RS-485 using LAN connectors?

Look up the PoE spec for the pairing used for power and data.

No problem: More:

Dunno. UL only cares if the device or connectors fail in some way, such as starting a fire, electrocuting the user, or exploding. There's not enough power in RS-485 to be able to do any of these, so I guess you would be safe. However, methinks it best to ask someone with more experience in obtaining the blessings of UL.

Screw terminals? That's really stone age. I thought the standard connector for RS-485 is a DE9 connector. (No, it's not a DB9).

There are 4 uncommitted wires in the RJ45 jack which can be used for PoE. However, you'll probably need to implement some kind of complex safety protocol to keep UL happy, such as 802.3AF or AT. You can buy that off the shelf, so there's no reason to reinvent the wheel.

Also, think about what might happen if you plug your 48V RS-485 contrivance into one of the numerous interface and wiring standards found on an RJ45 jack. For example, if your low current and fused 48V power unloads its power filter capacitor into the winding of a datacomm line xformer, you run a good chance of blowing the xformer winding before the fuse will blow.

Also, what's a fuse? These daze, we use self resetting OCPD (over current protection device) gizmos such as PTC thermistors:

I'm trying to forget the allegedly good olde daze.

The problem is what happens when you plug your RS-485 into an Ethernet interface by mistake.

Since Ethernet is always transformer coupled, it is unlikely that there would be any permanent damage in normal cases.

This complicates things especially when you also consider PoE. Contrary to common belief PoE does _not_ use a separate pair for DC pair, but rather uses a phantom like feed system. The Ethernet transformer are center tapped. The center tap in one pair is connected into positive DC supply and the negative DC to the other pair transformer CT. Thus one pair has a positive voltage, the other a negative DC potential.

One idea would be to use the "unused" pair for RS-485 and use one Eth pair for positive and the Eth other for negative DC voltage.

Check what happens, when the other end uses 1GbE or 10Gbe is used, where all pairs are used.

Any chance that the device would also support Modbus/TCP ?

If not, use a simple Eth/serial converter close to the device. Run Modbus RTU frames over PoE. Use a PoE power extractor close to the converter and use it to power the converter and the Modbus RTU device.

One issue: you'll want 8-pin crossover cables, unless you like connecting RX to RX and Tx to Tx... either that, or some careful labeling ; tag one end of each cable (marked end to box with 'straight'' switch position, unmarked end to box with 'crossover' switch position).

Otherwise, you're stuck with a host/slave dichotomy of devices.

It effectively creates a DC bias on two (or four) of the pairs.

The actual PoE standards are complicated - they are highly over-engineered, IMHO, with negotiation between the supplier and the device about current requirements, and multiple versions using different wire pairs to maximise incompatibilities.

It is not uncommon to have a "passive PoE" solution in closed systems, which means simply putting a fixed DC voltage on some of the pairs. Unfortunately, there is no standard here. But since this is a RS-485 system, not an Ethernet system, there is no problem.

DC bias, regardless of the choices, should not cause problems for normal Ethernet or for standard PoE if someone connects an Ethernet cable into the port. As long as you have reasonable protection against short-circuits then passive PoE mistakes won't be a problem either.

That's the normal way to do it. If the RS-485 is not isolated, then it is by far the simplest way.

Nothing bad.

He has RS-485, not RS-232 or RS-422. He will want DO connected to DO and NDO connected to NDO. That means straight-through cables.

Where are these devices being used? Is it unlikely there are any existing ethernet devices or lan infrastructure?

I could see a case where someone decides there is no reason to run a ethernet cable because they already have a cable from a switch to that location. So they plug your device A into the switch and your device B into the far end cable. At minimum you have a customer calling about the devices not working.

I remember all the hell we would have when a field guy would plug a ethernet cable into a Cisco RJ45 console port.

In a building. (several, actually.) All terminations are in our proprietary cabinets, and I do think it is very unlikely that someone would come along and want to plug any type of Ethernet or PoE device into the modular jack. There are no Ethernet devices in our cabinets.

.... but you never know. People do stupid things. If we sell enough of these, somebody somewhere will eventually try to plug our RS-485 device into a LAN router.

I want to at least wire the modular jacks in such a way as to minimize bad outcomes. I think we've got the answer, and I'll post it next week (for archive) after I confer one last time with our senior designer on this build. Thanks everyone!!

That reminds me of Walmart's directive for new stores in which all switches in the retail space were to be mounted at a height of 10+ feet inside a closed cabinet. The NOC told me that Walmart mounted switches in such an awkward manner in order to keep the public well away from the switch. Because in times past, someone would pull the power on a more readily accessible switch in order to plug in a coffee maker, a fan, or some other appliance. Sometimes the whole store went offline. If your unlocked cabinets are within reach, sooner rather than later, someone will connect your RS-485 4-pair to a data network. StarTech sells these PoE injectors:

It may very well be that the most important job done by the standard green PCB terminal block is to warn people "this is not a data network jack."

Thank you, 73,

That reminds me of Walmart's directive for new stores in which all switches in the retail space were to be mounted at a height of 10+ feet inside a closed cabinet. The NOC told me that Walmart mounted switches in such an awkward manner in order to keep the public well away from the switch. Because in times past, someone would pull the power on a more readily accessible switch in order to plug in a coffee maker, a fan, or some other appliance. Sometimes the whole store went offline. If your unlocked cabinets are within reach, sooner rather than later, someone will connect your RS-485 4-pair to a data network. It may very well be that the most important job done by the standard green PCB terminal block is to warn people "this is not a data network jack."

Correction: the StarTech device in my earlier followup is not a PoE injector. Apparently there are no PoE injectors that work with RS-485:

Thank you, 73,