Timeline for RS-485 bus with PoE bus
Current License: CC BY-SA 3.0
9 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 20, 2016 at 19:55 | comment | added | Michael Karas | My points regarding the common mode voltage at the receiver due to shifts in the GND line are separate issues than the ones you start talking about due to the I R drop of the cable data lines them selves having to conduct the termination current. | |
| Jan 20, 2016 at 13:32 | comment | added | HilarieAK | From reading the spec sheet on the driver I'm looking at, it puts out 3v +/- on the pair, and needs to receive a difference of at least 200mv between the two, so I can suffer a 80% loss on the signal and still receive it reliably. Vdrop = Ohms * Current (in amps) So if I have 24 AWG copper wire at 26 ohms per 1k feet and the driver pushes 90 ma @ 1v (Max rating) .090amps * 26ohms = 2.34 soooo Yeah, You are right and I really need to focus on the math for this stuff (and probably throw an full duplex transceiver in here or there for a few extra dollars to boost the signal) | |
| Jan 20, 2016 at 13:04 | comment | added | Michael Karas | To put this into perspective lets say you have good quality 100% copper wire. Cat 5 would be 24 AWG at about 2.5 ohms per 100 feet. If you were running a distance of 200 feet and distributing 1A of current on the 48V supply the net voltage drop in the GND wire would be about 5V (i.e. 5ohms * 1A). (The V+ wire would have the same drop so the net voltage delivered to the destination would be ~38V). As you can see this 5V GND drop is a significant percentage of the RS485 common mode voltage range. Also consider the typical 3V common mode range produced by a typical RS485 driver. | |
| Jan 20, 2016 at 12:48 | comment | added | Michael Karas | (continued from above) up or down [depending which end is the receiving end]. The RS485 receiver will be designed to be able to extract the differential signal as long as the common mode voltage stays within a reasonable range. If I recall the normal RS485 specification this is supposed to work in common mode range of -7V to +12V. | |
| Jan 20, 2016 at 12:39 | comment | added | Michael Karas | No. I am not talking about the voltage delivery wires coupling noise to the RS485 lines - although that could happen I suppose if the voltage wire had current variation that was out of sync with the changes of the RS485 signal itself. My main point is that there will be voltage drop along the wires between the power source entity and the destination power consumption entity. This voltage drop can get significant when small gauge wires are at play over long distances. The voltage drop in the GND wire of the power delivery system will shift the common mode voltage at the receiver (continued) | |
| Jan 20, 2016 at 7:49 | comment | added | HilarieAK | Oh, I think I may have misread what you were saying. Are you saying that running 48V through the same line as the RS-485 may mess up the RS-485 signal? | |
| Jan 19, 2016 at 18:18 | comment | added | Michael Karas | Large capacitors will only help to the extent that you are able to isolate the local power circuit from the current flowing in the power source in the cable. You should setup some tests to evaluate this over the longest lengths of cable runs that you want to support. It can be very helpful to reference a multi-channel scope to one cable end GND and then monitor the other end of the cable's ground with a scope probe to see how the ground shifts happen whilst the RS485 is transmitting in both directions. | |
| Jan 19, 2016 at 17:12 | comment | added | HilarieAK | Thank you, for some reason I didn't read deep enough into the chip I was thinking of using(MAX1483)'s data sheet. I now see that it will pull 240ma. I'm thinking though with a few large capacitors I should be able to smooth out any drop, only 1 slave will ever be transmitting at once so with lossless conversions I should be able to pull this off with only 1 pair for the PoE. | |
| Jan 19, 2016 at 14:44 | history | answered | Michael Karas | CC BY-SA 3.0 |