Series 3700A Switch and Control Cards Reference Manual
Section 13: Model 3750 multifunction control card
3700AS-909-01 Rev. A / Preliminary
13-5
The factory default jumpers are set as follows:
•
Connects the digital lines on each bank to the internal pull-up resistors.
•
Connects those pull-up resistors to the in5 V.
•
Connects VEXT1 to VEXT2. Removing the jumper allows for two different external voltage
sources. They are grouped according to the D-SUB connector from which they are accessed (that
is, channels 1, 2, and 4 are on one D-SUB, while channels 3 and 5 are on the other).
Figure: Simplified schematic of a digital I/O channel
Figure 68: Simplified schematic of a digital I/O channel
The figure shows a simplified schematic for one bit of a digital I/O channel. Each I/O bit has an
optional pullup resistor in parallel with a diode that is used to clamp flyback voltages from inductive
devices like electro-mechanical relays. However, when the optional jumper is removed, both the
pullup resistor and diode are removed from the circuit. The pullup resistor is considered “weak” and
can easily be overdriven by the external circuit.
CAUTION
When driving the digital I/O channel with 5 V or higher, be sure to avoid driving the line higher than
the pullup voltage (5 V or VEXT). Otherwise, the internal flyback diode will become forward biased,
possibly causing a high current situation.