Pull-up resistors in parallel

[liyin]

The circuit for the IgorPlugXP plugin for Girder has a 3.3KΩ pull-up resistor to keep the IR receiver output line high (5V).

If R1 is removed Girder doesn't recognize the circuit. If R1 is replaced with a wire Girder recognizes the circuit, but shows no activity when pressing the remote's buttons.

My question is why is this resistor necessary if the IR receiver has an internal 80KΩ pull-up resistor connecting the output line to the Vcc line.

 

[IanP]

Here is how an RS-232 pin can be described as a receiver pin:

A receiver may present a load ranging from 3000 ohm to 7000 ohm and expects signals to swing from below -3 volt to above +3 volt.

80kΩ is just not enough to pull the voltage (@ the quoted load) up, therefore you need to add a decent pull-up resistor and 3k3 seems to be the right value .. ..

IanP

 

[liyin]

If you are saying the resistance must be between 3,000Ω and 7,000Ω, I can see 80KΩ + 3.3KΩ resistors in parallel = 3169Ω (between range). But still don't understand the relationships and the wording used in your post.

What's the meaning of "present a load"?

And of "not enough to pull the voltage (@ the quoted load) up"?

If resistors cause a voltage drop, reducing the resistance by adding another resistor in parallel will make the drop smaller, hence closer to 5V. Is this the way to understand it?

I knew about pull-up resistors used to prevent too much current going to ground, while allowing and input to stay high (Vcc).

But in this context their use seems a bit different, they are used with an "open-collector" output (according to TSOP IC datasheet).

 

[IanP]

Have a look at the attached picture: Z[in] represents LOAD ..
Or one can say: input pin presents itself as a load ..

If resistors cause a voltage drop, reducing the resistance by adding another resistor in parallel will make the drop smaller, hence closer to 5V. Is this the way to understand it?

More the less ..

I knew about pull-up resistors used to prevent too much current going to ground..

Pull-ups do not prevent anything, see what they do in the next sentence ..

But in this context their use seems a bit different, they are used with an "open-collector" output ..

OC can only sink current to the ground ..
If you assert a “1” to the OC output - it floats, and the only way of defining the voltage is by adding an external pull-up hooked up to a Vcc ..

IanP