Campbell Scientific SDS122 Two-Way Serial Data Switch - Manual (Page 9)

SDS-122 SERIAL DATA SWITCH
5
result in errors on low-level analogue
measurements. The SDS-122 can be set to
provide opto-isolation to prevent such ground
loops, but this mode of operation is not
suitable for all RS232 devices, either because
the device needs to source power from the
datalogger or because it is not able to provide
power to the output electronics of the SDS-
122.
WARNING: The opto-isolation provided
by the SDS-122 is not designed, nor
should it be used, for the purpose of
providing a safety protection barrier.
Internal protection devices will cause a
breakdown of the isolation if the potential
difference between the datalogger and
RS232 ground exceeds 47V.
To work in isolated mode, the RS232 device
must provide power to the SDS-122 by
holding at least one of the input handshaking
lines at a positive voltage during
communications (pins 4 or 20 in DCE mode,
pins 5 or 6 in DTE mode). The voltage input to
these lines must be <9V or be current limited
to an effective source impedance of 1Kohm. If
a suitable handshaking line is not available, a
power source (6-20V) can be connected to pin
15 (referenced to pin 7G).
In non-isolated mode, the SDS-122 can
provide power from the datalogger via the
handshaking lines, to power external
interfaces. See details on jumpers PL50,
PL51 and PL56 below for information on the
power available. When this mode is enabled,
no isolation is provided by the SDS-122.
Some devices, such as the RAD-SRM short
haul modem, and most telephone modems
provide their own isolation barrier. Care
should be taken when selecting third party
devices to ensure that they provide some
form of barrier to prevent ground loops.
4.1 JUMPER CONFIGURATION
It is envisaged that most applications will be
with a telephone modem connected to Port B
and a personal computer connected directly to
Port A. The SDS-122, as supplied, has its
jumpers set to this default configuration.
The locations of the jumpers on the SDS-122
circuit board are shown in Figure 5.
The description of each jumper and its default
setting is given in Section 4.3. Note that some
jumpers work in combination to achieve the
state required.
4.2 SPECIAL MODES OF OPERATION
The SDS-122 contains a programmable chip,
which, in conjunction with the jumper settings
(see Section 4) determines the way in which
the ports are selected. Currently, the program
allows one or other of the two communication
ports to be connected to the datalogger.
There is no state where neither port is
connected. For special applications, the
program can be modified to provide other
modes of operation. Please contact Campbell
Scientific for more details.