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ISO1228: Increasing the current in Source Mode

RKG
Prodigy 10 points
Part Number: ISO1228


Tool/software:

Hello,

I am interested in operating the ISO1228 in source mode, but would like to consider the possibility of increasing the current.

I see that the device supports Type 1 for 2.5mA and 3.5mA, and Type 2 for only 2.5mA (3.5mA is stated in table 8.2 as "not supported").

Question 1:

It is clear to me that Rpar can be changed in order to set the current...

  • So, what is the consequence of adjusting Rpar until the device sources 5mA or 10mA of current, for example?
  • I'm assuming it no longer conforms to IEC 61131-2, but is it safe to operate the device with higher currents, set by Rpar?
  • Is there a way to characterize the voltage thresholds and behavior if this is done?

Question 2:

I am anticipating that Type 2 operation is one possible solution, and that two channels need to be connected in parallel.

  • Is this supported in source mode?
  • If using the parallel interface output, do I read the status of just one of the two OUTx lines, or connect them together?

Thank you in advance for your help.

  • +1
    TI__Mastermind 32255 points

    Hello RKG,

    1. Question 1
      1. The resistor RILIM controls the current limit of all eight channels. ISO1228 does not support higher current limits than 3.5mA due to thermal limitations. 
      2. For higher currents and input voltages please try ISO121x. The design of ISO121x allows for a shunt resistor that can take the excess current (see 9.2.1.2.4 Designing for Input Voltages Greater Than 60 V)
    2. Question 2
      1. Correct, the ISO1228 supports 4-channels of Type 2 operation by connecting two channels in parallel which will enable 7mA current limit. However, this is only available in sink mode. 
      2. You would only need to read one of the OUTx lines. The parallel input would have a redundant signal. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 10 points

    Hi Andrew,

    Okay thank you, that is all very clear.

    I have a stupid question about Type 2 mode.

    When you say "connect two channels in parallel", is this connection before or after the threshold resistor?

    i.e. For a type 2 implementation of ISO1228 providing only 4 channels (as 2 are paired), does it still have 8 Rthr and 8 RPar, or 4 Rthr and 4 Rpar?


  • +1 in reply to RKG
    TI__Mastermind 32255 points

    Hi RKG, 

    This a good question (not stupid at all). The Type 2 digital input requires a 7mA current limit. The ISO1228 can implement this by using two type 3 (3.5mA current limit) inputs

    To configure type 2 inputs, tie the two input pins together (for example, IN1 and IN2). Then, configure each pair of pins using the type-3 input's resistors (RILIM = 1kΩ, RPAR = 9.76kΩ, (IL = 3.5mA)). 

    Best,
    Andrew