2020/04/07
WCC Lite user manual
Table 15: Modbus Master parameters for
Signals
tab
Parameter
Type
Description
Mandatory
signal_name
string
Userfriendly signal name
Yes
device_alias
string
Device alias from a Devices tab
Yes
signal_alias
string
Unique alphanumeric name of the signal to be
used
Yes
enable
boolean
Enabling/disabling of an individual signal
Yes
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
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 resgister(s) or coil(s). Can
be described in HEX or DEC formats
Yes
number_type
string
Type of a number (FLOAT, DOUBLE, DIGITAL,
etc.)
Yes
log_size
Integer
Size of this signal’s log in Event log.
scan_rate_ms
integer
If
scan_rate_ms
in devices tab is not provided
or is a positive number, read or write job will be
executed within this timeframe in milliseconds
Yes/No
pulse_short_time_ms
integer
Time interval for short output pulse to stay
active
No
pulse_long_time_ms
integer
Time interval for long output pulse to stay active
No
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 (17.2.4).
Modbus job or tag (as a task to be completed) can be built in a 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.
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, two columns should be filled with particular values. To a newly created additional signal one
should make
job_todo
equal to
device_status
and
tag_job_todo
equal to
communication_status
.
Communication error status is set when a predefined count of messages (three by default, defined
in
poll_retry_count
column) fail to be received or are considered invalid.
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1.4.0 | ELSETA | Modbus
