modbus addresses (as required). The table below shows the exact equation used for each
group of data.
DL205 User Manual, 4th Edition, Rev. B
4–38
Chapter 4: System Design and Configuration
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Discrete Data Types
DL260 Memory Type PLC Range (Octal)
Address Range
(484 Mode)
Address Range
(584/984 Mode)
Modbus Data Type
Global Inputs (GX)
GX0 – GX1746 1001 – 1999 10001 – 10999 Input
GX1747 – GX3777 --- 11000 – 12048 Input
Inputs (X) X0 – X1777 --- 12049 – 13072 Input
Special Relays (SP) SP0 – SP777 --- 13073 – 13584 Input
Global Outputs (GY) GY0 – GY3777 1 – 2048 1 – 2048 Output
Outputs (Y) Y0 – Y1777 2049 – 3072 2049 – 3072 Output
Control Relays (C) C0 – C3777 3073 – 5120 3073 – 5120 Output
Timer Contacts (T) T0 – T377 6145 – 6400 6145 – 6400 Output
Counter Contacts (CT) CT0 – CT377 6401 – 6656 6401 – 6656 Output
Stage Status Bits (S) S0 – S1777 5121 – 6144 5121 – 6144 Output
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