DS26C32AM Cold Sparing

Hi Richard,

Input of DS26C32 has high input impedance to create a fail/safe mechanism to prevent self oscillation when the cable is not connected or when the input is not being driven. The input has a high value pull up Resistor and pull down biased to create a high output when the input is not connected. When the source is enabled and the device is powered off, the device attempts to power up through the high pull up R. It turns on partially because the input impedance is very high, compared to the termination Resistor, and because of the high input impedance the voltage is not high enough to power the device. Also, the current limit on the driver kicks in much sooner than the device powers up through the high value pull up Resistor. So as long as the source is another RS422 driver we do not see any problem under this condition. You can monitor the Vdd pin to measure the voltage and indirectly measure the current being drawn by the device.

Regards,,nasser

Hi Nasser,

Thanks for the response. It was very helpful.

One of the issues I’m trying to resolve is the amount of current on the VDD rail of the unpowered DS26C32AM part due to the powered interfacing RS-422 driver.   I’d like to know more about the implementation of the input signal resistor networks and ESD diodes in the DS26C32AM part. I have attached a “pdf” file of what I think is the implementation of the part’s input signal resistor networks and ESD diodes. The “pdf” also shows expected current paths when the DS26C32AM part is unpowered and the interfacing RS-422 driver is powered and enabled.

Based on the assumed implementation, there are two likely possibilities for the current flow:

    1. The current flows through the DS26C32AM's protection diodes and returns through chassis since the both UUT's secondary returns (VDD_RTN) are tied to chassis.

    2. The current flows through the DS26C32AM 's resistors that are pulled up to the VDD rail and returns to chassis as described in the first possibility. This would be the case if the protection diodes have already failed open due to high currents in this partially powered configuration.

If the protection diodes are present at the DS26C32AM receiver inputs, then the current could have been up to the output short-circuit current limit of the driver. If no protection diodes, then the current is limited by the high resistance of the pull-up resistors.

So, my questions are as follows:

    1. Your initial reply did not mention protection diodes. Do the DS26C32AM receiver inputs include protection diodes? If so, it would allow for greater current to flow to the VDD rail than the high resistance pull-up resistors.

    2. Are the resistor networks implementations in the attached “pdf” file match the ones in the DS26C32AM inputs? Are the resistance values close to the ones in the actual part’s networks?

Thanks,

Richard

1805.Assumed Input Structure of DS26C32AM Receiver - 14 Aug 2013.pdf

Hi Richard,

1). Yes indeed you are correct. There is ESD diode protection and these are designed to take far greater current than the fail/safe Resistors can handle.

2). I believe you have the right idea. We have the ESD diode and then you would see the fail/safe Rs in order ot K Ohms.

Regards,,nasser 

Hi Nasser,

Thanks for the reply.

I have a few more questions about the DS26C32AM’s inputs. I just want to make sure I understand how the inputs are structured.

    1. You answered my question #2 by stating that the resistor network implementations in the earlier posted “Assumed Input Structure of DS26C32AM Receiver - 14 Aug 2013.pdf” file “has the right idea.” But your answer also stated that “We have the ESD diode and then you would see the fail/safe Rs in order ot K Ohms.” Does this means that each DS26C32AM (+) input has a diode and resistor in parallel and that both are tied to VDD?

    2. The diagram in the “Assumed Input Structure of DS26C32AM Receiver - 14 Aug 2013.pdf” file shows the inputs having 10.6k-Ohm series resistors before the diodes. Do the DS26C32AM’s inputs also have series resistors between the inputs and the protection diodes?

    3. Referencing the diagram in the “Assumed Input Structure of DS26C32AM Receiver - 14 Aug 2013.pdf” file, which resistors would be considered “fail/safe resistors”? For the (+) input, would resistor R0 be the “fail/safe resistor” with R3, R4 and R5 not being present in the circuit?

    4. Is it possible for you to provide me with a diagram/schematic of the DS26C32AM’s input protection circuit?

Thanks, Richard

Thanks for all of the responses, Nasser.

If you come across the input diagram for this part at sometime in the future, please upload it to this post.

Thanks,

Richard Nichols