Hach 911, User Manual

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SCADA-Modbus® System Guidelines
D.11 Response
The 940 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 940. 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.
D.12 940 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 is
given to reply to Modbus queries via this serial port.
Communication Handshaking
The 940 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 Equipment (DTE) must assert and hold
high the DTR line of the DB9 connector (DSR of meter). The 940
Flow Meter does not
support RTS/CTS hardware handshaking. Note that DTE must be capable of handling a
12-second maximum response lag.
Table 19 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>
Pin Description Pin Description Pin Description
Pin 1 Data Carrier Detect (DCD)
1
Pin 4 Data Terminal Ready (DTR) Pin 7 Request to Send (RTS)
Pin 2 Received Data (RD) Pin 5 Signal Ground (SG) Pin 8 Clear to Send (CTS)
Pin 3 Transmitted Data (TD) Pin 6 Data Set Ready (DSR) Pin 9 Ring Indicator
1
Not used.