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Newbie Question - Pullup Resistors

noworld

Just getting into electronics and had a question about the ZX Spectrum 2.0 video. About 12 minutes in there are pullup resistors on the data bus. What is the purpose of these resistors? If all the lines are pulled high (1) then how is data put on the bus? Is the specific location of the resistors on the bus important?

 

Thanks!

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  • I'm not familiar with the ZX Spectrum itself, but the usual reason for pull-up resistors on a bus is to force the signals high if nobody is actively driving the bus.  Digital signals have two defined values: "1" (greater than a "high" threshold voltage) and "0" (less than a "low" threshold voltage).  Depending on the transistor technology, it can be a bad thing to have a signal between the threshold voltages for an extended period of time.  Pull-up or pull-down resistors are a way to ensure that a signal goes to a well-defined "0" or "1" if it's not being driven by a pull-up or pull-down transistor.  A transistor is stronger than a typical pull-up resistor, so it wins it it's turned on.

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  • I'm not familiar with the ZX Spectrum itself, but the usual reason for pull-up resistors on a bus is to force the signals high if nobody is actively driving the bus.  Digital signals have two defined values: "1" (greater than a "high" threshold voltage) and "0" (less than a "low" threshold voltage).  Depending on the transistor technology, it can be a bad thing to have a signal between the threshold voltages for an extended period of time.  Pull-up or pull-down resistors are a way to ensure that a signal goes to a well-defined "0" or "1" if it's not being driven by a pull-up or pull-down transistor.  A transistor is stronger than a typical pull-up resistor, so it wins it it's turned on.

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