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.