MTAC-GPIO Electrical Requirements

External Inputs 1-4:

These signals are optically isolated from the rest of the system. The External Input Ground pin is also isolated from the rest of the main board. Current draw of the input is approximately 500 uA for a 3VDC input, increasing to 8.5 mA for a 30 VDC input. In order to see a “1” on a External Input pin, you need to supply a voltage between 3VDC and 30VDC to turn on the optoisolator input.

Equivalent Circuit:


External Outputs 1-4:

These signals are open collector with maximum working voltage Of 55VDC and maximum current draw of 300mA. These signals share a common ground with the system, and cannot be used on isolated signals.  If an output is used as an “digital” signal, an external pull up resistor is required for proper operation. If an output is being used to drive an inductive load, such as a relay, a clipping diode is recommended to protect the output circuitry from inductive spikes.

Equivalent circuit:

DOUT Equivalent Circuit

Analog Inputs 1-3:

These signals are analog inputs fed through an operation amplifier buffer to a 12 bit Analog to Digital converter (ADC). The ADC on the GPIO card is a ADS7950S. The +Vref is being generated by a LMV431A precision source set to 2.48V. The ADC has a =Vref * 2 setting for 4.960V full scale. This is hardcoded by the firmware. The inputs are multiplex, so only one voltage is converted per system call. The input is scaled to accept a 0-5VDC input. These signals share a common ground with the main board, and cannot be used on isolated signals. Input max is recommended to be no greater than 5.2V.

Equivalent circuit: