SN65HVD1785: SN65HVD1785

Part Number: SN65HVD1785

Using this to chip to output proprietary code on the RS-485 bus. The signal is received by another '485 transceiver down the line. The output voltage levels for both +(A) and -(B) lines is 2.5V and signal transmits properly. After a while, the +(A) line goes dead down to a few millivolts. It remains dead with no load applied. Replacing the chip makes it work again. This only happens on +(A).

  • John,

    I've notified an expert of this thread and they will respond accordingly. Thanks for your patience.

    Regards,

    Eric Hackett 

  • John,

    Can you check the power supply to see if any corresponding drop occurred? Or is there any possible connection issue? The last question is: can you advice where you purchased the parts? If you suspect you get bad parts, you can contact your vendor for failure analysis.

  • There was no corresponding drop in the power supply. This SN65HVD1785 is being driven by a micro and all voltages in the circuit remained normal including the voltage to the transceiver. The Tx line from the micro to the transceiver sends the proper code, but only the A(+) line on the bus side of the chip doesn't work. These parts were purchased from Digikey. 

  • If you don't mind, can you share your schematic? Will the chip recover by itself, or do you need to power cycle, or will A(+) line no longer behave normally? How many parts show this issue? If you put a problematic chip in a new board, would it show the same behavior? (I want to take the other factors out of equation.)

  • Here is a partial schematic showing the transceiver outputs connected to pins 1 & 2 of a Cat5 modular jack. You can also see there are 2 points available for connecting bias resistors, however none were connected in this application. The chip does not recover, and replacing with a new chip solves the problem. 

  • John,

    Thanks for more information. The schematic looks good to me. Does the issue happen only when the SN65HVD1785 transmits? It sounds like there is no load either. Can you confirm it's true? Again how frequently do you have bad parts?

  • It is not known when this issue happens, but 6 devices failed on one particular jobsite. The devices were each connected to another '485 transceiver over a bus line using Cat5 cable. The '485 transceivers to which these devices were connected did not fail. The characteristic of the failure is that the devices work when installed, but begin to fail over a period of time. I have other devices on different jobsites which have not failed.

  • If you don't mind, can you share which devices were connected? SN65HVD1785 has high bus fault voltage, which can sustain high DC. I suspect the issue might come from unexpected ESD events. But since it's A pin always gets damaged, is it possible to check there is any risk specifically related to this side of the wire?