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SDM-SIO1A and SDM-SIO4A Serial Input/Output Modules
8.2
Maintenance
26
Some confusion can be caused in debugging such problems because it is
possible for the communication to partly work even with such mismatches, for
instance:
Mismatched parity settings will often show many valid characters
•
interspersed with invalid characters or "?" which the SDM-SIO1A
will store if there are parity errors.
•
Incorrect connections resulting in inverted signals can also result in
some valid characters being received, but usually with no normal data.
An RS-485 receiver can often read data transmitted at RS-232 levels,
•
although the reverse will not work.
Where you see some serial activity which is invalid, check the documentation
and configuration of both the device the SDM-SIO1A is connected to and the
datalogger. Also, check the wiring carefully.
In the case of RS-485/RS-422, there is also a big risk of confusion being
caused by there being several different ways of labelling the transmit and
receive lines. Some devices may even have reversed meaning for some lines—
for example, A and B are sometimes used and transposed. Generally, for these
lines it is safe to try swapping them around if there is doubt as to whether the
same terminology is being used.
If you are sure the configuration and wiring of the datalogger and serial device
is correct, it is possible to check what the datalogger thinks it is transmitting
and the data it is receiving by connecting to the datalogger with a computer and
entering the terminal mode. You can then use the "W" command to "sniff" data
passing to and fro through any serial port including SDM-SIO1A ports. The
data can be displayed in ASCII or binary (HEX) modes. This is a powerful tool
for checking programming errors and the response of remote devices. See the
datalogger manual for more information on terminal mode.
Either during setup or at some point later, it is possible that erratic
communication errors may be seen. The normal cause of this will be noise
pickup, especially for long cable runs. The likelihood of such errors can be
reduced by using screened cable, making sure there is a ground connection
between the two systems, and installing termination resistors for RS-485/
RS-422 links. It is worth noting that textbooks often say termination resistors
are only needed for long cables at very high baud rates, which is true when
considering noise caused by reflections. However, even a single termination
resistor at one end of the link can suppress induced noise caused by pickup
from other sources such as nearby mains cables. As a general rule, if you have
a noise issue, it is worth trying to add a cheap 100-120 Ω resistor across
RS-485 lines.
No regular maintenance is needed for the SDM-SIO1A; however, occasionally
operating system upgrades are released to add enhanced features or to correct
bugs. The operating system is normally published in the Support section of our
website at www.campbellsci.eu.
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