HOWEVER...

After performing a "long power cycle" (i.e. leaving EVERYTHING unplugged overnight) the exact same connection is now working perfectly. The manufacturer of the RS-232/485 converters I'm using told me to "just use PuTTY" to test the devices. Apparently the port on one of the machines saw an XOFF and simply stopped transmitting. When I powered on I set both copies of PuTTY to no flow control before starting any testing.

Sorry if I wasted your time. I'd somehow gotten the ports into a bad state and had nothing that would show me that state. But everything is OK now.

I am connecting Pin 2 to TX- on the Serial Tap and Pin 3 to RX+. I've tried with and without the Pin 4 connected to GND and7 seen no change in behavior.

This is where it goes wrong. For a two-wire RS485, you should use the RX+/RX- pair (or TX+/TX-, doesn't matter. But not TX-/RX+ or TX+/RX-). Connecting GND is not strictly essential, but is recommended. See more here: https://ioninja.com/forum/topic/14/rs485-modbus-rtu-comms-snooping/2

Using the 485 header, I've managed to capture the data bytes being transmitted, but in between I see 40 or 50 FRAMING ERRORs. The test programs are opening the ports at 8,N,1/38400 baud and I've set IO Ninja to use the same settings.

The 485 pinout diagram for two wire the RS-232 to RS-485 converters I'm using show the following:

Pin 8 = TD(A)-
Pin 3 = TD(B)+
Pin 2 = RD(A)-  [tied to TD(A)-]
Pin 7 = RD(B)+  [tied to TD(B)+]
Pin 4 = GND

I am connecting Pin 2 to TX- on the Serial Tap and Pin 3 to RX+. I've tried with and without the Pin 4 connected to GND and seen no change in behavior.

So, in the absence of any documentation that is clear enough for a moron like me, I need to know if I have connected things correctly and if not, what is the correct way to connect a two wire RS-485 to the Serial Tap?

[EDIT] It just occurred to me to try connecting Pins 2&3 backwards from the labels, just to see what happens: It works much better! I still see a framing error between almost every message, and BREAKs scattered throughout, but the pages of framing errors are at least gone. Adding the GND connection appeared to make the capture quality lower, so I'm just using Pins 2&3. Am I doing this right or is there a better/more correct way? Thanks!

[EDIT #2] I just stopped and started both the program sending data and the IO Ninja. Now I'm getting continuous FRAMING ERROR messages ... even when I stop the data generator and close its serial port. When I also stopped the receiver and closed its serial port, IO Ninja started continuously recording zero bytes and BREAKs with a strange sort of heartbeat pattern. I will try to attach a short session showing the strange pattern of null characters and BREAKs. Nope. Can't figure out how to attach a file, but you can get it from IO Ninja Session Null-Break.zip if you want to look at what I see.

My only real question here is why don't I see the 'z' in the log or any characters at all in the information pane?

If this is expected, (and it very well may be) I just need to learn why.

For reference, here's a photo of the Serial Tap so you can see what's lit up (the connection is idle -- no traffic at all):