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Original question:

TIDA-00333: Dual Isolated Half-Duplex RS-485 Repeater: Question on Max Data Rate

TIDA-00333: Data rate inquiry

Saeed Danesh
Prodigy 30 points
Part Number: TIDA-00333
Other Parts Discussed in Thread: ISO3082, ISO1410, ISO3080

Tool/software:

Dear Experts,

I have a question about the TIDA-00333 design. The transceiver used in this design is the ISO3082, which, according to the datasheet, has a data rate of 200 kbps. However, in the TIDA-00333 guideline, it is mentioned that the data rate is limited to 100 kbps.

My question is: Why is the data rate reduced to 100 kbps? My required data rate is 115200 bps. I understand that you recommend ISO1410 for higher data rates, and its data rate is specified as 500 kbps. Based on the new design, what is the actual achievable data rate?

Repeater with ISO3082:

www.ti.com/.../tidu590.pdf

Repeater with ISO1410:

www.ti.com/.../slyt480a.pdf

Additionally, I have another question. In the design with ISO3082, a 100pF capacitor is connected from the Driver Input to GND for both transceivers, and a 300Ω resistor is placed in series with this capacitor (CD = 100pF). What is the reason for adding this resistor?

I would appreciate your insights.

Thanks,

Saeed

  • 0
    TI__Mastermind 29205 points

    Hello Saeed, 

    The TIDA-00333 reccomends a data rate less than 100kbps due to the high node count and long cables expected in the application. Also, ISO3082 is an older device, and the drive strength is not as strong. The device can transmit at 200kbps; however, longer cable lengths may reduce the maximum data rate. I would expect 115200kbps to be okay for most applications.

    However, ISO1410 uses the newest isolation technology and may provide more stability ate 115200kbps with longer cable lengths. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 30 points

    Hi Andrew,

    Thanks for your response.

    In the new design with ISO1410, what is the data rate for the entire design? Has it been tested?

    Does only the cable length affect the data rate, or does the Schmitt trigger also have an impact?

    Additionally, could you please answer my previous question?

    In the design with ISO3082, a 100pF capacitor is connected from the Driver Input to GND for both transceivers, and a 300Ω resistor is placed in series with this capacitor (CD = 100pF). What is the reason for adding this resistor?

    Thanks,

    Saeed

  • 0 in reply to Saeed Danesh
    TI__Mastermind 29205 points

    Hello 

    Saeed Danesh said:
    In the new design with ISO1410, what is the data rate for the entire design? Has it been tested?

    ISO1410 has not been tested in the reference design, however, it is p2p with improved propagation delay. The max speed should improve and the RC time constant will need to be recalculated according to step 3, figure 4 in slyt480. Of course it is okay to continue using ISO3080. 

    Saeed Danesh said:
    Does only the cable length affect the data rate, or does the Schmitt trigger also have an impact?

    Data cable length will have the largest effect on data rate (see figure 1). The propagation delay of the Schmitt trigger is only a few nano seconds. 

    Saeed Danesh said:
    Additionally, I have another question. In the design with ISO3082, a 100pF capacitor is connected from the Driver Input to GND for both transceivers, and a 300Ω resistor is placed in series with this capacitor (CD = 100pF). What is the reason for adding this resistor?

    The 300Ω and the 100pF are the Rd and Cd that set the time constant for the direction control in figure 4.

    Best,
    Andrew