Micromod MOD: MODBUS RTU Communications Guide User Manual
Page 25

MODBUS RTU
COMMUNICATIONS GUIDE
21
Table 12. MODBUS Reference Information for MOD30 ML and Modcell Communications
Code
Function
Action
Data Types
Max Points per
Command
Address
Range
From
To
01
Read Coil Status
Read one or more
consecutive points
Discrete
99
00001-00499
CL
DRWIN1-5
02
Read Input Status
Read one or more
consecutive points
Discrete
99
10001-10499
CL
DROIN1-5
03
Read Holding
Registers
Read one or more
consecutive registers
All except ASCII and HEX -
One Register per Point
32
40001-40499
ML
SRWIN1-5
Any - Uses as many
Registers as req'd for Data
125
41001-45999
CL
MRWIN1-5
04
Read Input
Registers
Read one or more
consecutive registers
All except ASCII and HEX -
One Register per Point
32
30001-30499
ML
SROIN1-5
Any - Uses as many
Registers as req'd for Data
125
31001-35999
CL
MROIN1-5
05
Force Single Coil
Write one point
Discrete
1
00001-00499
CL
DRWIN1-5
06
Preset Single
Register
Write one register
All except ASCII and HEX -
One Register per Point
1
40001-40499
ML
SRWIN1-5
Any - Uses as many
Registers as req'd for Data
1
41001-45999
CL
MRWIN1-5
08
Loopback Test
Return of Query (00)
For Diagnostic Use Only
N/A
N/A
N/A
N/A
15
Force Multiple
Coils
Write one or more
consecutive points
Discrete
16
00001-00499
CL
DRWIN1-5
16
Preset Multiple
Registers
Write one or more
consecutive registers
All except ASCII and HEX -
One Register per Point
16
40001-40499
ML
SRWIN1-5
Any - Uses as many
Registers as req'd for Data
16
41001-45999
CL
MRWIN1-5
65
Extended Modbus Read and/or Write any
Data Type
Any - Data in buffer, rather
than Coils or Registers
Note 6
n/a
CL
FGLISTIN
NOTES:
1. Placing discrete points into Holding or Input Registers uses an entire 16 bit register for one point. This
may be more efficient when only a small amount of discrete data is required, along with analog data. If a
significant number of discrete points are required, communications and memory efficiency would be
improved by placing them into the Coil or Input Status area. Another possibility would be to pack 16
discrete points into a count and store it in an Input or Holding Register. This would be dependent on the
host device's ability to accept the packed data format.
2. Date, Floating Point and mSec Time are converted to 16 bit integers when tied to an ML register.
3. It is recommended that no more than the number of points supported per command be put into each Slave
ML block. This will prevent the Master device from accessing more than the allowed number of points in a
single command. The instrument will not return any data if the maximum number of registers is exceeded
or an invalid register is included. Some HMI packages, such as Fix DMACS, allow poll records to be
broken into groups to avoid this problem.
4. When using Extended Modbus, group control loop data together in CL blocks for greatest efficiency.
5. Some peripheral equipment that is designed for Modbus communications may not support Extended
Modbus (function code 65).
6. The instrument's foreground can hold up to 2400 bytes of data. The number of points that can be handled
per command depends on the host software. Connecting the CL block to BGLISTIN may provide
increased performance with some host configurations. This is the case when using Database Caching
with PC-30 or Genesis for Windows. Without Database Caching enabled, a connection to FGLISTIN is
recommended.
7. Each Modbus List block can contain up to 99 points, and each type of connection provides inputs for five
lists. This is also true for Configured List blocks for Standard Modbus. When using Extended Modbus,
Configured List blocks may be nested, allowing more than 99 points to be connected to FGLISTIN or
BGLISTIN.