Xylem H-4191 - Manual (Page 4)

1-2 Operation
H-4191
Figure 4 Monitor a data recorder Figure 3 Interface between a PLC and
two SDI-12 Sensors
1.1 Applications
Figure 1 shows a monitoring application where a laptop computer is used to monitor the SDI-12
communication between a data recorder and two SDI-12 sensors. Figure 2 shows the connections
when using the H-4191 as a gateway between a PLC and two SDI-12 sensors.
1.2 Operation
The H-4191 has a microprocessor with both RS-232 and SDI-12 interface ports. The H-4191
normally operates as a SDI-12 master device: it issues SDI-12 commands and collects responses.
Under certain conditions the H-4191 can respond to host commands itself as a “pseudo” SDI-12
sensor (see Section 1.7).
The RS-232 interface port is connected to a personal computer or other “host” device. The H-
4191 provides two way communication between the RS-232 host and the SDI-12 bus. During
normal operation, the host device sends an address together with a command to a SDI-12 sensor.
The sensor then replies with a "response".