How to add remote testpoints to non-multidrop RS422 without causing reflections?

Question

I have a RS422 non-multidrop connection between two connectors on a PCB (CLK+/- and DATA+/-) clocked at 1MHz, and currently routed with adapted edge-coupled microstrips (120Ohm differential impedance).

I would like to add measurement stubs, for example with oscilloscope probe tip connectors - but without degrading the quality of the bridge I am measuring.

The existing connection is about 10cm long, but the measurement stub will have to be 20 to 30cm long to reach a maintenance panel.

• Because of the way the PCB is designed, I cannot just add testpoints ON the existing RS422 connection.
• I thought about designing microstrips with differential impedance much greater than the signal I want to measure and leave the testpoint not terminated, but it's impossible for me to go above 350 Ohm.
• I also thought about making a 120Ohm stub terminated with a 240Ohm resistor (since the equipment is not multi-drop, I suspect it won't handle 2 stubs with 120Ohm termination each - more likely to handle 50% more load), which currently is my baseline but I'm worried about the reflections from the 120->240Ohm transition and potentially from equivalent 60 OHm to 120 Ohm (see below).

What is the best way to handle this in your opinion?

Don't two 120Ohm stubs look like a 60 Ohm path at the fork, from the perspective of a signal upstream of the fork - therefore causing reflections?

Answer 1

clocked at 1MHz

As a general rule of thumb when looking into data corruptions on mis-terminated lines anything up to the 10th harmonic would be considered. So if the basic data rate is such that it looks like 1 MHz we might consider 10 MHz to be a good basis for analysis.

The wavelength of 10 MHz is 30 metres and, another the rule of thumb is that any transmission line longer than about one-tenth of a wavelength could potentially be problematic if badly terminated. That's a line length of 3 metres.

What is the best way to handle this in your opinion?

Hence, there isn't really going to be a problem to either the data down the stub (30 cm) or the original data path (10 cm). If in doubt, connect to the stub via 1 kΩ resistors and use a differential amplifier to monitor the data signal.

Simulation

1 MHz output signal (Vout): -

I don't think the edge disruptions are significant for RS-422 integrity. If you do something like this: -

You'd be hard-pressed to see any degradation on your main RS-422 signal at all. And the monitor signal also looks good but, as expected is attenuated by 10:1.

Remember, my sims have used unbalanced signals and coax so, you'd need to split the 1 kΩ resistor into two 500 Ω resistors for proper RS-422.

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As an afterthought...

If I made the monitor line 3 metres long and didn't use any resistors, I'd see this disruption on the main signal: -

It's still OK and wouldn't throw up any significant corruptions.

If I made the monitor line 10 metres long, the main signal is starting to look crappy: -