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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A
B
C
D
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