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Isolated signals for MSP and LPT

Mikolaj Filar
Expert 1550 points

Hello all,

I have 5V optical encoder and I want to isolate signals from it and provide them to MSP430 and LPT port.

My first idea:

- use opto-isolator: NPN transisotr's emitter grounded, collector connected to MSP power supply through low value resistor (~200 ohm), two outputs (from the collector) with diodes (one for LPT and one for MCU).

Disadvantages:

Much current when phototransistor is open (higher resistor value will lower this current but longer time of response will be required).

My questions:

Is this idea correct?

Do you know better solutions? Maybe is there any TI product will simplify the circuit and provide safe isolation?

  • 0
    Expert 2740 points

    Do you need high voltage isolation for safety or are you looking for a logic level shifter.

    Are the 2 supply grounds different?

    Peter

  • 0 in reply to Peter Dvorak
    Expert 1550 points

    I need isolation for safety.

    Ground is common.

  • 0 in reply to Mikolaj Filar
    Mastermind 7395 points

    It will depend on the speed of the signals that you want to achieve. I don't understand the reason for the diodes, you can connect the collector directly to the two ports... Unless you are worried of the LPT driving 5V to the MSP... but even in that case, there are some I/Os of the MSP which are 5V tolerant.

    I think that the resistance doesnt have to be that low.

  • 0 in reply to Avi Chami1
    Expert 1550 points

    When will we read in opto- isolator datasheet: higher resistance makes timings longer. I will have two signals from optical encoder (high resolution but I don't remember how exactly). Of course I will try to choose the best resistance when I will know details.

    Albert_83 said:
    I don't understand the reason for the diodes, you can connect the collector directly to the two ports... Unless you are worried of the LPT driving 5V to the MSP... but even in that case, there are some I/Os of the MSP which are 5V tolerant.

    Physicists will use this device and they are very "creative" so I prefer to protect the MSP, but the diodes aren't a big problem.

    I am also worried if they connect LPT line to GND or somwhere with lower voltage I will lose a signal on MSP.

  • 0 in reply to Mikolaj Filar
    Guru 227245 points

    Mikolaj Filar said:
    higher resistance makes timings longer

    Of course a stronger signal on the sender side of the isolator will result in a quicker (steeper) curve on the receiver side. If you use a low current to drive the sending diode of an optocoupler, it takes much longer to saturate the photo-sensing base of the receiving phototransistor than when you flood it with light.

    The question is: how fast do you need it? And how slow is it with different currents. The coupler datasheet may provide diagrams that help.

    An alternative is usign the AdUM series of isolating signal transmitters. They work with inductive transmission. The cheaper ones are good for up to 1MHz signal frequency, but there are higher speeds available. There is a small delay, of course, but it is constant, so it works for most applications.
    One big advantage is that you provide two different supplyvoltages, so you can power one side with 5V, the other with 3.3V (for the MSP). Different parts have several drivers in one or both directions. The magnetic coupling provides good isolation.

    We use them for coupling an ADE energy metering chip (directly connected to AC) with an MSP using SPI signals.

    Mikolaj Filar said:
    I am also worried if they connect LPT line to GND or somwhere with lower voltage I will lose a signal on MSP.

    Well, GND loops are always a source of problems. Best isolaiton is void if you connect gounds where they are not meant to be connected. :)

  • 0 in reply to Jens-Michael Gross
    Expert 1550 points

    I know details now. I don't need to care about timings, anyway I am going to provide possibility of improving performance, so I am going to use faster optocouplers (probably 6N137, inasmuch I need 5V source for encoder) .

    Jens-Michael Gross said:
    An alternative is usign the AdUM series of isolating signal transmitters. They work with inductive transmission. The cheaper ones are good for up to 1MHz signal frequency, but there are higher speeds available. There is a small delay, of course, but it is constant, so it works for most applications.
    One big advantage is that you provide two different supplyvoltages, so you can power one side with 5V, the other with 3.3V (for the MSP). Different parts have several drivers in one or both directions. The magnetic coupling provides good isolation.

    I'm just reading datasheets. These devices are really nice. TI should start to design own ones :)

    Jens-Michael Gross said:
    Best isolaiton is void if you connect gounds where they are not meant to be connected. :)

    :)

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