vdocuments.net_modbus-slave-modbus-master-in-s7.ppt

Modbus Slave
&
Modbus Master
in S7
Refer to the Modbus Serial Pack v2 zip.
-Project Modbus v2
-Manuals for drivers and hardware

© Siemens AG 2008 - Subject to modifications
Industry Automation
Preparation………………..
To enable the Modbus functionality of the CP cards, you must do
three things:
1. Install Point to Point software from
http://support.automation.siemens.com/WW/view/en/27013524
2. Install Modbus Master and Slave drivers from
3. Fit the Modbus dongle to the rear of the CP
4. Install the driver to the CP
Preperation

Industry Automation
Summary of the GOULD-MODBUS Protocol
The type of data exchange between MODBUS systems is controlled by
Function Codes (FCs).
The following FCs can be used to carry out data exchange bit-by-bit:
FC 01 Read coil (output) status,
FC 02 Read input status,
FC 05 Force single coil,
FC 15 Force multiple coils.
The following FCs can be used to carry out data exchange register-by-
register:
FC 03 Read holding registers,
FC 04 Read input registers,
FC 06 Preset single register,
FC 16 Preset multiple registers.
Gould-Modbus

Industry Automation
Summary of the GOULD-MODBUS Protocol
Function Code Modbus Address Range Access
Output Coils
Function Codes 01, 05, 15
00001 - 09999 Read/Write
Input Coils
Function Code 02
10001 - 19999 Read Only
Internal Registers
Function Code 04
30001 - 39999 Read Only
Holding Registers
Function Codes 03, 06, 16
40001 - 49999 Read/Write
It will save considerable time and frustration to have a good idea upfront of
the mapping of Modbus data to S7 data. The Modbus slave will define the
function codes to be used by virtue of the fact that specific data is held in
specific Modbus registers.
Gould-Modbus

Industry Automation
Configure the Hardware as Modbus Slave
With the software installed you can now open the Hardware Configuration
and configure the CP341 in your rack. Double click the CP to open its
properties and click on the Parameter button. The Point to Point
configuration software will now open.
From the drop down box at the top left, select Modbus Slave as the
Protocol. Next double click the Protocol icon in the middle of the screen.
S7-300
Slave

Industry Automation
Configure the Driver……..
You are now presented with a number of tabs. The first tab "General" shows
the driver versions in use. This is for information only. The second tab
"Modbus-Slave" allows you to define the communications properties of the
system, baud, stop bits etc.
S7-300
Slave

Industry Automation
On the tab labelled FC01, 05, 15, you can enter the Modbus address ranges
to be mapped into M area, Outputs, Timers and Counters. If any areas are
not used, enter 0.
S7-300
Slave

Industry Automation
On the tab FC02, you can define the mapping for M area and Inputs to be
read from.
S7-300
Slave

Industry Automation
On the tab FC03, 06, 16 you only need to enter a DB number. These
function codes use the address range starting from Modbus address 40000.
The DB number specified is the start DB of a range of DB's that are
generated by the user and must be 512 words long. If your Modbus address
is say 40513, you would need to generate another consecutively numbered
DB of 512 words long. Modbus address 40513 would be the first address in
the second DB (word 0 of the second DB, the 513 word in total).
S7-300
Slave

Industry Automation
The tab labelled FC4 is configured in the same way but for the 30000
address range.
S7-300
Slave

Industry Automation
The next tab is labelled "Limits". This is where you can define the range of
DB's, Memory and Outputs to be written to by FC05, FC15, FC06 and
FC16. If you were to use the Modbus address 40513, this would reside in
the second DB at word 0. You would therefore need to set the limits here to
DB1 and DB2 (assuming DB1 is the DB selected for use by FC03, 06, 16).
This is similar for M and Q areas. The final tab defines any electrical
properties.
S7-300
Slave

Industry Automation
Download Hardware Configuration and Modbus Driver……..
Save and Compile the Hardware Configuration then download to the
controller. Once the configuration is downloaded we can install the driver to
the CP. To do this you must have an online connection and use the Load
Drivers button from the Driver Parameters (as we have just configured).
S7-300
Slave

Industry Automation
Movement of Modbus Data between the CP to the User Program…….
Now the driver is configured. It knows what to do when it receives a Modbus
telegram – read or write data to or from a specific Simatic area. Now that
the Modbus data can be interpreted by the Modbus driver on the CP341, we
need to move the data from the CP to the CPU of the PLC. This is done by
use of FB80 MODB_341. This block can be found in the library entitled
"Modbus". The block is called as part of the cyclic user program via OB1. It
needs to know the logical address (LADDR) of the CP341 from the
hardware configuration which can be found by highlighting the CP in the
rack in Hardware Configuration. The address will be shown under the I-
address column of the bottom half of the window.
S7-300
Slave

Industry Automation
Block Dependencies:
When you add the FB80 block to the user program, you will be required to
generate an Instance Data Block. BEFORE generating this block, ensure
that the FB7 and FB8 blocks are also present in the blocks folder (these are
found in the same Modbus library). This is because FB80 uses these blocks
to communicate with the CP.
S7-300
Slave

Industry Automation
OB100 – Startup OB
The only coding that the user must perform is in OB100, where we set a bit
on and a bit off to initialise the FB on start-up of the CPU, used at the
parameters CP_START and CP_START_FM.
S7-300
Slave

Industry Automation
OB1 – Main Cyclic OB
Call FB80 in OB1 and assign addresses
for status reporting. Also assign the
CP_Start and CP_Start_FM bits
generated in OB100.
Download all program blocks to the
controller and test!!!
S7-300
Slave

Industry Automation
Configure the Hardware as Modbus Master
To configure the Modbus Master, we start in Hardware Configuration.
Perform the same steps as for the slave (selecting Modbus Master as the
Protocol) and click on the Protocol icon in the middle of the screen.
S7-300
Master

Industry Automation
Here you have three tabs. The first, "General" identifies the driver version,
as in the slave. The second, "Modbus Master" is where you define the
communications properties, stop bits, baud etc.
S7-300
Master

Industry Automation
The final tab defines any electrical properties. This is the hardware
configured for operation. We must now send and receive data to/from the
CP341.
S7-300
Master

Industry Automation
In the cyclic user program we must
call two blocks, P_SND_RK and
P_RCV_RK, found in the Modbus
library. The Modbus Master will send
a request for data to its slave
(reading values) or simply send it
data (writing).
S7-300
Master

Industry Automation
The send block is used to send a number of bytes that are structured in a
specific way and populated with specific data, depending upon the function
code to be used. For example, FC03 requires the first byte to contain the
slave address, the second byte to contain the function code number, the
third and fourth bytes (DBW2) must contain the Modbus start address and
bytes five and six (DBW4) contain a value representing the number of
registers required.
*(All of the structures of data for the various function codes can be found in section 5 of the Modbus master manual).
S7-300
Master

Industry Automation
You will need to define a DB
that can be used to receive
the data into. You call the
receive block, and populate
its parameters similarly to the
send block, telling it the
LADDR of the CP341 and the
DB to which the data is to be
received.
S7-300
Master

Industry Automation
With these blocks now configured, you can download to the
hardware and you should have a working Modbus master. Of
course, the blocks will need code writing around them for timing the
send and receives and also error handling, but I recommend using
a VAT (variable table) to manually trigger sends etc until you are
happy that the system is functioning. This way you are in control of
the requests and also the data that is sent.
S7-300
Master

Industry Automation
S7-400 Modbus Master and Modbus Slave……
The configuration is similar when using the S7-400. Differences include the
possibility to install two interface modules to one CP441-2, which can be
configured independently for different functions. These interface modules
are fitted to the front of the CP, with the type dependant upon the electrical
connection to the system (RS232, RS485, RS422). The driver configuration
is identical to when using the S7-300.
S7-400

Industry Automation
There are differences in the blocks
used for both slave and master
functions in the S7-400.
Firstly, the slave uses block FB180
MODB_441. This can be found in the
same location as the block used in
the S7-300.
Secondly, the master uses different
blocks to the S7-300. The S7-400
uses the System Function Blocks
SFB12 and 13 (BSEND and BRCV).
S7-400

Industry Automation
The main difference with both the master and slave blocks is the requirement of a
connection between the devices. This connection is configured in Netpro and the ID of
this connection is entered as a parameter to the send and receive blocks. The
connection type is Point to Point. Once a new connection of the type Point to Point is
added to the configuration, you can view the properties of it. Here you can set the CP to
be used and select which of the CP’s interfaces are to be connected. You also find the
connection ID, whose value needs to be entered to the send and receive blocks.
S7-400

Industry Automation
Diagnostics
Sometimes you may have configured the system but devices do not
communicate in the manner you expected. Things may appear to be healthy
but data is not being successfully transferred. If this is the case then you
have a number of options available.
The first place to check would be on the block calls. The blocks will generate
status and error information.
Secondly, you can check the Module Information of the CPU and CP for any
diagnostic entries.
Error codes generated on the block calls and in Module Information can be
found in the manuals included in the Modbus Serial Pack.
A further option is to monitor the network using a network analyzer. This will
enable you to see the raw data present on the network media and determine
if it is present and correct. You should be seeing telegrams with the format
matching that described in chapter 5 of the Modbus master manual,
depending upon the function code being implemented (byte 1 should
contain the slave address, byte 2 should contain the function code etc).
Diagnostics

Industry Automation
Multipoint Connection Details…..
In the RS422 mode CP341 and CP441-2 can only be used as a Master“
RS422
Multipoint

Industry Automation
RS485
Multipoint

Industry Automation
The following applies for both modules:
• GND (PIN 8 by CP341 / CP441-2) must always be connected on
both sides
• The casing shield must be installed everywhere
• A terminating resistor of approx. 330 Ω is to be soldered into the
connector on the last receiver of a node sequence
• Recommended cable type: LIYCY 3 x 2 x 0,14 R(A)/R(B) and
T(A)/T(B) twisted pairs
• A wiring with “Stub“ is not allowed
Multipoint

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