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AM26LS32AC: Differential Input Resistance

Go Bravos
Expert 6090 points
Part Number: AM26LS32AC
Other Parts Discussed in Thread: AM26LS31

Team,

Two manufacturers made a subassembly I am testing. One used the correct 26ls31 driver, and the other used two 54S240 inverters (one input is inverted by 54LS04). The subassembly that receives this circuit has a 26LS32DM , with 6.8 to 9k input resistance (calculated or inferred from other tests). I am told this correctly generates the expected outputs for both manufacturers at the box and system level. I used the AM26LS32 in test receiving circuit, which has input resistance of 12 to 15K. It only passes the manufacturer that used the 26ls31 driver.

 

Page 15 has the equivalent circuit for AM26LS32AC. http://www.ti.com/lit/ds/symlink/am26ls32ac.pdf

The test circuit has the normal 1k serial terminations, which can be removed easily. The 26ls31 or 54S240 do not have any serial termination resistors in their respective subassemblies. Is getting an IC with the lower input resistance the only option? Is there an issue with PCB footprint dimensions between the commercial AM26LS32 and the mil qualified parts with the lower input resistance?

Thank you.

  • 0
    TI__Mastermind 25705 points

    Bravos,

    Is this your setup?

    Do you have any termination resistors on the bus? What voltage do you expect to have? With a smaller receiver input resistance, you would see a smaller voltage on the bus. Is it what you wanted?

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 20 points

    no parallel terminations

    Manufacturer A subassembly                           cable                 Manufacturer C

    26LS31 Y output – no serial termination – twisted pair wire1 – 1k ohm serial – 26LS32 A input

    26LS31 Z output – no serial termination – twisted pair wire2 – 1k ohm serial – 26LS32 B input

    Manufacturer B subassembly                           cable                 Manufacturer C

    54S240+ – no serial termination – twisted pair wire1 – 1k ohm serial – 26LS32 A input

    54S240- – no serial termination – twisted pair wire2 – 1k ohm serial – 26LS32 B input

    This apparent doesn't drive correctly, as the software at test masks the digital failure.

    Is there a termination scheme that can be added at Manufacturer B or C (1k can be removed)?

    Or is the only option to take one 54S240 outputs and use a 26LS31?

     

    Thanks,

     

    Nick

     

     

     

     

     

     

  • 0 in reply to Nicholas Piccinich
    TI__Mastermind 25705 points

    Nick,

    Can you elaborate "doesn't drive correctly"? Do you mean the voltage amplitude is too small? Among all the scenario you listed, which ones work and which ones don't?  If you use AM26LS31 as the driver and AM26LS32 as the receiver, the expected voltage is larger than 1.5V with 120 termination resistors on both sides (per RS-485 standard). Without termination, the voltage level would go up. I'm not familiar with 54S240's driving capability. Do you have any comparison results between AM26LS31 and 54S240?  

    Regards,

    Hao

  • 0 in reply to Hao L
    Prodigy 20 points

    Hao,

    I don't have the ability to measure when the two subassemblies are together.

    I'll just have to come up with a separate harness for test setup that will verify the single ended logic of the 54S240.

    Brad Davis or Steve Widener,

    Can you close the question?

    Thanks,

    Nick

  • 0 in reply to Nicholas Piccinich
    TI__Mastermind 25705 points
    Nick,

    Thanks for your update. Please let me know if you get any new questions.

    Regards,
    Hao