Hanna Instruments HI 504901 - Manual (Page 26)

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<CAN> answer is sent back if the required channel is not
configured in the instrument or if the baud rate does not match
the value set in the interface memory for the corresponding
channel. When command is correctly recognized, the local PC
and the selected remote instrument are connected directly and
everything is works like a direct point-to-point connection (if the
delay issues are excluded). No need to change baud rate value
on PC side as modem buffers handle automatically the baud
rate difference (remember that the baud rate must be kept at
9600 bps in remote communication on PC).
To change RS485/RS232 channels for selecting another instru-
ment without interrupting the call, the application software will
issue the “DSC<CR>” command three times (9600 bps baud
rate): after the third reception of this command, the phone in-
terface will answer with “+++” and then, after several seconds,
with <ACK>. If the syntax is wrong, no answer will be sent
back by the phone interface. After that disconnection, another
channel can be connected as described above (sending
“CNR<CR>” command at 9600 bps baud rate).
Local PC connection closure
The application software can close a local PC connection pull-
ing down the DTR line of the serial port at any moment.
Remote PC connection closure
The application software can close a remote PC connection
hanging up the data call sending to the modem “+++” and
then, after “OK” reception, “ATH<CR>” commands at any
moment.
An automatic disconnection on HI504901 side takes place if
no character is received in 3 minutes.
select which channel and baud rate the interface uses for com-
municating with the target device (i.e. the baud rate used by
interface during polling procedure and set during last interface
configuration). To do this, the CNR<CR> command with the
proper values (channel N and rate R) has to be sent to cellular
phone interface. For example, if target device is attached on
RS232-1 port and its communication rate is 2400 bps, then the
command C0B<CR> must be issued. The cellular phone in-
terface will send back <ACK> if the command is correctly
recognized, otherwise <NAK>. <CAN> is sent back if the
required channel is not configured in the instrument or if the
baud rate does not match the value set in the interface memory
for the corresponding channel. When command is correctly
recognized, baud rate in the local PC and the selected instru-
ment are connected directly and everything works like a direct
point-to-point connection (if we exclude the delay issues).
However, it is also possible to change RS232/RS485 channel
for selecting another instrument without interrupting the con-
nection. If selection of another instrument is requested and one
connection is already in place, the application software must
issue the “DSC<CR>” command three times at the baud rate
of the present channel (channel disconnection). HI504901
answers with <ACK> if the command sequence is correct, but
no answer will be sent back by the phone interface if the syntax
is wrong.
After that disconnection, another channel can be connected as
described above (using the “CNR<CR>” command at 9600
bps baud rate).
Remote channel selection
Also in remote communication session the application software
needs to select a channel, but the procedure is a bit different
from the local one. Before communicating with one of the in-
struments attached to HI504901, the application software on
remote PC needs to select which channel and baud rate are
used by the interface for communicating with the target device.
To do this, the CNR<CR> command with the proper values
(channel N and rate R) has to be sent to cellular interface. The
phone interface will send back “+++” (the “+++” is needed
to return temporarily to command mode in the cell phone to
change baud rate) and then, after several seconds, <ACK> if
the command is correctly recognized, otherwise <NAK>.