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SN75176A: Recommended capacitive bypass for 19.2k baud

May
Genius 9880 points

Part Number: SN75176A

Hi,

My customer encountered ESD problem in their internal network application that uses RS-485 drivers. According to them, they want to bypass all their 7 connectors on the network.  How much capacitive loading can they can add for 19.2K Async serial?

Thank you.

Regards,
May

  • 0
    TI__Guru 54042 points

    Hi May,

    1. Where is the capacitive loading being attached? Do you have a reference schematic - as I am bit unsure why there are seven connectors to the device - if they are all on different pins than answer may change as well as how these capacitors are oriented will also impact the system.

    2. Could you explain the issue that is being seen in the system? Is there a glitch? Where is the issue seen? 

    Please if you could add the above information it will be helpful as there is no capacitive loading spec on these devices as its highly system and use case dependent. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Genius 9880 points

    Hi Parket,

    Below just received response from customer and they only provided the schematic.

    Thank you.

    Regards,
    Maynard

  • +1 in reply to May
    TI__Guru 54042 points

    Hi Maynard,

    1. There are no units on the capacitors - what is the unit because as read it looks like 100mF which is definitely not going to work. If it is 100nF (.1uF) it still is not a good idea. 

    2. There is no termination resistors - which is also not recommended. 

    3. If the capacitors are at 100nF (which is what I am assuming - but this should be confirmed)- with 5 caps to ground on each line (I am not sure where 7 is coming from because its not on the schematic). That is 500nF in parallel and at 19.2Kbaud (9.6KHz) that is an impedance at fundamental frequency of ~33 Ohms which is well below what you can place on this line. (This should be no less than 375 Ohm including the input impedance of the other transceivers on the line to ground). At most you can have about 7.5nF to 8nF for each cap - including cable/transmission line capacitance so if there is a long transmission distance that capacitance will get smaller. When capacitors to ground are used they are in the pF usually for EMC reasons (and even then there are more preferred EMC solutions that caps to ground) 

    Without anymore information I cannot give possible solutions - because I do not suggest this one (or lowering the capacitance a lot ) . If they are willing to share what the problem is I will be able to see if we have known workarounds or to combat any misunderstandings on how to design with this device. If they provide more information I may be able to zero in on the problem and give a workable solution but if not this solution won't work and the max loading on the bus is 375 Ohms to ground - which must also include other devices + cable length + capacitance per unit length. So there isn't a max capacitive load - but there is a minimum impedance which is violated unless you shrink the caps a lot - but I am skeptical this will fix the issue.

    Best,

    Parker Dodson