CS-3G Digital Transceiver Kits (including the COM111 modem)
34
modem. Many modems have a default timeout of 30 seconds which may not
be long enough for connection over busy or distant networks. Adding
“S7=45” to the end of the modem initialisation string sets the timeout at 45
seconds and Loggernet will increase its own timeouts to suit. Normally you
are allowed to increase this setting up to 60 (seconds) if a call is seen to take
such a long time to connect. Setting too long a delay may, however, slow
down the retry sequence in the event of a failed connection.
To add this setting, use the Phone modem option in Loggernet where you can
edit the modem initialisation string in the setup screen. You cannot amend
this setting for a TAPI interface in a way that Loggernet can also detect.
Campbell Scientific communications software includes other time-outs
which are used to check for hung communications links. These are set to be
quite short for land-based modems, as the response to any command sent
from the PC should be almost instantaneous. A link to a remote COM111
modem normally passes through several nodes of a digital data network, all
of which can add delays. Usually these delays are less than one second and
so have no effect on the communications process. However, on a long
distance call or busy network, the built in time-out delay for the PC software
may need to be extended.
In Loggernet or PC400W, a station-specific delay can be added by
increasing the time in the ‘Extra response time’ box shown on the data
logger hardware set-up screen. This delay is entered in seconds. Loggernet
has a default, built-in, delay of ~2 seconds, which is adequate for use with
the COM111 modem in most cases. On busy networks, or with poor
communication links, a value of 3 or even 4 may be required, to allow the
error correction process to function.
Where a COM111 modem is used as the base station modem, it is advisable
to increase these delays even more by perhaps another 1-2 seconds, if
communications are proving to be unreliable.
Entering too large a delay can slow down the entire
communications process, and so an optimal value should be
determined by trial and error.
10. Preventing COM111 connection problems
The COM111 monitors its connection state to the network. The modem will
reset itself if it gets deregistered and will often re-establish its connection to
the network. Generally, this will provide a reliable connection long term.
However, there is a very small chance that the COM111 modem could crash
as a result of electrical spikes on the power supply or local electrical storms.
If the COM111 modem is powered continuously, such a crash will often
prevent it answering an incoming call again until it has been reset.
To help overcome these problems, the data logger can be programmed to
either reset the COM111 modem by sending it commands (see Appendix A)
or to shut down the COM111 modem at regular intervals using a power
switch as described in Appendix A. This will save power, but also ensures
that the COM111 modem is reset into a known state and make it re-register
itself on the network. To be sure that the COM111 modem resets properly, it
must be turned off for at least 10 seconds. How often the COM111 modem is
turned off will depend on the normal calling schedule, considerations of
power consumption and of how long you could afford to lose communication
with the COM111 modem if it did go offline.
NOTE