Current limiting a buffer IC's outputs

[ultrageek.lloyd]

I'm currently interfacing a microcontroller to a router via EJTAG, and per the EJTAG standard, the current draw on the EJTAG port is not to exceed 8mA per pin.  I've placed an sn74hc244 buffer IC between the microcontroller and the router to reduce loading on the microcontroller pins, but I'm wondering if I need to do any current limiting on the buffer output pins, and if so, what's a good way to do so.  Any help would be appreciated.

Thanks,
ultrageek

[Stephen Hill]

A simple resistor in series with the opamp output would do the trick. Assuming 5V, that would be 625ohm so a standard 680ohm resistor would do.

[ultrageek.lloyd]

A simple resistor in series with the opamp output would do the trick. Assuming 5V, that would be 625ohm so a standard 680ohm resistor would do.

Thanks for the reply.  I thought of that, but I was afraid that the voltage drop across the resistor would cause the router to see less than Vcc, and hence never see a logic high.  Now that I think about it though, for inputs, the router is the current source, meaning that for a logic high current shouldn't be flowing, and for a logic low, current will flow from router to buffer, meaning the voltage drop doesn't matter.  I guess it's just a case of me not understanding how digital signals work =].  Thanks for your input, I think that solution should work.

ultrageek

[poptones]

You put the output resistor inside the feedback loop. Voila! It compensates for the voltage drop and you still get protection.