Campbell Scientific Sentek EnviroSCAN EasyAG SDI-12 Manual v 3.4 - Manual (Page 11)

EnviroSMART & EasyAG SDI-12 Probe Manual
Copyright © 1991 – 2003 Sentek Pty Ltd All rights reserved Page 6
Figure 2: EnviroSMART® and EasyAG® SDI-12 probe interface communication power sequence
Timing
Figure 3 shows voltage levels on the SDI-12 line during transmission of an Acknowledge Active
command (a!) using the “?” (wildcard) address.
After the break condition has been sent to the EnviroSMART or EasyAG SDI-12 probe interface by
holding the line high (above 3.5V, spacing) for 12mS, the line is held low (below 1 V, marking) for a
minimum of 8.3mS before the first character is transmitted.
Each character transmitted consists of a Start Bit (spacing), followed by 7 data bits, followed by an even
parity bit, followed by a stop bit (marking). Characters are transmitted least significant bit (LSB) first
with each bit in the character being 0.833mS wide.
EnviroSMART or
EasyAG SDI-12 probe
interface in sleep mode
15 second timeout
process command
(including response)
break
break
no
yes
sample sensors
sleep mode
standby mode
standby mode
sampling mode
250
µ
A
66
m
A
66
m
A
100
m
A
(Moisture)
130mA
(Salinity)
valid
communication
received?