The Common Mode Range for RS-485 is -7V to +12V, i.e. +/-7V on top of the 0-5V signal range. A good receiver, or one that adheres to the standard should not insist on a midpoint of 2.5V
Agreed.
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
In 230/400 V mains environment, with data lines running in parallel with mains wires for long distances, the worst that would happen is the data bus swinging between +320 V and -320 V due to capacitive coupling.As long as the data isolation voltage is larger than that swing, who cares.
In practice the resultant capacitive field from a mains _cable_, is much less since much of the external field is canceled by the neutral or other phases and the coupled voltages are much lower.
Does the century old unipolar 20 mA current loop need a separate ground wire in addition to one conductor carrying current to the receiver and an other back from the receiver ? No it doesn't, look for instance at the telephone connector.
RS-422/485 with a single 100 ohm terminator resistor and minimum
+/-200 mV swing, is essentially a +/- 2mA bipolar current loop. In practice, the loop current is closer to +/- 20 mA and hence the voltage swing larger. Why would a current loop need a ground voltage reference ?If bipolar transistors are used to sense the voltage across the termination resistance, the issue is providing some base current to the bipolar transistors.However, with "fail-safe" resistors installed, there is a current flowing
The transistor bases can rob enough current to bias the input transistors. Do no need for a separate ground reference wire.
On 2018-11-02 Tauno Voipio wrote in comp.arch.embedded: [...]
With "fail safe" receivers, a termination resistor alone will keep the differential voltage low enough for the receiver to see 'idle'. But if ypou can ad bias, why not do it?
Why is Pozz trying to do this without termination? On RS485, use termination! Never had trouble with termination, have had trouble without.
Just remembered an occasion where I had to source a resistor in a remote location with no electronics shop anywhere nearby. And then climb a 70m ladder to install it. Then you really wonder why they originally did not install a termination resistor. :-(
-- Stef (remove caps, dashes and .invalid from e-mail address to reply by mail) I have not yet begun to byte!
The only case I can understand is power consumption especially in battery powered systems. There is quite lot power dissipated in the termination resistor and in drivers. This is a real problem with RS-422 in which the transmitters are always active in idle Mark state.
In RS-485 the transmitter is active only when a specific node wants to transmit,otherwise all transmitters are in tree-state.
Putting a big capacitor in series with the termination resistor reducing idle consumption especially in RS-422 without hurting transients,
Then why not use series termination as used in all low power systems I've ever come across?
-- Mike Perkins Video Solutions Ltd www.videosolutions.ltd.uk
Series termination is really only appropriate for a point-to-point system (one driver, one receiver) as it is based on the signal going to half level until it reaches the far end when the reflection makes it become full strength, and then the reflection comes back to the driving end and the termination keeps it from bouncing back, so middle points see 1/2 levels for a period until the reflection comes back.
One thing that can be done is to terminate with a series R-C network so their is no idle current, but the active edge still sees the appropriate impedance to avoid reflections. You may then add a much higher resistor across the capacitor for the fail-safe bias network (with significantly higher resistances than for a pure restive termination to provide a clean DC bias in idle.
With parallel termination doesn't every point see a significantly lower level all the time? The drivers aren't voltage sources, they still have significant series resistance, no?
Rick C.
Most RS-485 chips can deliver at least +/-20 mA, so there is +/-1 V across the 100//100 ohm termination, Long wires with high resistance will reduce this. The threshold is +/-0.2 V.
While the voltage is lower, significant current will flow and power dissipated. This is a brute force method, but helps keeping the interference out of the connection by improving signal-to-noise ratio.
Il 02/11/2018 23:57, Stef ha scritto:
I try to clarify my questions on termination resistors.
I can really use them. The higher power dissipation is interesting in my battery-powered application, but I think I will have enough energy. So this isn't a problem.
I will use a termination resistor on both sides. My concerns were on the resistor correct value. If I need a termination, I need to use a correct resistor value. Is it critical?
The other problem with signal ground, I know it is better to connect it together with A and B. However it would be much more simple and cheap to use a simple couple instead of a full CAT5E cable.
Most twisted pairs are around 100 Ohms, so you use a terminating resistor near there. Being slightly off isn't normally an issue, it just says that you get a bit of a reflection. RS485 isn't that high speed (or fast edge rate) so that makes things even a bit less critical, and ring tolerances higher.
As has been said, the key requirement for the ground is to keep the common mode voltage in range. This means that when using it for long distance links you need to pay attention to grounding issues, and often need to worry about isolation to avoid ground currents.
Depending on the application, you may also be able to save power by disabling the transmitter when you're not transmitting. You'll need to think about the implications for the receiver: the failsafe logic, etc. But if you're only transmitting 1% of the time, you only need to be dumping current into those termination resistors 1% of the time.
-- Rob Gaddi, Highland Technology -- www.highlandtechnology.com Email address domain is currently out of order. See above to fix.
I have converted from RS232/RS422/RS485 to single-mode fiber over tens of k ilometers. In fact I had that much bandwidth for RS232 that I would just cr eate virtual wires from the 8 signals so baud rate etc wasn't important. Wh at changed on one end would change on the other. For megabaud RS485 rates t hough I would buffer the data and send it in high speed packets to reduce l atency and also allows for control signals.
However for short distances up to a 1000m or so I would just use TP with pr oper termination which is easy if it is essentially point-to-point. Most of my systems though are multidrop and I have even used 4-core phone cable to carry DC and 485 to nodes inside a building. In one of my recent designs I had about 40 nodes connected over about 15m within a large cabinet with I DC cable and with the 485 signals on the two inner pairs, communicating at
2M baud. Despite the huge amounts of electrical noise it worked perfectly.I am really surprised though to find that in 2018 people are still talking about RS485 "bias" resistors. Doesn't anyone use fail-safe type 485 chips? I use many different ones such as 65HVD75 or VP3082/3088 etc and these prov ide failsafe in both line open and line short conditions without the need f or bias resistors. They are also 1/4 or 1/8 load and also have much more se nsitive receivers that work down to an acceptable 80mV hysteresis. There ar e also other types that have huge common mode range. Besides many multidrop systems have problems because techs add termination and bias to nodes that "are missing it".
But I'm in the two wire camp and I am fully aware of the technician's think ing that you "need a ground" and the logic that says there could be huge gr ound differentials also ignores the fact that if this is so, then connectin g grounds/earths across these systems would and does result in huge noisy g round currents through the same cable conveying the signal, and so the syst ems better have a very good earth or none at all (isolated).
Isolation (solid-state) and 2-wire is better but over long distances you ne ed lightning protection too, and I use combinations of transzorbs, ferrite beads, polyfuses, and miniature gas discharge tube protection and in these cases there is not a ground, but there is a very good earth.
Getting back to low speed RS485 and just how much you can get away with. I once ran RS485 at 19,200 baud multidrop over many branches of figure eight fire proof cable up and down lift wells over 26 floors and simply relied up on blindly repeating check-summed packets three times so that the master co uld address all lifts or groups of lifts simultaneously. Somehow it just wo rked and kept on working :)
So no, you don't need fiber for 500m, just protect and isolate in hazardous and noisy environments. You can even get away with good optos at such a lo w speed and just use those tiny 4-pin isolated DC-DC converter modules to p ower the RS485 chip. You don't have to change anything with the MCU and UAR T methods. I prefer using solid-state isolators though such as the ADuM3211 ARZ as they are much easier to use.
BTW, without the third wire the current flow through the differential recei ver's inputs and through its ground is its reference so that one input is g reater or less than the other. Think of the inputs as being resistors conne cted to the receiver's ground. Try connected a small battery across the rec eiver terminals and then swapping it to change the signal, none of which re quires a third wire (for the signal). LTSpice this if you are in doubt.
Cheers, Peter
In the context of lightning protection, what is a good earth ?
With two buildings with separate mains connections and individual earthing electrodes, you do not even need a direct lightning hit, just a hit to the mains leads and some arc suppressions to earth at mains entry and you have several kA flowing into the earth. With earthing resistance as low as 1 ohm, you will have a ground potential bounce of several kV in one building. I very much doubt that ordinary isolation and some protection will handle this potential difference.
When connecting two buildings with individual earthing electrodes, I would definitively use some fiber.
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