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I have the following circuit for an isolated serial port. 6N137 is a high speed opto, so baud rate is not an issue.

The problem with this is that it works only 1 way i.e. CNB -> CNA transmission works ok however CNA->CNB TX does not work.

Swapping them around does not solve the issue. Transmission works only 1 way.

I have tested this on a breadboard at low baud rates (9600 baud). After eliminating some initial loose connections the problem still persists. I have individually tested and measured every component in this circuit and all of them are fine.

Any suggestions are appreciated on the following points:

1. Is this circuit correct or does it need some additional pull-ups/pull-downs/caps?
2. Are R15,R17, R16, R18 values correct or does it need tweaking around? Datasheet for 6N137 states that R17, R18 should range between 330-4k ohm. I have experimented around all values between that range with no luck.
3. Suspecting damaged 6N137 I have tried with multiple other ICs with no luck. All ICs work fine when individually tested.
4. I do not have access to a scope. Will probing with a scope help here?

EDIT: The answer below is not valid as I am using 6N137-L which works on 3.3V. I have tried this circuit with both 6N137/6N137-L with 3.3/5V supply. Same behavior, only works 1 way.

I have the following circuit for an isolated serial port. 6N137 is a high speed opto, so baud rate is not an issue.

The problem with this is that it works only 1 way i.e. CNB -> CNA transmission works ok however CNA->CNB TX does not work.

Swapping them around does not solve the issue. Transmission works only 1 way.

I have tested this on a breadboard at low baud rates (9600 baud). After eliminating some initial loose connections the problem still persists. I have individually tested and measured every component in this circuit and all of them are fine.

Any suggestions are appreciated on the following points:

1. Is this circuit correct or does it need some additional pull-ups/pull-downs/caps?
2. Are R15,R17, R16, R18 values correct or does it need tweaking around? Datasheet for 6N137 states that R17, R18 should range between 330-4k ohm. I have experimented around all values between that range with no luck.
3. Suspecting damaged 6N137 I have tried with multiple other ICs with no luck. All ICs work fine when individually tested.
4. I do not have access to a scope. Will probing with a scope help here?

I have the following circuit for an isolated serial port. 6N137 is a high speed opto, so baud rate is not an issue.

The problem with this is that it works only 1 way i.e. CNB -> CNA transmission works ok however CNA->CNB TX does not work.

Swapping them around does not solve the issue. Transmission works only 1 way.

I have tested this on a breadboard at low baud rates (9600 baud). After eliminating some initial loose connections the problem still persists. I have individually tested and measured every component in this circuit and all of them are fine.

Any suggestions are appreciated on the following points:

1. Is this circuit correct or does it need some additional pull-ups/pull-downs/caps?
2. Are R15,R17, R16, R18 values correct or does it need tweaking around? Datasheet for 6N137 states that R17, R18 should range between 330-4k ohm. I have experimented around all values between that range with no luck.
3. Suspecting damaged 6N137 I have tried with multiple other ICs with no luck. All ICs work fine when individually tested.
4. I do not have access to a scope. Will probing with a scope help here?

I have the following circuit for an isolated serial port. 6N137 is a high speed opto, so baud rate is not an issue.

The problem with this is that it works only 1 way i.e. CNB -> CNA transmission works ok however CNA->CNB TX does not work.

Swapping them around does not solve the issue. Transmission works only 1 way.

I have tested this on a breadboard at low baud rates (9600 baud). After eliminating some initial loose connections the problem still persists. I have individually tested and measured every component in this circuit and all of them are fine.

Any suggestions are appreciated on the following points:

1. Is this circuit correct or does it need some additional pull-ups/pull-downs/caps?
2. Are R15,R17, R16, R18 values correct or does it need tweaking around? Datasheet for 6N137 states that R17, R18 should range between 330-4k ohm. I have experimented around all values between that range with no luck.
3. Suspecting damaged 6N137 I have tried with multiple other ICs with no luck. All ICs work fine when individually tested.
4. I do not have access to a scope. Will probing with a scope help here?

EDIT: The answer below is not valid as I am using 6N137-L which works on 3.3V. I have tried this circuit with both 6N137/6N137-L with 3.3/5V supply. Same behavior, only works 1 way.

I have the following circuit for an isolated serial port. 6N137 is a high speed opto, so baud rate is not an issue.

The problem with this is that it works only 1 way i.e. CNB -> CNA transmission works ok however CNA->CNB tx does not work.

Swapping them around does not solve the issue. Transmission works only 1 way.

I have tested this on a breadboard at low baud rates (9600 baud). After eliminating some initial loose connections the problem still persists. I have individually tested and measured every component in this circuit and all of them are fine.

Any suggestions are appreciated on the following points:

1. Is this circuit correct or does it need some additional pull-ups/pull-downs/caps  ?
2. Are R15,R17, R16, R18 values correct or does it need tweaking around  ? Datasheet for 6N137 states that R17, R18 should range between 330-4k ohm. I have experimented around all values between that range with no luck.
3. Suspecting damaged 6N137 I have tried with multiple other ICs with no luck. All ICs work fine when individually tested.
4. I do not have access to a scope. Will probing with a scope help here  ?

I have the following circuit for an isolated serial port. 6N137 is a high speed opto, so baud rate is not an issue.

The problem with this is that it works only 1 way i.e. CNB -> CNA transmission works ok however CNA->CNB TX does not work.

Swapping them around does not solve the issue. Transmission works only 1 way.

I have tested this on a breadboard at low baud rates (9600 baud). After eliminating some initial loose connections the problem still persists. I have individually tested and measured every component in this circuit and all of them are fine.

Any suggestions are appreciated on the following points:

1. Is this circuit correct or does it need some additional pull-ups/pull-downs/caps?
2. Are R15,R17, R16, R18 values correct or does it need tweaking around? Datasheet for 6N137 states that R17, R18 should range between 330-4k ohm. I have experimented around all values between that range with no luck.
3. Suspecting damaged 6N137 I have tried with multiple other ICs with no luck. All ICs work fine when individually tested.
4. I do not have access to a scope. Will probing with a scope help here?