framing, or overrun error is detected. These errors are only flagged for RS-232/RS-485/RS-422
data coming into the SDM-SIO2R.
These types of errors are usually caused by a connection issue or sometimes a mismatch of the
interface type, baud rate, or data format (parity, etc.).
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:
l
Mismatched parity settings will often show many valid characters interspersed with invalid
characters or “?” which the SDM-SIO2R will store if there are parity errors.
l
Incorrect connections resulting in inverted signals can also result in some valid characters
being received, but usually with no normal data.
l
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-SIO2R is connected to and the data logger. 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 labeling 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 data logger and serial device is correct, it is
possible to check what the data logger thinks it is transmitting and the data it is receiving by
connecting to the data logger 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-SIO2R 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 data logger
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
SDM-SIO2R Serial Input/Output Module with Switched Sensor Power
38
