S7-300/400 Tip S7-300 as Modbus Secondary Tip No. 5
Copyright 1997 by SIEMENS page 1 / 29 Modsec_t.doc
Status: 3/97 Version 1.0
SIMATIC S7-300/400 customers have free use of the application tips. These tips are only a general approach to using the S7-300/400 with various
applications. Your specific application may be different. It is your responsibility to use the SIMATIC S7-300/400 properly in your applications.
Group Topic
5 S7-300 as Modbus Secondary
Overview
This Applications Tip describes a method for getting an S7-300 family PLC to act as a Modbus
RTU secondary device. This tip will not work for S7-400 PLC’s because the current versions of
the CP441 card (6ES7 441-1AA00-0AE0 and 6ES7 441-2AA0-0AE0) do not support the ASCII
driver.
Hardware Requirements:
• S7-300 PLC. The Modbus and CP340 code and data size is about 7 Kbytes, so a S7-313 or
better is required.
• CP 340 Point-to-Point Communications Processor
Supported Functions:
• Modbus Function 3 - Read Holding Registers
• Modbus Function 16 - Preset Multiple Registers
Both of these functions map Modbus holding registers starting at Modbus address 40001 into a
single S7 data block.
Configuring the CP340
To work with the Modbus communications software, the CP340 card must be configured to use
the ASCII driver. You do this using the STEP 7 Hardware Configuration utility. Once the
CP340 card is added to your S7-300 rack, double-click on the card to set its properties. STEP
7 will open a window where you can set the CP340 properties.
SIMATIC
S7-300/400 Tips
