GPIO-24 adapter has weak pull-up resistors on some of the 24 pins. Weak pull-up resistors are used to ensure that adapter digital inputs assume a high logic value (logical one) while external devices are disconnected.
The USB-I/O adapter pull-ups are automatically turned off when pins are configured as digital outputs or analog inputs. In other configurations the pull-up resistors are software controlled in GPIO-24 adapter.
All the pins with pull-up resistors are divided into two groups. The first group consists of A0, A1, A2, A3, A4, B0, B1, B4 pins. The second group consists of A6, A7, B7, C0, C7 pins. The weak pull-ups can be turned on/off only for the whole group of pins. It is impossible to turn on/off pull-ups for individual pins.
To enable/disable the weak pull-ups for any group of pins, use GPIO_SET_PULL_UPS command. The state (on/off) of the weak pull-ups can be retrieved withGPIO_GET_PULL_UPS command.
GPIO-24 adapter inputs have very high resistance. It is a positive feature because they will not affect the circuit where they are connected. However, it also means that unconnected inputs can easily pick up electrical noise and rapidly change between high and low states in an unpredictable way. This will make the pulse and frequency counters count pulses caused by such bouncing. It is advisable to turn on the weak pull-up resistors to prevent such problems.