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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, master device request a cluster of slave devices for data. If any device understand that this message is addressed for it – it replies 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

To use Modbus Master in WCC Lite, it has to be configured via an Excel configuration. This configuration contains two Excel sheets where parameters have to be filled in - Devices and Signals

Modbus Master 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


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 IP address of TCP slave device Yes -


port integer

TCP communication port

Yes

-

502



bind_address string

IP address of network adapter used to connect to 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 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 rate - all reads and writes will be executed within this timeframe (parameter scan_rate_ms in Signals tab will be ignored)

Yes Yes 300

retry_count integer

Number of requests, before 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 slave device and restarting connection (in milliseconds)

(Default: 500)

No

-

500



update

boolean

Enable to keep updating the tags even if they have the same value.

No

No

0

0

1

Modbus Master parameters for 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 single request

Yes Yes


tag_job_todo

string

Similar format to job_todo field. Address and length must be a subset of 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

Size of this signal’s log in Event log.

No No 0

pulse_short_time_ms

integer

Time interval for short output pulse to stay active

No No 0

pulse_long_time_ms

integer

Time interval for long output pulse to stay active

No No 0

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 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 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 are based on the same logic. If signal returns value of 0, it means 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 a 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 - user can select a more convenient way for him:

  • hexadecimal format with every single byte separated by | 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 configuration for Modbus Master is set up, handler for protocol will start automatically. If configuration is missing or contains errors, 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
-r [ –raw ] Show raw telegram data
-f [ –frame ] Show frame data
-s [ –serial ] Show serial port data 
–tcp Show tcp packets
–ascii Show ASCII messages
–rtu Show RTU messages
-e [ –redis ] Show redis debug information 
-R [ –readyfile ] Ready notification file

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 command line interface and find out why link is not functioning properly. To launch a debugging session, a user should stop modbus-master process and run modbus-master command with respective flags as shown above.