Hanna Instruments HI 504 - Manual (Page 41)

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The modem connection (both with HI504900 and HI504902)
allows the user to ask the controller from a remote position
about its status, measurements and to change some param-
eters, while for receiving alarm indications it is necessary to
install the HI504900 cellular module.
Many devices can be monitored through a remote modem
connection, simply connecting all the devices and the modem
or cellular module to the same RS485 network.
Only one device has to be configured through O.31 to an-
swer to modem calls. That device will be the one controlling
the modem or the cellular module. To avoid conflicts, the
following must never be done:
• put more than one modem or cellular module in the same
RS485 network;
• set O.31 to “On” in more than one device in the same
RS485 network;
• set P.00 to “CELL” in more than one device in the same
RS485 network (SMSs can not be sent from more than one
HI504);
• set O.31 to “On” in one device and P.00 to “CELL” in
another one within the same RS485 network;
• put a PC monitor within the RS485 network.
Provided the above, any Hanna instrument with RS485 port
can be attached to the network and monitored remotely.
When making a call, after the data connection is established,
the “NNPWD...” command (where “NN” is the address of
the device controlling the modem, i.e. the one with O.31 set
to “On”) must be issued by the remote computer within 15
seconds. When the data connection is up, the RS485 protocol
for a remote connection is all the same as for a local network
(see the previous section). An automatic disconnection takes
place if no character is received in the RS485 network in 3-4
minutes.
When a modem connection is up, the cellular module does
not send any SMS. If an error for which the SMS sending is
enabled is active after closing the modem connection, an
alarm SMS will be sent by the HI 504 after disconnection.