Connecting an external reference voltage to P2 pin A1 of VME 2120-175 Digital Output board. In order to protect this signal, installing a fuse or current limiting diode in series with it.
The input characteristics of this signal and the current limit for this signal when using 28VDC as the reference voltage.
The VMIVME-2120-175 option has no pull-up resistors installed. In this case, the external 28VDC source provides a reverse bias to the clamp diodes built into the driver outputs. See the Product Manual page 34. As long as the load impedance connected to the driver outputs is also connected to 28VDC or less, the clamp diodes will always be reverse biased and never draw significant current from the 28VDC external source during normal operation. Make sure that Jumper JC is set to the EXTV position. For non-inductive load applications, the current draw should be much less than 1mA. A 1/8A fast blow fuse or equivalent current limiting diode could be used to protect against catastrophic component failure or accidental short circuits to the backplane.
If the load is inductive, a transient voltage will be created when the output driver turns off. This transient may cause the clamp diode to conduct, limiting the transient voltage to 28V at the driver output, but passing a transient current back into the 28VDC source. The magnitude of the transient current is a function of the load inductance, driver output switching time, and circuit impedance. A well-designed low impedance voltage source with suitable connecting cables will have the ability to absorb transient currents produced by the inductive loads. The source should be fused or have fold-back current limiting to protect against catastrophic failures.
If your application will use pull-up resistors, the worst case current flow from the 28VDC would occur when all output drivers are turned on simultaneously. The current would be 28VDC/(parallel combination of all pull-up resistors). For 28VDC operation a resistor value of 6800 ohms or greater is recommended to keep power dissipation in the resistors at acceptable levels.