Selecting The Version Of The Modbus Rtu Instructions; Maximum Number Of Supported Modbus Slaves - Siemens SIMATIC S7-1200 System Manual
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13.5.3.2
Selecting the version of the Modbus RTU instructions
There are three versions of the Modbus RTU instructions available in STEP 7:
● Version 1.1: Compatible with V4.0 and later CPUs and V2.1 and later CMs
● Version 2.1: Compatible with V4.0 and later CPUs and V2.1 and later CMs
● Version 3.0: Compatible with V4.0 and later CPUs and V2.1 and later CMs
For compatibility and ease of migration, you can choose which instruction version to insert
into your user program.
You cannot use both versions of the instructions with the same module, but two different
modules can use different versions of the instructions. Do not use both 1.x and 2.y
instruction versions in the same CPU program. Your program's Modbus RTU instructions
must have the same major version number (1.x, 2.y, or V.z). The individual instructions
within a major version group may have different minor versions (1.x).
When you use the instruction tree to place a Modbus RTU instruction in your program, a new
FB instance is created in the project tree. You can see new FB instance in the project tree
under PLC_x > Program blocks > System blocks > Program resources.
To verify the version of a Modbus RTU instruction in a program, you must inspect project
tree properties and not the properties of a box displayed in the program editor. Select a
project tree Modbus RTU FB instance, right-click, select "Properties", and select the
"Information" page to see the Modbus RTU instruction version number.
13.5.3.3
Maximum number of supported Modbus slaves
Modbus addressing supports a maximum of 247 slaves (slave numbers 1 through 247).
Each Modbus network segment can have a maximum of 32 devices, based upon the loading
and drive capabilities of the RS485 interface. When you reach the 32-device limit, you must
use a repeater to expand to the next segment. You need seven repeaters to support the 247
slaves connected to one master for RS485.
Siemens repeaters work only with PROFIBUS; their function is to monitor PROFIBUS token
passing. You cannot use Siemens repeaters with other protocols. Therefore, you require
third party repeaters for Modbus.
Modbus timeouts are long by default; the use of multiple repeaters does not create a time-
delay problem. The Modbus master does not care if a slave is slow to respond or if multiple
repeaters delay the response.
S7-1200 Programmable controller
System Manual, V4.2, 09/2016, A5E02486680-AK
Click the icon on the instruction tree task card to enable the headers and columns
of the instruction tree.
Communication processor and Modbus TCP
13.5 Modbus communication
To change the version of the Modbus RTU
instructions, select the version from the drop-
down list. You can select the group or indi-
vidual instructions.
1193
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