13 Modbus
13.1 Introduction
Modbus is a communication protocol for use with its programmable logic controllers (PLCs). Modbus has become a de facto standard communication protocol and is now a commonly available means of connecting industrial electronic devices. It was developed for industrial applications, is relatively easy to deploy and maintain compared to other standards, and places few restrictions other than size on the format of the data to be transmitted.
Modbus enables communication among many devices connected to the same network, for example, a system that measures temperature and humidity and communicates the results to a computer. Modbus is often used to connect a supervisory computer with a remote terminal unit (RTU) in supervisory control and data acquisition (SCADA) systems. Many of the data types are named from industry usage of Ladder logic and its use in driving relays: a single-bit physical output is called a coil, and a single-bit physical input is called a discrete input or a contact.
WCC Lite supports both Modbus Master and Slave protocols. One can select between transmission over TCP/IP or serial connection (RS-485/RS232). Bytes to transmit can either be encoded according to both RTU and ASCII parts of the standard.
13.2 Modbus Master
Modbus communication contains a single Master and may include more than 1, but not more than 247 devices. To gather data from peripheral devices, the master device requests a cluster of slave devices for data. If any device understands that this message is addressed to it, it will reply with data. As no timestamp is sent along with data, having recent data requires frequent polling. WCC Lite can be configured to acquire data periodically in custom-defined intervals.
Configuring datapoints
Modbus Master in WCC Lite has to be configured via Excel. This configuration contains two Excel sheets where parameters have to be filled in - Devices and Signals
Modbus Master parameters for the Devices tab
| Parameter |
Type |
Description |
Required |
Default Value (when not specified) |
Range |
||
|---|---|---|---|---|---|---|---|
| TCP | RTU/ASCII | Min | Max | ||||
|
name |
string |
User-friendly name for a device |
Yes | Yes | |||
|
description |
string |
Description of a device |
No | No | |||
|
device_alias |
string |
Alphanumeric string to identify a device |
Yes | Yes | |||
| enable | boolean |
Enabling/disabling of a device |
No | No | 1 | 0 | 1 |
| protocol | string |
Protocol to be used |
Yes | Yes | Modbus RTU, Modbus TCP | ||
| ip | string | The IP address of the TCP slave device | Yes | - | |||
| port | integer |
TCP communication port |
Yes |
- |
502 |
|
|
| bind_address | string |
The IP address of the network adapter used to connect to the slave device (Default: ”0.0.0.0”) |
No | No | 0.0.0.0 | ||
| id | integer | Modbus Slave ID | Yes | Yes | |||
| mode | string | Choosing between RTU (”rtu”), ASCII (”ascii”) and TCP(”tcp”) modes. ASCII is the same as RTU, but with ASCII symbols. |
No | No |
TCP (for TCP) RTU (for Serial) |
rtu, ascii, tcp | |
| timeout_ms | integer |
Response timeout in milliseconds |
Yes |
Yes |
10000 |
|
|
| device |
string |
Communication port (”PORT1”/”PORT2”) |
- |
Yes |
|
PORT1 |
PORT2 |
| baudrate | integer |
Communication speed, baud/s |
- |
Yes |
9600 |
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 |
|
| databits | integer |
Data bit count for communication |
- |
Yes |
8 |
6 |
9 |
| stopbits | integer |
Stop bit count for communication |
- |
Yes |
1 |
1 |
2 |
| parity |
string |
Communication parity option |
- |
Yes |
none |
none, even, odd |
|
| flowcontrol |
string |
Number of requests, before the link is considered lost (device status signals are changed) and reconnect attempt will be issued |
- |
Yes |
none |
none |
|
| scan_rate_ms | integer |
If provided and positive - all jobs will have similar scan rates - all reads and writes will be executed within this timeframe (parameter scan_rate_ms in the Signals tab will be ignored) |
Yes | Yes | 300 | ||
| retry_count | integer |
Number of requests, before the link is considered lost (device status signals are changed) and reconnect attempt will be issued |
No | No | 3 | ||
| serial_delay | integer |
RS485 delay between read and write operations in milliseconds |
- |
Yes |
50 |
|
|
|
keep_alive_timeout |
integer |
Time interval for sending a keep-alive packet (in milliseconds) |
No | No | 60 | ||
|
modbus_multi_write |
boolean |
Use 15/16 functions to write 1 register/coil (Default: 0) |
No | No | 0 | 0 | 1 |
|
comm_restart_delay |
integer |
Time delay between disconnecting from the slave device and restarting the connection (in milliseconds) (Default: 500) |
No |
- |
500 |
|
|
Modbus Master parameters for the Signals tab
| Parameter |
Type |
Description |
Required |
Default Value (when not specified) |
Range |
||
|---|---|---|---|---|---|---|---|
| TCP | RTU/ASCII | Min | Max | ||||
|
signal_name |
string |
User-friendly signal name |
Yes | Yes | |||
|
device_alias |
string |
Alphanumeric string to identify a device |
Yes | Yes | |||
|
signal_alias |
string |
Unique alphanumeric name of the signal to be Yes used |
Yes | Yes | |||
| enable | boolean |
Enabling/disabling of an individual signal |
No | No | 1 | 0 | 1 |
|
job_todo |
string |
Request to send according to Modbus specification without device address and checksum. This field can be identical on several tags to fetch them in a single request |
Yes | Yes | |||
|
tag_job_todo |
string |
Similar format to the job_todo field. Address and length must be a subset of the job field. Defines the individual tag’s register(s) or coil(s). Can be described in HEX or DEC formats |
Yes | Yes | |||
|
number_type |
string |
Type of a number (FLOAT, DOUBLE, DIGITAL, etc.) |
Yes | Yes | |||
|
log |
integer |
If the log parameter is 1, the signal will be visible in the events log tab, if 0, signals will not be logged. |
No | No | 0 | ||
|
pulse_short_time_ms |
integer |
The time interval for short output pulse to stay active |
No | No | 0 | ||
|
pulse_long_time_ms |
integer |
The time interval for a long output pulse to stay active |
No | No | 0 | ||
|
periodic_update_ms |
integer |
Signal value will be published periodically according to the value set. |
No | No | |||
Note: If the number type is more than 16 bits (this means FLOAT, SIGNED32, DOUBLE etc.), modbus multiwrite should be enabled (1).
Device status signals
Modbus Master has an additional signal which can be configured to show communication status. It is used to indicate if the slave device has disconnected from the master (WCC Lite). To configure such signal for Modbus protocol, job_todo and tag_job_todo fields with string values are required. For Modbus master required parameters for the status signal will be signal_name device_alias, signal_alias, number_type, job_todo and tag_job_todo. Job_todo value must be device_status and for tag_job_todo there are 4 variations: communication_status, device_running, device_error, uknown_error. Each signal has 4 possible values and is based on the same logic. If the signal returns the value of 0, it means an unknown error has appeared, 1 – device or protocol connection is on and working properly, 2 – device is off or protocol is disconnected, 3 – error or service is down.
Different device vendors can have different implementations of a Modbus protocol stack. A register table can be one of the primary differences. WCC Lite Modbus Master transmits the most significant word (byte) first, however, devices from some vendors might require transmitting the least significant word (byte) first. If that is the case, make sure to switch bytes as needed. To find out more about setting a correct number format, one should consult a section number_type.
Modbus job or tag (as a task to be completed) can be built in two different formats - the user can select a more convenient way for him:
- hexadecimal format with every single byte separated by a | symbol. Device address, bytes containing output information and CRC (LRC) bytes should be excluded from the message;
- decimal format containing function number, first address and address count, separated by ; symbol. All other information should be excluded from the message;
job_todo can group several tag_job_todo’s. That way one Modbus message can be used to extract several tags. Grouping is accomplished dynamically meaning that if several identical jobs are found, their tags are grouped automatically.
Debugging a Modbus Master application
If the configuration for Modbus Master is set up, a handler for the protocol will start automatically. If the configuration is missing or contains errors, the protocol will not start. It is done intentionally to decrease unnecessary memory usage.
Modbus Master command line debugging options
modbus-master
-h [ –help ] Display help information
-V [ –version ] Show version
-d<debug level> Set debugging level
-c [ –config ] Config path
-e [ –redis ] Show redis debug information If Modbus Master does not work properly (e.g. no communication between devices, data is corrupted, etc.), a user can launch a debug session from the command line interface and find out why the link is not functioning properly. To launch a debugging session, a user should stop the Modbus-master process and run the Modbus-master command with respective flags as shown above.
To run a debug session:
- Step 1: Service must be stopped by entering the following command into the wcclite:
/etc/init.d/modbus-master stop - Step 2: After the service is stopped it must be started with the preferred configuration file (JSON files found in the /etc/ folder) and a debug level 7: modbus-master -c /etc/modbus-master/modbus-master.json -d7 Additional output forming options described in the table above.
- Step 3: Once the problem is diagnosed normal operations can be resumed with the following command: /etc/init.d/modbus-master start
13.3 Modbus Slave
WCC Lite can act as one (or several) of slave devices in a communication line. This can be used to transmit data to SCADA systems or other RTU devices. It can reply to messages from Modbus Master with matching devices and register addresses.
Configuring datapoints
Modbus Slave in WCC Lite has to be configured via Excel. This configuration contains two Excel sheets where parameters have to be filled in - Devices and Signals
If TCP/IP is used as a transmission medium, only devices with IPs predefined in the host column are allowed to connect. All other connections are rejected
Modbus Slave parameters for Devices tab
| Parameter |
Type |
Description |
Required |
Default Value (when not specified) |
Range |
||
|---|---|---|---|---|---|---|---|
| TCP | RTU/ASCII | Min | Max | ||||
|
name |
string |
User-friendly name for a device |
Yes | Yes | |||
|
description |
string |
Description of a device |
No | No | |||
|
device_alias |
string |
Alphanumeric string to identify a device |
Yes | Yes |
unknown
|
||
| enable | boolean |
Enabling/disabling of a device |
No | No | 1 | 0 | 1 |
| protocol | string |
Protocol to be used |
Yes | Yes | Modbus serial Slave, Modbus TCP Slave | ||
| host | string | Space-separated host IP addresses of master device |
Yes | - | |||
| port | integer |
TCP port to listen for incoming connections |
Yes |
- |
|
|
|
| bind_address | string |
The IP address of the network adapter used to connect to the slave device (Default: ”0.0.0.0”) |
No | No | 0.0.0.0 | ||
|
keep_alive_timeout |
integer |
The minimum time a connection can be idle without being closed in milliseconds |
No | No | 60 | ||
| mode | string | Choosing between RTU (”rtu”), ASCII (”ascii”) and TCP(”tcp”) modes. ASCII is the same as RTU, but with ASCII symbols. |
No | No |
TCP (for TCP) RTU (for Serial) |
rtu, ascii, tcp | |
| device |
string |
Communication port (”PORT1”/”PORT2”) |
- |
Yes |
|
PORT1 |
PORT2 |
| baudrate | integer |
Communication speed, baud/s |
- |
Yes |
9600 |
300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200 |
|
| databits | integer |
Data bit count for communication |
- |
Yes |
8 |
6 |
9 |
| stopbits | integer |
Stop bit count for communication |
- |
Yes |
1 |
1 |
2 |
| parity |
string |
Communication parity option |
- |
Yes |
none |
none, even, odd |
|
| flowcontrol |
string |
The communication device’s flow control |
- | No |
none |
none |
|
Modbus Slave parameters for the Signals tab
| Parameter |
Type |
Description |
Required |
Default Value (when not specified) |
Range |
||
|---|---|---|---|---|---|---|---|
| TCP | RTU/ASCII | Min | Max | ||||
|
signal_name |
string |
User-friendly signal name |
Yes | Yes | |||
|
device_alias |
string |
Alphanumeric string to identify a device |
Yes | Yes | |||
|
signal_alias |
string |
Unique alphanumeric name of the signal to be used |
Yes | Yes | |||
| enable | boolean |
Enabling/disabling an individual signal |
No | No | 1 | 0 | 1 |
|
number_type |
string |
Type of a number (FLOAT, DOUBLE, DIGITAL, etc.). This defines the size that will be read. |
Yes | Yes | |||
|
log |
integer |
If the log parameter is equal to 1, the signal values and attributes will be seen in the events log. |
No | No | 0 | ||
| slave_id | integer | Address of a slave device | Yes | Yes | |||
| function | integer | Function number | Yes | Yes | 1, 2, 3, 4 | ||
| register_address | integer | Register address | Yes | Yes | |||
| periodic_update_ms | integer | Signal value is published periodically according to the value set. | No | No | - | - | - |
Device status signals
Modbus slave has an additional signal which can be configured to show communication status. It indicates if the master device has disconnected from the slave (WCC Lite). To configure such signal for Modbus protocol, job_todo and tag_job_todo fields with string values are required. For Modbus slave required parameters for the status, the signal will be signal_name device_alias, signal_alias, number_type, slave_id, function, register_address, job_todo and tag_job_todo. Job_todo value must be device_status and for tag_job_todo there are 4 variations: communication_status, device_running, device_error, uknown_error. Each signal has 4 possible values and is based on the same logic. If the signal returns the value of 0, it means an unknown error has appeared, 1 – device or protocol connection is on and working properly, 2 – device is off or protocol is disconnected, 3 – error or service is down.
Mapping values to registers
Internally stored values aren’t organised in a register-like order, therefore mapping should be done by the user. This mapping includes setting the address of the device WCC Lite is simulating as well as the function number, register number and how many 16-bit registers are used to store a value. These values should be set in common_address, function, info_address and size columns respectively in the Excel configuration.
To find out how many registers should be used for storing values, and how values can have their values swapped, a user should consult a section number_type.
If a Modbus master device requests data from a register that is mapped but doesn’t yet have an initial value, an ILLEGAL DATA ADDRESS error code will be returned. The same error code is returned if the requested size of the value is bigger than defined or if the register is not configured at all.
Debugging a Modbus Slave application
If the configuration for Modbus Slave is set up, a handler for the protocol will start automatically. If the configuration is missing or contains errors, the protocol will not start. It is done intentionally to decrease unnecessary memory usage.
Modbus Slave command line debugging options
To run a debug session:
- Step 1: Service must be stopped by entering the following command into the wcclite:
/etc/init.d/modbus-slave stop - Step 2: After the service is stopped it must be started with the preferred configuration file (JSON files found in the /etc/ folder) and a debug level 7: modbus-slave -c /etc/modbus-slave/modbus-slave.json -d7 Additional output forming options described in the table below.
- Step 3: Once the problem is diagnosed normal operations can be resumed with the following command: /etc/init.d/modbus-slave start
-h [ –help ] Display help information
-V [ –version ] Show version
-d<debug level> Set debugging level
-c [ –config ] Config path
-e [ –redis ] Show redis debug information If Modbus Slave does not work properly (e.g. no communication between devices, data is corrupted, etc.), a user can launch a debug session from the command line interface and find out why the link is not functioning properly.
To launch a debugging session, a user should stop modbus-slave process and run modbus-slave command with respective flags as shown above.