Modbus Slave

1
Modbus SlaveModbus Master in S7

• Refer to the Modbus Serial Pack v2 zip.
• Project Modbus v2
• Manuals for drivers and hardware

2
Preparation..

• To enable the Modbus functionality of the CP
  cards, you must do three things
• Install Point to Point software from
  http//support.automation.siemens.com/WW/view/en/2
  7013524
• Install Modbus Master and Slave drivers from
  http//support.automation.siemens.com/WW/view/en/2
  7774018 http//support.automation.siemens.com/WW/v
  iew/en/27774276
• Fit the Modbus dongle to the rear of the CP
• Install the driver to the CP

Preperation
3
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
4
Summary of the GOULD-MODBUS Protocol
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.
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
Gould-Modbus
5
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
6
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
7
Configure the Driver..

• 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
8
Configure the Driver..

• On the tab FC02, you can define the mapping for M
  area and Inputs to be read from.

S7-300 Slave
9
Configure the Driver..

• 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
10
Configure the Driver..

• The tab labelled FC4 is configured in the same
  way but for the 30000 address range.

S7-300 Slave
11
Configure the Driver..

• 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
12
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
13
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
14
Movement of Modbus Data between the CP to the
User Program.

• 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
15
Movement of Modbus Data between the CP to the
User Program.

• 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
16
Movement of Modbus Data between the CP to the
User Program.

• 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
17
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
18
Configure the Driver..

• 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
19
Configure the Driver..

• 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
20
Movement of Modbus Data between the CP to the
User Program.

• OB1 Main Cyclic OB
• 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
21
Movement of Modbus Data between the CP to the
User Program.

• 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.

S7-300 Master
(All of the structures of data for the various
function codes can be found in section 5 of the
Modbus master manual).
22
Movement of Modbus Data between the CP to the
User Program.

• OB1 Main Cyclic OB
• 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
23
Movement of Modbus Data between the CP to the
User Program.

• 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
24
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
25
S7-400 Modbus Master and Modbus Slave

• 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
26
S7-400 Modbus Master and Modbus Slave

• 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 CPs
  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
27
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
28
Multipoint Connection Details..
RS422 Multipoint

• In the RS422 mode CP341 and CP441-2 can only be
  used as a Master

29
Multipoint Connection Details..
RS485 Multipoint
30
Multipoint Connection Details..

• 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 O 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