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SCADA-Modbus® System Guidelines
Appendix E
in the address field of the message frame to let the master know which slave
is responding.
The device address of the 980 Flow Meter is set via the front keypad in the
980 Communications menu.
1. From the Main Menu select
OPTIONS > ADVANCED OPTIONS >
COMMUNICATIONS SETUP > MODBUS SETUP
2. Enter a value between 0 and 247.
Function Field
The function code field of an ASCII message frame, ranging from 1 to 255
decimals, consists of two characters that represent the type of action the
master is requesting from the slave. Of these functions, the 980 Flow Meter
currently supports function 3 (Read Holding Registers). When a message is
sent from the master to a slave device, the function field tells the slave what
kind of action to perform. For example, this may include reading the channel
values of Level and Velocity. When the slave responds to the master, it echoes
the function code field to indicate a normal response. In the event of an error,
such as parity error, LRC error, or a request that cannot be handled, the slave
will not respond and the master will eventually process a time-out condition.
Data Field
The data field of an ASCII message frame consists of n pairs of ASCII
characters that represent data sent to or from a slave device (flow meter). The
data field contained in the master request contains additional information that
is required by the slave before any action takes place. This may include
channel register addresses, the number of registers to read, and the actual
byte count in the data field. For example, if a master requests that the flow
meter read the current status of a group of channels (function code 03), the
data field specifies the starting register and how many registers are to be
read. If no error occurs, the data field of the response from the meter to the
master contains the data requested.
LRC Field
The LRC field of an ASCII message frame consists of two ASCII characters
that provide an additional level of error checking to verify the integrity of the
communication media. The LRC field is one byte that contains an 8-bit binary
value. The LRC value is calculated by the transmitting device, which appends
the LRC to the end of the message. The receiving device recalculates the
LRC and compares it against the LRC value of the incoming message. If the
two values are not equal, an error condition occurs. The LRC is calculated by
adding together successive 8-bit bytes of the message, discarding any
carries, and then complementing the result. The LRC is calculated by
summing all values in the ASCII message except for the leading ‘colon’ and
ending <CR><LF>.
11:00 AM 21 - APR - 01 MODEM SETUP
ACCEPT
DEVICE ADDRESS:
RETURN
1 CLEAR
ENTRY
ENTER 0-247