UNIDATA
Unidata Manual 6293 - Precision Water Level Instruments Issue 2.1
18
In the commands:
•
“a” is the sensor address.
•
“!” terminates the command.
•
“<CR><LF>” terminates the response from the instrument.
When requested via sdi-12 the 6541B will return the water level value shown on the
LCD. The returned ASCII string will exclude any leading zeros. The exception to
this is where the display has a 0 to the left of the decimal point. Hence if the LCD is
displaying “000.09” the value returned is “0.09”.
4.6 Current Loop Interface (4-20mA).
The 6541B Water Level Instrument is available with a current loop output option.
This provides a 4 to 20mA current loop output which is driven by a 16 bit converter
on the main 6541B PCB.
The output loop compliance is from 0V
Î
V
loop supply
-2.5V .
The zero level (4mA) and span level (20mA) points are programmable and once set
are stored in non volatile memory. If these points require alteration, simply repeat the
set up process entering the revised zero and span levels.
4.6.1 Wiring Connections.
The 4/20mA output wiring shall be configured as below:
•
The power for the current loop is to be provided from the data logger end of the
loop.
•
This voltage shall not exceed 18V.
•
The output loop compliance is 0
Æ
(Vcc-2.5V)
It should be noted that the three current loop connection terminals on the 6541B
Water Level Instrument are electrically isolated from the remainder of the water level
instrument circuitry via optocouplers.
