| Circuit for driving digital outputs |
DataTakers and the ADAM 4050 have the digital outputs needed to drive external devices, such as the Sable gas stream multiplexers. However, attaching these devices is sometimes trickier than with a Sable UI2 or UI3. This is because the Sable multiplexers require digital high (+5 volts) on their control lines, and DataTakers and the ADAM 4050 use open-collector outputs. Open collector outputs can 'drive' a digital high signal to digital low (~ 0 volts) on command: when active, they are low-resistance paths to ground; when inactive they have very high resistance. Thus they don't provide positive voltage directly.|
The DataTaker's digital channels incorporate pull-up resistors (to ~3.3 to ~5 V) and this may provide sufficient voltage to drive directly-connected Sable multiplexers. However, on many DataTakers (the model seems to be irrelevant), you need to provide the necessary positive voltage on the three digital output lines (bits 1, 2, and 3). You will definitely have to do this with the ADAM 4050.
The simplest way to do this is with the DataTaker's DC power source terminals (these provide +5 V on the DT500 and + 12V on the DT800), along with some 10K resistors. With an ADAM 4050, use the DC power supply for the unit itself, provided it doesn't much exceed 12 V (if it does, use a voltage divider to reduce it to about +5 to +8 volts). For more detail on the ADAM 4050, see this page.
Once you have a voltage source for each of the three digital leads, connect them as shown on this diagram:
Note that on the ADAM 4050, the output lines are DO0, DO1, and DO2 instead of 1, 2, and 3.
This circuit keeps the digital output lines high unless they are 'pulled' low by activating the open collectors. LabHelper understands this and controls the signals accordingly.