114
SCADA-Modbus
®
System Guidelines
For example, to read the level channel of the 950 Flow Meter, the query must be as
indicated in
The master queries the flow meter using a Read Holding Registers request, which
implies a 4XXXX register reference, to slave device address 01. The message requests
data from holding registers 40007–40008 to obtain the level information, which requires
two registers to store the floating point value. Note that registers are referenced from zero
in the data field.
Response
The 950 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 950. 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.
950 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
Table 45 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 46 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>
Содержание Sigma 950
Страница 2: ......
Страница 6: ...4...
Страница 20: ...18 Introduction...
Страница 34: ...32 Basic Programming Setup...
Страница 50: ...48 Sensor Installation...
Страница 82: ...80 Maintenance...
Страница 83: ...81 Appendix A Program Flow Charts Figure 22 Overview of Basic Program Menus...
Страница 84: ...82 Program Flow Charts Figure 23 Setup Flow Chart...
Страница 85: ...83 Program Flow Charts Figure 24 Options Flow Chart...
Страница 86: ...84 Program Flow Charts Figure 25 Alarms Menus Flow Chart...
Страница 87: ...85 Program Flow Charts Figure 26 Calibration Menus Flow Chart Page 1...
Страница 88: ...86 Program Flow Charts...
Страница 102: ...100 Programming Features...
Страница 106: ...104 Primary Devices Head Measurement Locations...
Страница 126: ...124 SCADA Modbus System Guidelines...