Four open collector outputs. Transistors are rated for the maximum voltage of 24V and the maximum per-channel current of 0.5A. Note that the maximum current should not be exceeded even at lower voltages. Do not apply negative voltage! To open a transistor, set the corresponding control line LOW. When left unconnected, control lines default to HIGH (and, hence, transistors will be closed). Combine this Tibbit with terminal block devices — #20 (nine terminal blocks) or #21 (four terminal blocks). Note that #21 doesn't provide the ground line, and using this Tibbit requires the ground line between the outside world and your system. Hence, if you use Tibbit #21 you will need to 'steal" the ground somewhere else.