SN75179B: Idle state of VOD

Hi Uchikoshi, 

After reviewing the schematic and waveforms, I have come up with a couple of suggestions. 

I still do not have the signal frequency from the data you have given. I would assume in the scope captures since the differential voltage is hanging for roughly 5us that the signal rate is in the Hz - kHz range. So the signal rate is quite low. Lower signal rates may indicate that you are trying to communicate across a long length of cabling. 

How much cable is between SN75179 and the AM26LV32CDR? If there is a lot of cable between these two devices, I could see how a ground potential difference may be significant enough to effect the data cables, or even cause the transceivers to fail. This GPD could also affect the voltage across PTZ15B Zener diodes due to common mode voltage differences.

Consider this grounding diagram: 

Note that as grounds shift due to communication across further distances, ground loops can occur affecting your data lines. Example A being the worst, example B is better but could cause data errors, and example C adhering to the RS485 standard. 

Since the SN75179B is an older TI device, it is not as electrically efficient as newer devices today. Look at ICC (supply current) from the datasheet:

I would check to make sure that there are no current limits to the device. This device operates off around 57mA typical. If there is a current limiting device limiting supply current below typical then the transceiver will have less power to turn on internal driving circuitry. This could explain the 5us delay you are seeing since the device is having a hard time driving internal transistors.

If none of what I said above is helping to solve your problem, would you be able to setup a test circuit to test your SN75179B driver circuitry to see if it is meeting proper differential output delay time? 

A quick drop in replacement test with another SN75179B device might be helpful here too. We can test to see if the problem occurs on another chip. 

Please let me know if this information helps! 

Regards,

Tyler