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open collector output logic

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tellMeDaBasix

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open collector output

I confused with open collector output logic IC:

Let us consider following 3 parts:

1. 7405: Hex Inverter with open collector outputs
2. 7406: Hex Inverter Buffer/Driver with 30V open collector outputs
3. 7416: Hex Inverter Buffer/Driver with 15V open collector outputs

I have questions:

1. Why have 3 IC with different voltage rating?
Why not one IC with 30V open collector output?

2. For 2 and 3, it given that output can go to 15V and 30V, but how to know about 1? Is it (Vcc + 0.5)?
Where to find information in datasheet?

3. If I connect pullup (example 4.7k) to each output pin, then would not 7405 become EXACTLY like 7404?

4. Is the difference between 7405 and 7404 ONLY that 7405 not have output MOSFET source connected to pullup resistance?

I be very happy with answers!
 

betwixt

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open collector

1. simply the manufacturing of the device has to be different, the lower voltage ones are cheaper to produce. It is a trade off between cost and need to use the higher voltage.

2. The maximum voltage should be in the data sheet. For devices that do not have open collector pins, the pin produces the voltage so it is set by the device, usually very close to VCC.

3. No, the difference is that open collector relies on the current through the pull-up resistor to produce the logic high. In a normal device, the output stage drives the pin to a high state. Although it may seem the same at first, consider that the effective value of the transistor driving the pin high may only be a few ohms so the pin voltage will rise very quickly. A resistor will most likely give a slower rise time.

4. The 74xx series are TTL, they do not use MOSFET transistors but basically you are correct. The top transistor in the output is not to connect an internal pull-up resistor though, it is to connect the output as directly to the VCC pin as is possible.

Brian.
 

tellMeDaBasix

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open collector outputs

But 74HC is FET, no?

Also, from Brian answer 3, does it mean OC IC output pin never destroy if short to ground but 74HC04 pin burn if short to ground?
 

betwixt

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74hc open collector

Don't be :|
I am not sure where you are but please remember that while some of us are working during the day, some are sleeping at night.

You didn't mention the 'C', 'HC', 'AC', 'HCT' and other families in your original post so we are both right. The base 74 series is all bipolar though.

You are correct about not damaging OC outputs by connecting them to ground, that is one of the reasons they are used.

If you imagine the output pins of two non-OC devices connected together, if one turned the top output transistor on to drive its output pin high and the other turned the bottom transistor on to drive its pin low, the two conducting transistors would burn out. They would be in series directly across the VCC to ground rail and both turned on.

Now look at it with OC pins, only the pull-up resistor can source current so provided it has a sensible value, the current is limited and the devices are safe. Either device can pull the line low safely and they can both pull low at the same time safely. OC is often used in situations where there are many devices connected this way, it is called wire-OR configuration because it mimics the logic OR function.

Brian.
 

tellMeDaBasix

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connect to open collector

betwixt said:
Don't be :|
I am not sure where you are but please remember that while some of us are working during the day, some are sleeping at night.
I not understand :-(

betwixt said:
If you imagine the output pins of two non-OC devices connected together, if one turned the top output transistor on to drive its output pin high and the other turned the bottom transistor on to drive its pin low, the two conducting transistors would burn out. They would be in series directly across the VCC to ground rail and both turned on.
Ok!
 

betwixt

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open collector output

One of your posts just had a :| before I replied. The message then disappeared, I thought it was you being impatient for a reply.

It is easy to forget when using the internet that it covers so many time zones. I often get people complaining they don't get immediate replies when they forget it could be the middle of the night here.

Brian.
 

tellMeDaBasix

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what is open collecter

Audioguru, I see your advice most time good.

You say I should use GAL?
 

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