Xylem H-4191 - Manual (Page 5)

H-4191
Operation 1-3
1.2.1 Commands
The RS-232 host interface is fixed at 9600 baud. The H-4191 inputs SDI-12 commands (ASCII
characters) from the host and places the character string in a buffer. Up to 60 characters can be
buffered. The command can be sent at any speed, even hand-typed commands will work. The
backspace character (08h) is supported and deletes the previous character in the buffer. Note: the
SDI-12 protocol requires upper case characters. When the H-4191 detects the “!” SDI-12
command terminator character the H-4191 generates a SDI-12 break and transmits the contents
of the buffer to the SDI-12 port in a contiguous burst with the proper parity and bus timing. The
H-4191 also accepts the character (ENTER Key) as a command terminator and substitutes
the character with the required “!” SDI-12 command terminator. This makes the H-4191
easier to use for computer users who more commonly complete a message with the ENTER Key.
If you use the ENTER Key to terminate the message, do not type the “!” character.
The H-4191 automatically generates SDI-12 compliant parity and timing. After transmitting the
command, the H-4191 waits for up to 40mS for a sensor response. If the sensor response fails to
arrive, the H-4191 performs a retry sequence specified by the SDI-12 protocol. The H-4191
generates up to four breaks with up to four retries per break (16 retries total).
1.2.2 Sensor Response
The response from a SDI-12 sensor is input by the H-4191 at 1200 baud and output to the RS-
232 port at 9600 baud. Any sensor service requests or other SDI-12 bus traffic are also forwarded
from the SDI-12 port to the RS-232 port. This feature allows the H-4191 to be used to passively
monitor the activity on a SDI-12 bus between a data recorder and its sensors.