Response; Flow Meter Response Time - Sigma 980 Operating And Maintenance Manual

Appendix E

Response

980 Flow Meter Response Time

Page 138
SCADA-Modbus® System Guidelines
Table 38 Channel Query to Read Level (Example)
Start
Slave Address
Function
Starting Address High
Starting Address Low
No. of Registers High
No. of Registers Low
LRC
Stop
The 980 Flow Meter responds with the following transmission reflecting a level
reading of 15.0 inches:
Table 39 Transmission Response that Reflects a 15 in. Level Reading
Start
Slave Address
Function
Byte Count
Data High
Data Low
Data High
Data Low
LRC
Stop
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.
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
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
':'
01
03
00
06
00
02
F4
<CR><LF>
':'
01
03
04
00
00
41
70
47
<CR><LF>
®
application