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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
option.

- 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 Yes 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


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

modbus-slave

-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 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.