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SCADA-Modbus® System Guidelines
Appendix E
Response
The 980 Flow Meter responds with the following transmission reflecting a level
reading of 15.0 inches:
The flow meter response echoes the address and function code, which
indicates that no problems exist in the communication between the master
and 980. The ‘Byte Count’ field specifies how many 8-bit data items are being
returned in the data field. With Modbus ASCII, this is one-half the actual count
of ASCII characters transmitted in the data portion of the response.
The contents of 40007 are shown as two byte values of 00 00 hex, and the
contents of register 40008 are shown as two byte values 41 70 hex.
Together, these values represent the floating point IEEE representation of
the level status.
980 Flow Meter Response Time
As a result of time lags associated with data acquisition, instrumentation
could conceivably take up to 12 seconds to respond to a SCADA RS232
request. Therefore, the SCADA system must be designed to accommodate
this potential communication lag. For example, in a Wonderware
®
application
running a Modbus ASCII DDE server, the com port reply time-out must be set
to 12 seconds. This is the amount of time that the meter will be given to reply
to Modbus queries via this serial port.
Communication Handshaking
The 980 Flow Meter contains minimal communication handshaking. For the
meter to identify an RS232 connection from an outside source, and to keep
the RS232 hardware active once communicating, the Data Terminal
Table 38 Channel Query to Read Level (Example)
Start ‘:’
Slave Address 01
Function 03
Starting Address High 00
Starting Address Low 06
No. of Registers High 00
No. of Registers Low 02
LRC F4
Stop <CR><LF>
Table 39 Transmission Response that Reflects a 15 in. Level Reading
Start ‘:’
Slave Address 01
Function 03
Byte Count 04
Data High 00
Data Low 00
Data High 41
Data Low 70
LRC 47
Stop <CR><LF>