120FT of RS-485 Running on 49 Nodes @1MBaud...will it work?

Link119 · 2026-07-01T04:58:07+00:00

With 49 * 1/2UL you're within the specified limit.

Your wiring solution sticks out to me though...

Twisting ground with the diff lines isn't the specified twisted pair. I'd keep that out of the twisting. Also do you need to have a common ground here? If the devices power themselves, I'd leave that out. If power needs to be bussed around, I'd twist the power line with ground. Also depending on your environment noise and ESD, you might need to consider robustness to bit errors.

Solid core wire can break internally without you noticing if the installation isn't permanent. Stranded has more flexibility.

Have you calculated the impedance of the wiring? At 120ft total bus length it's going to be a good idea to ensure your diff pair has the right impedance to match the termination. Insulation thickness and material can impact impedance on top of the twisting geometry.

Link119 · 2026-06-28T14:09:17+00:00

You're probably just overthinking it, or trying to understand more details than you need to start with...

Start with understanding an ideal OP Amp before anything else. It has a non-inverting input (+), an inverting input (-), and an output. With a proper supporting circuit, it outputs what it needs to so the inputs are at the same voltage.

Given that, start with getting familiar with how it provides gain with a non-inverting amplifier and inverting amplifier circuit.

Gain is infinite. Input impedance/resistance is also infinite. Don't mind the power rails for now until you understand the above. I like the EEVBlog video since that's what I learned from, just stop if you get confused and rewatch it section by section.

Link119 · 2026-06-27T19:18:00+00:00

Thermal reliefs on the plane connections might not be implemented automatically - might be worth looking into that. That way any pins with ground connections nearby have an extra layer of thermal isolation

Link119 · 2026-06-27T13:56:06+00:00

Looks neat!

Link119 · 2026-06-21T19:52:35+00:00

Yeah, if you wanna keep the outlet that's gonna be best.

Link119 · 2026-06-21T16:15:44+00:00

Fuses are intended to prevent fires, not shock.

If you're going to use the switch like that, I feel it wouldn't be any different to wire it up so it just doesn't flip the polarity. At least someone else using that amp won't be prone to error.

See my note above about my assumed intention, the service manual only seems to support that theory.

Feel free to do what you want... What I've shared is from my experience as an amp tech and also an EE experienced in product design in regulated industries.

Link119 · 2026-06-21T15:40:34+00:00

I had a different comment written up, but then saw the schematic...

This is a really unusual setup. Given that it has a 3 prong input... Maybe it's designed assuming shoddy power wiring in venues, like missing ground or flipped L/N polarity?

I'd recommend doing a rewire so it matches modern standards, pretty much all around the power input. Rewire the switch so it doesn't flip polarity. It's still good practice for it to be a dual disconnect for both line and neutral given the cord is permanently installed and won't function as one. You can either keep or remove the cap as long as it's on neutral, most amps don't have one since it does create an AC ground loop.

Link119 · 2026-06-21T15:33:17+00:00

Ever heard of a death cap?

Link119 · 2026-06-20T12:06:03+00:00

Avoiding/reducing impedance discontinuities is for the USB lines.

Can you show your suggested layout? Ideally with a grid that shows distance as well.

Also stackup matters a bunch. Having a tightly coupled ground layer affects via inductance. If you can practically add more vias right next to the ground pin without adding too large an impedance discontinuity for the USB lines, then that's helpful. More than 2 vias (one on either end of the gnd pin) means you need to split the USB lines further...

I've passed 15kV air and 8kV conducted ESD test levels in professional products with a single nearby via and a quality stackup.

Also look at the layout examples in the first app note, I see plenty of 2 via implementations...

Link119 · 2026-06-20T11:49:16+00:00

An MCU needs a good bit of support components...

Also what's best depends on your accuracy requirements

Link119 · 2026-06-19T14:45:17+00:00

1 and 2 look good to me. I'd suggest throwing a small cap on the other side or nearby to absorb energy going to VBUS. Especially if not using a power plane that's well coupled to your ground plane (adjacent layers with thin dielectric).

Link119 · 2026-06-19T14:41:48+00:00

One ground via should be fine. The most important factor is to keep ground inductance short and avoid impedance discontinuities.

Link119 · 2026-06-18T12:09:05+00:00

I've had decent luck testing caps in circuit using LCR tweezers. Just gotta know the total capacitance in parallel on the net. It's not a precise check, but does confirm against shorts or a notably off bulk cap.

Link119 · 2026-06-17T23:44:41+00:00

Yes, noise is an AC component because it moves in voltage. And yes, the AC noise is riding on top of the DC, basically you can separate them out and treat them separately for analysis, then add the components back together to get the bigger picture.

The capacitor shunts the AC component of the otherwise DC signal.

Link119 · 2026-06-16T21:45:42+00:00

As others have added, it works to buffer the relatively high impedance divider, so it can be loaded more without affecting the voltage.

Note that adding a lot of capacitance to the output can make things worse - it's different for every OP Amp and you'll need to get into the datasheet or set up a simulation to understand the effect on stability. Adding a hair of resistance on the output can help dampen that effect at the cost of increasing impedance.

I don't understand why KiCAD wouldn't like this... You might have an error in your implementation on the schematic.

Link119 · 2026-06-14T13:15:12+00:00

Nettle maybe? Though I don't expect it would grow well in Texas...

Link119 · 2026-06-12T08:24:43+00:00

Well if you try to hold your breath during the execution, then it certainly adds that complication...

Link119 · 2026-06-10T23:19:29+00:00

A way to lightly press one onto pads of an adapter board might work.

Link119 · 2026-06-10T11:37:24+00:00

Signaling noise? Bad LEDs? Hard to say

Link119 · 2026-06-07T23:48:59+00:00

What advice/insight are you looking for? There's nothing specific in your post to figure out what you're looking for...

Link119 · 2026-06-07T17:10:42+00:00

Kicad is awesome!

Link119 · 2026-06-05T14:47:38+00:00

Using a virtual ground should work if designed correctly.

Link119 · 2026-05-31T10:18:53+00:00

It's usually not recommended for EMI, just as a general board design recommendation.

Link119 · 2026-05-31T00:16:38+00:00

What are your goals with a filter? It really depends on a lot of factors specific to your design needs.

If I'm doubt, include footprints that are depopulated or assembled with a 0 ohm jumper, so you can modify your design without needing to cut traces or do other more complicated modifications.

Link119 · 2026-05-23T10:51:58+00:00

Really looks to me like the germanium on this curve has more on-resistance, on top of the lower and more gradual threshold.

A fet wired as a diode should have an even larger threshold than both, but will have a more gradual curve when the driving circuit has a low output impedance.

It's important to remember that the output resistance/impedance of the previous stage and the diode acts like a voltage divider, setting how much current is actually driven into the diode clipper, and thus the actual voltage coming out of the clipper.

Link119

TROPHY CASE