Measurement Computing USB-2408 Series - Manual (Page 19)

USB-2408 Series User's Guide Functional Details
19
External pull-up/pull-down capability
You can also place an external pull-up resistor on any of the DIO bits, which enables you to pull the DIO bit up
to a voltage that exceeds the internal +5 V pull-up voltage.
When using external pull-up resistors, be aware of the following:
 You should either remove the J6 jumper, or store it by attaching it to one of the three pins.
 When using external pull-up resistors, be aware that the internal resistors cause a slight voltage impedance
shift to digital lines in the "on" state as various lines change between the on/off states.
Figure 13. Digital I/O external resistor configuration
Counter input
The CTR0 and CTR1 terminals are 32-bit event counters that can accept frequency inputs up to 1 MHz. The
internal counter increments when the TTL levels transition from low to high.
Ground
The analog ground ( AGND ) terminals provide a common ground for all analog channels.
The digital ground ( GND ) terminals provide a common ground for the digital and counter channels and the
power terminal.