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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 a messages from Modbus Master with matching device and register addresses.

Configuring datapoints

To use Modbus Slave 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

If TCP/IP is used as a trasmission medium, only devices with IPs predefined in 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

Min

Max

name

string

User-friendly name for a device

Yes


description

string

Description of a device

No


device_alias

string

Alphanumeric string to identify a device

Yes


enable boolean

Enabling/disabling of a device

No 1 0 1
protocol string

Protocol to be used

Yes
Modbus serial Slave, Modbus TCP Slave
host string Space separated host IP addresses of
master device
Yes (for TCP).


port integer

TCP port to listen for incoming connections

Yes (for TCP)




bind_address string

IP address of network adapter used to connect to slave device

(Default: ”0.0.0.0”)

No (for TCP) 0.0.0.0

keep_alive_timeout

integer

Minimum time a connection can be idle without being closed in miliseconds

No (for TCP) 60

mode string Choosing between RTU (”rtu”), ASCII
(”ascii”) and TCP(”tcp”) modes. ASCII
is the same as RTU, but with ASCII
symbols.
No

TCP (for TCP)

RTU (for Serial)

rtu, ascii, tcp
device

string

Communication port (”PORT1”/”PORT2”)

Yes (for serial)


PORT1

PORT2

baudrate integer

Communication speed, baud/s

No (for serial)

9600

300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200

databits integer

Data bit count for communication

No (for serial)

8

6

9

stopbits integer

Stop bit count for communication

No for serial)

1

1

2

parity

string

Communication parity option

No (for serial)

none

none, even, odd


flowcontrol

string

Communication device’s flow control
option.

No (for serial)

none

none


Modbus Slave parameters for Signals tab
Parameter

Type

Description

Required

Default value

(when not specified)

Range

Min

Max

signal_name

string

User-friendly signal name

Yes


device_alias

string

Alphanumeric string to identify a device

Yes


signal_alias

string

Unique alphanumeric name of the signal to be Yes used

Yes


enable boolean

Enabling/disabling an individual signal

No 1 0 1

number_type

string

Type of a number (FLOAT, DOUBLE, DIGITAL, etc.). This defines the size that will be read.

Yes


log

integer

Size of this signal’s log in the Event log.

No 0

slave_id integer Address of a slave device Yes


function integer Function number Yes


register_address integer Register address Yes


Modbus Slave 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 or vice versa. To configure such a signal, one column should be filled with particular value. To create an additional signal, one should make a tag_job_todo column equal to device_status or communication_status. Communication error status is set when a predefined count of messages (three by default, defined in keep_alive_timeout column) fail to be received or are considered invalid.

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 an address of the device WCC Lite is simulating as well as function number, register number and how much 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 register should be used for storing a values, how values can have their values swapped, a user should consult a section number_type (18.2.4).

If a Modbus master device requests a data from a register that is mapped but doesn’t yet have initial value, ILLEGAL DATA ADDRESS error code will be returned. The same error code is returned if a requested size of value is bigger that defined or if register is not configured at all.

Debugging a Modbus Slave application

If configuration for Modbus Slave 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 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 command line interface and find out why 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.