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AM26LS32A: Different behavior on new replacement parts of the same family.

Enrique Sosa
Prodigy 15 points

Part Number: AM26LS32A

Original (Obsolete) Part Number: DS26LS32CN. Replacement Part Number: AM26LS32ACN.

Setup:

For the setup on the original part, the G is connected to +5VDC and the G-bar is connected to ground which translates to ACTIVE HIGH. Each of the input ports for each of the 4 channels have a 120 Ohm resistor across inputs A and B. For the original part, having no input signal on inputs A and B produces a HIGH output. The voltage on each of the input ports is +2.5VDC which triggers the HIGH output of +5VDC.

Problem:

On the replacement part, using the same setup, the input voltages produce only +1.8VDC to +2.4VDC. This causes the output to be LOW. For the operation of our equipment, we need the default state when there is no signal to be HIGH. I have read the datasheet, and it does say that the output should be undetermined due to not having a differential voltage across the input; however, all of the original parts consistently behave with the output HIGH in the undetermined start (no voltage differential across the inputs).

Request:

Is there a replacement you can recommend that would produce the HIGH output for this case? The original part number is now obsolete and the replacements are not behaving in the same manner.

  • 0
    TI__Guru 52262 points

    Hi Enrique,

    So this part is open fail-safe and it has weak biasing resistors (nominally 100K to VCC on A inputs and 100K to GND on B inputs) - but the fail-safe on this device is overridden when the device is terminated  - the older device you used didn't have fail-safe inputs but its typical threshold voltage was like right at 0V - which is most likely causing the difference in behavior.  

    For a possible solution - in quad channel devices like this you won't find a fail-safe part that really differs how fail-safe is done on this device. Really the closest option we would have is the AM26C32 - but its biasing resistors are even weaker - so I don't think you will find a benefit there.

    I see two potential options moving forward:

    1. Add external biasing resistors - essentially they are resistors that set the idle voltage to about 200mV so that during bus idle it will output high. We do have an app note how to size these resistors: https://www.ti.com/lit/an/slyt324/slyt324.pdf 

    A few notes:

    1a) This note is based on RS-485 not RS-422 - so some of the values will change.

    1b) Receiver Input Impedance is 12K (minimum  - use this value)

    1c) Minimum common mode loading is 400 Ohms - not 375 as indicated in note.

    1d) For RS-422 - only the last receiver in the chain should be terminated (one termination for entire bus) - so only one termination resistor should be considered  - usually this is 100 Ohms - but if you use 120 Ohms its not a big deal - RS-422 drivers can driver loads greater than 100 Ohms between differential outputs- but not less.

    If you only have 1 driver and 1 receiver per line - set the bias resistors to 680 (as that is going to be pretty close to what you calculate anyway) 

    2. The other option - which isn't ideal is to use multi-chips solutions using our modern devices with full integrated open/short/idle fail-safes. If you want to consider this direction please let me know the length of the bus, your intended data rate, and how many nodes are on the bus - because we definitely have devices that will meet your functional goals but it would require multiple IC's - so using the passive fail-safe from my first suggestion is probably the best choice.

    Please let me know if you have any other questions and I will see what I can do!

    Best,

    Parker Dodson