D-1
Appendix D. Using the TC35T with
Storage Modules
This appendix explains problems that may occur when using the TC35T to communicate with
storage modules connected to the datalogger and provides appropriate advice on how to work
around such problems.
D.1 Introduction
If a datalogger in a remote station is connected to an TC35T and a storage module
(SM192/SM716/SM4M or SM16M) at the same time, problems can occur when
trying to read data from the storage module owing to transmission delays in the
GSM link. Connections that use links that are not subject to significant delays are
not affected.
D.2 The Problem Explained
The problem is caused by delays that can occur when transmitting data across a
remote network. PC208W uses the 1M command to switch communication to the
storage module so that the module can communicate with the PC via the
datalogger. The module responds to the command in the correct way, and starts to
communicate with the PC, but if its response is delayed a little, PC208W misses
the response and writes an error message in the warning log.
The extra response delay parameter in PC208W is normally used to compensate
for such delays, but in this case it is not functional. If it is set at a value below
2000ms, then the program will eventually re-send the 1M command and data will
be collected correctly. However, because of the original failure, there will be an
extra delay of around 20 seconds before the data starts to be collected. If the extra
delay time is set to more than 2000ms, the call will always fail because PC208W
will be unable to establish a connection to the datalogger within its maximum
allocated dialling time, and so it hangs up.
D.3 Solutions
1.
One solution is, therefore, to ensure that an extra response time of slightly less
than 2000ms is used. The actual extra response time you set can be
determined by trial and error for the communication link being used.
However, even this may not work in all circumstances – for example when
your GSM link is such that normal delays approaching or exceeding 2000ms
are experienced, so that the extra response needs to be greater than 2000ms to
ensure normal communications with the datalogger.
2.
An alternative solution is to insert a ‘Generic’ modem between the base
modem and the datalogger in the network setup. Tick the ‘Half Duplex’ box
and check ‘Make RTS active’. Attach the storage module to that datalogger.
Set the telephone number to call in the Generic modem hardware option, and
leave the number in the datalogger hardware blank. This has the effect of
forcing the link into half-duplex mode. While this does not fully avoid the
problem – as there will still be an error message written to the log – it has the
effect of speeding up the process so that re-connection to the storage module
is made in 7 seconds rather than 20 seconds.
