Hello this is my first post so hope I am in the correct section. I have never wored a question like this before so apoligise in advance if its long winded.I require help to a problem that has got me stuck so hoping someone could help point me in the right direction.

I am needing to convert 2 external RS-422 signals from a piece of equipment 100 meters away into RS-232 so our PC can read it on the serial port., the latter being my board I am designing.

I am using an SN75157 IC for the RS-422 input which outputs to the input of an SP202E IC (There is an opto-isolater inbetween). I have LED indicators on the outputs of the SP202E to indicate data is outputting.

Now this is my issue, When I first power up my board with no external connection (RS-422 input), one of the LED indicators comes ON whilst the other stays OFF. I have worked backwards and noticed the SN75157 IC's outputs are starting up in 2 different states. 1 being HIGH and the other being LOW

When I connect the external RS-422 connection both LED's turn OFF and the data transfers no issue with blinking LED's

The moment the external piece of equipment is removed it goes back too 1 LED ON and 1 LED OFF, which will cause much confusion to the operator.

Is this common for the SN75157P to do this when first powered up? This is my first time ever using it.

Thank you in advance

 

The inputs are pulled up internally, so effectively the offset drives the
output state of the part. Seems like you could provide a small amount
of bias from a high z divider such that when inputs disconnected forces
the state you want. Not sure how effective this will be given the high
input current requirements vs loading tradeoff on noise margin the bias
network would present.

Spec shows offset from ground to be +/- 35 mV.

Regards, Dana.

 

The inputs are pulled up internally, so effectively the offset drives the
output state of the part. Seems like you could provide a small amount
of bias from a high z divider such that when inputs disconnected forces
the state you want.

Spec shows offset from ground to be +/- 35 mV.

Regards, Dana.

Thank you for the quick response Dana, that worked, much appreciated

 

There are many "fail safe" receivers available that guarantee well-defined receiver output state if the input is short circuited or open circuit. The alternative is to use a Thevenin termination that biases the input to the "mark" condition while providing termination of the transmission line at its characteristic impedance. This fixes the open-circuit problem but not the short-circuit issue. You can find lots of info about how to do this on the web, though it may be more productive to search for RS-485 than for RS-422 (very similar when it comes to receivers but 485 allows wider common mode voltage range).

 

There are many "fail safe" receivers available that guarantee well-defined receiver output state if the input is short circuited or open circuit. The alternative is to use a Thevenin termination that biases the input to the "mark" condition while providing termination of the transmission line at its characteristic impedance. This fixes the open-circuit problem but not the short-circuit issue. You can find lots of info about how to do this on the web, though it may be more productive to search for RS-485 than for RS-422 (very similar when it comes to receivers but 485 allows wider common mode voltage range).

Thanks for the advice ebp