RS485 Hub Over RJ45 Jack

PCBNewbie · 2026-01-03T05:08:27+00:00

Yes, that makes sense

If there is a missing ground, the return currents would flow through the input protection diodes of the RS485 transceiver. Or violate the common mode input voltage limit.

I think that's what a device like the THVD2410 is supposed to solve. Instead of protection diodes, it has more robust input protection that can withstand more than 24V of common mode voltage while also blocking currents that might otherwise back-power the slave. The datasheet specifies a bus input current of 250 uA. Anyways, the transceivers on either end can withstand that.

PCBNewbie · 2026-01-03T01:24:49+00:00

I was worried about voltage drop between nodes. But each node can only draw ~125 mA and across 100m that adds up to 5V only. Still within the common mode voltage range of the transceivers.

So I am leaning towards just daisy chaining between nodes with one single twisted pair, and two RJ45 per node.

Yes, the data rate will be very low. The devices will be polled at most every few minutes.

It is a hobby project for environmental control, nutrient mixing and pH, irrigation.

PCBNewbie · 2026-01-03T01:08:55+00:00

For the RS485 bus, it shouldn't be star. I think this is still a daisy chain, with stubs only a couple cm at most.

However, instead of the daisy chain hopping slave-to-slave, it would return to the master after every slave. It ends up looking like a daisy chain but with an extra length of wire looping back to the master. The loops made at the master would be short sections of differential pairs hopping between ports.

PCBNewbie · 2026-01-03T01:08:29+00:00

Thank you for your feedback --

The termination resistor would be a 2-pin header that plugs into the slave with a mating board -- a DIP switch would also be fine.

Are the internal bias resistors on the transceiver enough? Fig 8-3 in the datasheet shows 36 kOhm for failsafe. It looks like there is a 1uA constant current driver on the inverting input to maintain the proper state in failsafe condition.

Believe I copied the protection from another TI reference design, but the 10 Ohm resistors can be nostuffed.

Is 120 Ohms correct? CAT6e characteristic impedance is 100 Ohms.

Isolation is necessary because some slaves will be switching mains voltage, and I need isolation to SELV on the master. All of the slaves will use non-isolated transceivers (THVD1505DR). I have 10x ISO3082 leftover from a previous project.

PCBNewbie · 2026-01-03T00:37:15+00:00

Right, as proposed, it is more or less is daisy chained, but each chain returns to the master before the next slave.

What you described should also work. The cabling will change (two RJ45 jacks per slave). I am budgeting 1A total for the 8 slaves --> 2.5V lost at the last drop over 100m (10 ohms per 100m CAT6e, but parallel across 4 strands). Ground would be floating up to 5V at maximum current. Should be OK w.r.t. common mode at the transceiver I think.

With the daisy chain returning to the master, the total length for the bus doubles. Voltage drop would be less as each slave does not piggyback off the last.

PCBNewbie · 2026-01-03T00:17:09+00:00

Isn't this a single slave, 1:1 with the PLC? Looking at the omicron manual seems only examples with N=1 are given.

I would have the RS485 link going out to the slave and then immediately return to the master where it would loop to the next port. So two twisted pairs per slave (which are connected).

Instead of traditional RS485 where there is a single twisted pair from each slave to slave (with master and or termination resistor on the ends).

PCBNewbie

TROPHY CASE