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Sable Systems makes very nice 8-channel
multiplexers to switch airflows (4-channel versions have also been produced). These work quite well with
the LabHelper software, but for maximal effectiveness there are a
few points to keep in mind:
Because the Sable units are controlled with 3-bit word coding (you need to use this mode even if the multiplexer supports ASCII control), only one device
(i.e., one solenoid valve) can be switched on at a given time. Thus you might
expect that you could only monitor one animal at a time even if you have
more than one analyzer available. However, the design of the Sable
multiplexers is quite clever: each solenoid switches TWO
air streams. There are 8 connectors that are switched through the
'IN' port, and another 8 connectors that are switched through the
'OUT' port (these labels may differ on units of different vintage). These two sets of plumbing are independent of each other (except that a given valve's IN and OUT connections are both activated or
deactivated simultaneously). Therefore...... you can sample
from two respirometry chambers simultaneously as long as
both are attached to the same valve on the multiplexer (one to the
IN port and one to the OUT port).
It's
quite straightforward to connect just two animals. But
why have an 8 (or rather, 16) channel multiplexer if you can only do 2 animals
at a time? In theory, you could connect up to 14 animals and 2 references
air streams and, in periodic references mode, sequentially sample
from them two at a time. This has the potential for creating serious
confusion during data analysis unless you take steps to insure that LabHelper
records data only for channels that are actually being sampled by the analyzer.
In 'one line per device' mode, LabHelper switches the two valves
independently and each channel is associated with a different valve,
so it knows which channels are active and which are not. It can then
enter values of zero into channels that are not being sampled at a particular
time. However, when using the Sable multiplexer with two channels
connected through a SINGLE valve, LabHelper has no direct way of
automatically identifying the SECOND channel associated that valve.
You need to provide this information using the zero linking option.
Click the 'extra zero linking' button, and when you exit the EXTERNAL
DEVICE CONTROL window, the program will open a window showing the directly
switched channels. Click the appropriate 'additional channel'
buttons to 'zero link' other channels.
In this example, there are seven data channels. Four of these (#s
1, 5, 6, and 7) record oxygen consumption of different birds.
A dual-channel oxygen analyzer is in use, so we can measure two birds simultaneously.
Note that the hardware A-D inputs for the two 'halves' of the analyzer
are 1 (connected to data channels 1 and 6) and 6 (connected
to data channels 5 and 7). Finally, two valves are in
use. Valve 1 handles airflow from birds 1 and 2 (i.e., data
channels 1 and 5). Valve 2 handles airflow from birds 3 and
4 (i.e., data channels 6 and 7).
The primary data channels (in other words, the ones LabHelper
knows are linked to valves) are 1 and 6. When the program switches
from Valve 1 to Valve 2, it automatically zeroes channel 1 and activates
channel 6. But without further input it does not know that Valve 1
also switches data channel 5 and Valve 2 also switches data channel 7.
When valve 1 is on, the oxygen analyzer is reading gas streams from birds
1 (via one sensor) and 2 (via the other sensor). When valve 2 is on,
the analyzer is reading gas streams from birds 3 (via one sensor) and 4
(via the other sensor). Thus when valve 1 is on, both data
channels 6 and 7 should be set to zero. Similarly, when valve 2 is
on, both data channels 1 and 5 should be set to zero. This
is indicated by selecting the appropriate buttons in the "additional
channel" rows.
Cable connections The Sable multiplexers expect their
control signals to be coded in "3-bit word" mode, not "one
line per device" mode. Select this option in the top right corner
of the EXTERNAL DEVICE CONTROL window. You may also need to make
sure the wiring is correct:
- For the Sable UI2 and UI3: New-production multiplexers are shipped with a double-ended cable that plugs into both the UI2 / UI3 and the multiplexer.
Older versions came with a single-ended cable with bare wires on one end. Connect the wires to the corresponding outputs on the small 'daughter board' that comes with the main UI2 / UI3 unit.
Find the connector handling the lowest bit (bit 0) on the Sable daughterboard and connect
it to bit 0 on the multiplexer, and do the same for the other wires.
Then connect the Sable daughterboard's digital ground to the digital ground on the multiplexer. [All this may seem complicated,
but you only have to do it once.]
| NOTE: although the cable has standard DB-15 connectors (male and female), the multiplexer will not work if you try to use an off-the-shelf DB-15 cable!!
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- For the ADC-1 and DataTaker, you'll have to modify the double-ended cable that comes with new-production multiplexers, or add a 'converter' to the end that doesn't fit directly into the multiplexer. Then perform the same operation describe above.
In other words, find the connectors for the digital outputs on the A-D
device, using the instruction manual. Then connect the Sable's bit
0 wire to the lowest-numbered connector on the A-D device, and so forth.
- For the ADAM-4050: The simplest options are to either build a custom cable with the appropriate connections, or to use the cable that comes with the multiplexer, and make an appropriate matching male DB-15 connector attached to the ADAM-4050. NOTE: you must check the connections on the male end of the Sable cable -- they ARE NOT the same as on the female DB-15 on the multiplexer.
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