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Pull-up and pull-down resistors on enable pins of ISO15

Rohit Baranwal
Expert 1025 points
Other Parts Discussed in Thread: ISO15

I intend to use ISO15 RS485 transceiver for my application. I want to know how to decide upon the suitable value of pull-up and pull-down resistors on the REn and DE pins, if I intend to have jumpers installed in case I want to enable/disable the communication. Also, please tell me how to decide upon the cut-off frequency of the RC filters between the Rx, Tx pins of microcontroller and the corresponding pins of the ISO15 chip. An illustration is given in www.ti.com/.../slla299.pdf.

  • 0
    Expert 1025 points
    Hello, can anyone please help?
  • 0 in reply to Rohit Baranwal
    TI__Guru 55575 points
    Hi Rohit,

    DE & RE\ would require an input current of approx |10µA|. For VCCI of 5V, by calculation it would be a maximum of 500kΩ. Since larger values of pull-up / pull-down will result into larger rise / fall times, it is generally recommended to use resistors of values between 1kΩ & 10kΩ. This will also enable you to accommodate for VCC changes.

    Since the third harmonic will also constitute a significant component in the spectrum of a square wave, you could choose cut-off frequency to be after the third harmonic with a small guard band. For example if your max datarate is 1Mbps, the frequency of this digital signal would be 500kHz with the third harmonic being 1500kHz. Choosing cut-off at 1800kHz or higher should be safe. For low datarates you could also include 5th harmonic if seemed necessary.

    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Expert 1025 points

    Thanks for the reply, but one thing is not clear to me. How did you arrive at the frequency of 500 kHz for 1Mbps data rate? I don't know the underlying calculation, so could you please elaborate? Also, I thought that the RC filter is to suppress very thin pulses which could be caused by noise. Considering that, shouldn't the cutoff frequency be fairly high?

  • 0 in reply to Rohit Baranwal
    TI__Guru 55575 points
    Hi Rohit,

    1Mbps data rate will have a continuous bit pattern of 1s and 0s and the maximum variation can occur when the bit pattern has alternate 1s & 0s like (1010...). Two bits of one HIGH and one LOW would form a single cycle of a rectangular waveform hence the frequency is half. Any X data rate would be treated as equivalent of X/2 frequency.

    You are right, noise is generally formed of very thin pulses meaning a very high frequency pulses. To suppress all these high frequency noise and pass all low frequency data, you would need a low pass filter that passes all the frequencies below cut-off (data) and suppresses all the frequencies above cut-off (noise or interference signal). Hence the RC which is a LPF.

    Best Regards,
    Koteshwar Rao