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App Notes

Software & Driver


2.4 - D0-D15

Connections to 16 of the LabJack’s 20 digital I/O are made at the DB25 connector, and are referred to as D0-D15.  These 16 lines have no overvoltage/short-circuit protection, and can sink or source up to 25 mA each (total sink or source current of 200 mA max for all 16).  This allows the D pins to be used to directly control some relays.  All digital I/O are CMOS output and TTL input except for D13-D15, which are Schmitt trigger input.  Each D pin has a 1 MΩ resistor connected to ground.

These digital I/O can detect the state of a switch using the same circuit shown in Figure 2-5.

All digital I/O are set to input on power-up or reset.

Table 2.4.1. DB25 Pinouts

1: D0 6: D5 11: +5V 16: GND 21: D11
2: D1 7: D6 12: +5V 17: GND 22: D12
3: D2 8: D7 13: +5V 18: D8 23: D13
4: D3 9: NC 14: GND 19: D9 24: D14
5: D4 10: +5V 15: GND 20: D10 25: D15

Figure 2.4.2. Standard DB25 pin numbers looking at the female connector on the U12

Because the D pins have no overvoltage/short-circuit protection, the user must be careful to avoid damage.  A series resistor can provide substantial protection for these pins (see the CB25 datasheet).  The following are examples of things that could damage a D pin and/or the entire LabJack:

  • Shorting a high output to ground (or any potential other than +5V).
  • Shorting a low output to a nonzero voltage (such as +5V).
  • Exceeding the voltage limits specified in Appendix A.


The easy functions EDigitalIn or EDigitalOut are used to read or set the state of one digital line, and both take up to 20 ms to execute.

The functions AOUpdate and DigitalIO are used to set the direction, set the state, and/or read the state, of each D pin.  In addition, DigitalIO also returns the current state of the direction and output registers.  Both of these functions take up to 20 ms to execute, providing a maximum update rate of about 50 Hz per pin.