Measurement Computing BTH-1208LS - Manual (Page 17)

BTH-1208LS User's Guide Functional Details
17
Digital I/O
You can connect up to eight digital I/O lines to DIO0 through DIO7 . When configured for input, the digital
terminals can detect the state of any TTL-level input.
Digital input voltage ranges of up to 0 to 5.5 V are permitted, with thresholds of 0.8 V (low) and 2.0 V (high).
Each DIO channel is an open-drain, which can sink up to 50 mA for direct drive applications when used as an
output.
Figure 5 shows an example of a typical digital output connection.
Figure 5. Digital output connection (typical)
External pull-up capability
Inputs are pulled high by default to 3.3 V through 47.5
kΩ resistors on the circuit board. The pull-up voltage is
common to all 47.5
kΩ resistors.
You can place an external pull-up resistor on any of the DIO bits in order to limit source current to less than
50 mA., which requires a 100
Ω resistor minimum. A 2 kΩ or 5 kΩ resistor would be typically used, allowing
for a 1 mA current
You can also use an external pull-up resistor to pull the DIO bit up to a voltage that exceeds the internal 3.3 V
pull-up voltage (5.5 V maximum).
When using external pull-up resistors, be aware that the internal resistors cause a slight voltage shift to digital
lines in the on state as various lines change between the on/off states.