RS485: more on transmission line termination & material + reality check

By: Frank Rysanek of FCC prumyslove systemy s.r.o. [ rysanek AT fccps DOT cz ]

There seems to be a common wisdom saying that RS485 runs over just about any cable you happen to have around, and termination / grounding doesn't matter much. This article provides some practical observations / oscilloscope plots, implied by such approach. Finally, the highly theoretical transmission line mumbo jumbo (and conclusions, arcane termination networks and whatnot) are confronted with reality - practical findings as to what does actually help when an RS485 network just doesn't work. And no, common-mode termination is not the magic bullet :-) The work was guided and inspired heavily by the Circuit Cellar articles mentioned in the References.

Contents

• Oscillograms to start with (+explanation)
• Detour #1: RF transmission lines
• A few words on signal cables
• Detour #2: galvanic isolation of RS485 ports
• Wiring as suggested by the RS485 standard
• Reality check: on site, hands on (grounding and biasing to the rescue)
• References

My general approach to problems is to try to reproduce them in the lab first, and in the process, get the hang of the technical area of focus, maybe do some follow-up reading, figure out some test methods, hack together some reusable probes etc. In other words, do some thorough technical howework before I possibly depart for the customer's field site.
Ironically, but not really to our surprise, our hardware behaved fairly well in our lab. Under scrutiny, the UART's did show some marginal misbehavior (difficult to reproduce), but no complete show-stoppers of the sort described by our customer. So I decided to start by taking a hard look at RS485 itself, for the first time in my practice.
I actually have a nifty new USB oscilloscopey gadget, which I've recently complemented by a simple homebrew "refectometer probe" (pulse generator) - I figured it was time to give them a whirl. Others in my place would certainly suggest a jumbo mug of coffee, except that I don't drink that - but I still have to admit that I did sit back and think a lot.

My employer company doesn't really install RS485 (nor SDH/PDH nor CAN) in the field, so I don't have the genuine shielded/twisted cabling footage in stock. When testing RS485 gadgets in our lab, we tend to use CATx cabling (commonplace in our computer shop). Usually we use CAT5e UTP, or sections of single pairs extracted from that - but I also have some CAT7 S/STP left over from the last Ethernet upgrade (used it for the "gigabit backbone").

So I cobbled together a basic RS485 network: a 3rd-party Modbus/RTU slave (some discrete input module), some RS232 and USB converters for "eavesdropping" on the data via other computers, attached the culprit box as a Modbus master, added the basic rule-of-thumb termination (120 Ω at each end), and connected the scope probes.
I have a dual-channel digital scope (cheap USB thingy) with un-isolated un-balanced (single-ended) inputs, so I just attached the scope probes' ground clamps to the RS485 master's signal ground terminal and hooked up the probe tips to RS485 DATA+ / DATA-. The bus cabling consisted of maybe 14m of CAT5e UTP, in a coil on the lab table. There were several short stubs (50 cm or less) at the ends, but I figured that those wouldn't matter much (and I was right). Initially I didn't provide a signal reference ground for the bus (our vendors of RS485 equipment do not stress that point). So the various nodes either had a reference ground from the mains protective earth, or had no reference ground at all (some of the nodes were earth-isolated at the PSU or at the 485 port). The line parameters are 9600 8N1. I actually started with just hyperterminal at the several computers attached to the bus, and kept typing characters at the various terminals, to see if the data was getting through.

It was immediately clear that in my lab setup, the data was getting through just fine (as expected - it works even without terminators). Something did raise my eyebrows though - the oscilloscope traces.

1.

## loopey stuff - there's no explicit signal reference ground ##

An interesting aspect of the wobble is, that its wavelength is much too long to be caused by just the 14 m of transmission line. So the resonant frequency must result from something else.
The 14m coil of CAT5 cable and the mains ground clearly form quite a loop. In electric terms, loop = inductance. So I figured that the wobble was likely a sign of some LC resonance between this large loop (inductance) and the transceivers' paracitic input capacitance (which I later determined to be somewhere in the hundreds of pF, depending on hardware model).

Perhaps this wobble isn't outright detrimental to data integrity, and maybe it is that way by design / per definition. The wobble is not "tall enough" to push the differential RS485 receivers out of their common-mode operating range (approx. 0-7 V vs. GND) with unpredictable results (generally random signal inversions / clamping). Your general common sense tends to tell you that the longer the distances, the bigger the loop (inductance) - the greater the chance of adverse effects. But this appears to apply only for EMI ingress. As for the amplitude of our resonant wobble, that is probably determined by the amplitude of the initial common-mode edge (step-wise change of voltage), which is characteristic to the particular bus driver, and does not depend on line length / line+GND loop dimensions (inductance). The line+gnd loop dimensions do affect the resonant frequency and Q of the wobble (= duration of the wobble) - but as there is no periodic addition of energy to the "resonator", the amplitude can only decline after the initial edge.
Still the wobble is a pretty ugly sight, and it indicates sensitivity to EMI ingress into the "ground return path".

So, how do we suppress that resonant loop? The answer is that we need to provide a dedicated "signal reference ground" conductor, as closely aligned to the payload twisted pair as possible, in order to minimize the square of the resulting loop (to minimize its inductance). The reference ground is really a companion signal to the balanced twisted pair, helping to keep the individual nodes' transceivers "within the ballpark" of DC operating range. By minimizing the loop square, we minimize all sorts of "interference" offsetting the reference ground from the local optimum for each and every node.

2.

## explicit signal reference ground, at the cost of a solid ground loop ##

In my example, I just used another pair in the CAT5 cable for a reference ground. I connected the two conductors in parallel to avoid possible RF effects in the pair. I connected the ground at both nodes of interest straight to the relevant earth (I didn't install a "separation resistor") - so the grounds actually formed a hard ground loop (closed across the power supplies and mains earth), which traditionally is another big thing to avoid. Yet the effect of this grounding was profoundly positive - behold the whole second column in the "table of plots".

The ultimate right way around the loopy grounds problem is ground isolation - either at the PSU, or within the device, perhaps preferably at the RS485 port. This scenario is not depicted in my oscillograms (was not an option in my case - money always first, let alone the added complexity). Check the galvanic "detour #2" for further notes about galvanic isolation in the context of RS485.

My train of thought went in a different direction though. Note that on the first "grounded" oscillogram, there is still a noticeable common-mode glitch with a resonant fade-out.
The common-mode loop square is clearly smaller now, so the resonant frequency is higher and the oscillations fade out faster. Still it was making me wonder, how much of this is due to LC resonance, and how much could be due to an open-ended transmission line reflection (common mode).
Take a look at the relevant "TX OFF" plot. That's where both drivers (+/- or A/B) should "let go" of the line. Given how the balanced driver is wired, this "line release" produces a significant common-mode edge.

## composite 3wire transmission line (diff-mode and common-mode impedance components)##

The ground conductor (pair), following the live balanced pair in the same sheath, still comprises some "loop inductance" with the pair. But from a different perspective, it also comprises another transmission line. The balanced pair itself is a differential-mode transmission line. The balanced pair against the ground conductor (pair in my case) conceptually comprise another, common-mode transmission line. You can also think about them as a single, three-wire transmission line, with a differential-mode and common-mode component (aspect). And, just like the differential-mode TML should be terminated to prevent diff-mode reflections, the common-mode TML should also be terminated, to minimize common-mode reflections. Now how do we go about that?

If you really care this much about your transmission line :-) , the first thing you need to know is the common mode and diff mode impedance. If you don't know the figures for your cable, you can measure them. For many basic-performance twisted-pair cables, the vendor's datasheet doesn't even give you the differential-mode nominal impedance of each pair. And, even if the diff-mode impedance is given, the common-mode impedance is never mentioned (on high-performance STP cables, you can hazard a guess - see below). For unshielded TP cables, it's indeed not very fruitful or sane to ask about the impedance between pairs :-)
Anyway you can measure the impedances if you need to know - my favourite tool is a reflectometer. You pulse one end of the cable, terminate the other, and watch at the generator for a far-end reflection. If there's no reflection, your TML is terminated correctly (the resistor value is right).

## differential mode impedance measurement ##

## common mode impedance measurement ##

A few notes on the measurement setup:

1. My homebrew time-domain reflectometer probe (a trivial pulse generator, really) has single-ended / unbalanced output (perfect match for coax cables). Theoretically it is a sin to connect that to a balanced transmission line (one conductor to live output, the other to GND). Practically the mismatch is not too much of a problem. It would certainly be nice to have a balanced output on the reflectometer probe, but at the required slew rate / pulse length, that would pose other design challenges (how to make the output truly balanced, simple+cheap, without any time shift between the two live pins, and with configurable impedance all at the same time). In other words, connecting a balanced TML to a single-ended generator is an acceptable tradeoff.
2. You can use a standard high-impedance probe for the scope, but better yet (at higher sampling rates) is no standard probe at all. My homebrew reflecto-probe plugs straight to the scope's input, which has the typical high impedance (1 MΩ + ~30 pF).
3. The precise impedance matching of the generator (reflectoprobe) to the TML is not a big deal. As long as you focus on just the far-end reflection (when fine-tuning the far-end R_term), you don't need to have the near end precisely matched. Even differences as great as 50 Ω vs. 120 Ω don't matter much. On the time-domain waveform, a local-end mismatch causes an unimportant change in pulse amplitude and maybe some short glitches on the edges. It doesn't offset the far-end reflection in any important respect (as far as our measurement is concerned).

Once you know the impedances, you need to use an appropriate resistor network to terminate the 3wire line. That's right, a "network" - how else would you want to terminate the live pair conductors against each other, and at the same time terminate both the live pair conductors against the reference ground conductor?
In a general case, on a cable with a "poorly defined" reference ground conductor, the common-mode impedance will be on par or maybe greater than the diff-mode impedance in the balanced pair. Such as, in my case, both Zcomm and Zdiff were 100 ohms. You need to design a resistive circuit, that properly terminates both the diff-mode and the common-mode part of impedance.
Essentially you need three resistors and they can be connected in two different topologies: a star or a triangle ("T" vs. "delta").

## three-wire termination: star vs. triangle ##

The math is fairly basic.

• The star or "T" is really simple:
  
  
• The triangle or "delta" is slightly more difficult:
  
  

Perhaps one more touch is appropriate: AC-only coupling of the common-mode termination ("DC decoupling" ;-). The RS485 transceivers are balanced, but work with a permanent DC offset of something over 2 Volts. The basic resistive-only termination of the common-mode component would increase stress on the drivers (heat dissipation), and would hamper attempts at DC biasing (details of which will be discussed below). Note that with a good-quality STP cable,
Z_comm = Z_diff / 4
For a quality cable with Z_diff = 120 Ω, Z_comm = 30 Ω. This "phenomenon" will be discussed below, in the paragraph on my experience with CAT7.
2V / 30 Ω = 70 mA of additional load - that's quite a lot.

## three-wire termination, common-mode AC-coupled by capacitors ##

The easiest way to AC-couple (DC-decouple) the common-mode terminator is by adding an appropriate-sized capacitor. What size is appropriate? Note that the "RC timing constant" (Z_comm * C) of the terminator must correspond to the round-trip delay on the transmission line. The capacitor should have enough capacity to absorb the energy, that can be loaded onto the TML during the "propagation round-trip time". In other words, the longer the cable, the bigger the capacitor required. Given an open-space vacuum speed of light of 3 * 10⁸ m/s and a typical "contraction coefficient" of around 0.6 to 0.7, a round-trip time can be simply estimated as
1 m = 10 ns
or 10 cm/ns (which also happens to be my favourite reflectoprobe formula :-)

3.

## three-wire termination with delta's at both ends, common mode AC-coupled ##

In practice in my lab experiments, I just used some 10uF ceramic caps that I happen to have in a drawer (blocking caps from a scrapped motherboard). Combined with a Z_comm of 100 Ω, these produce a timing constant of 1 ms - that would theoretically cover 100 km of cable :-) so it's apparently quite an overkill for my 14m cable. Worked just fine for me anyway :-) The timing constant corresponds to about 1 character at 9600, as can be seen on the overview scope screenshot for CAT7 - note the upward slope.
Perhaps more importantly, note how the upgrade from "basic termination + reg gnd" to the "DC-decoupled delta" further suppressed the wobbling between columns 2 and 3 in the table of waveforms above (use the schematics in table header as a guide).

The next thing I tried, in my parade of imperfect solutions, was to add DC biasing resistors (as per the excellent Circuit Cellar articles) and watch the effect on scope traces. I upgraded my Delta at the master node with the suggested 560Ω pull-up/pull-down resistors. Actually I even re-calculated the delta itself (for the master node), to compensate for the added resistors, to keep the Z_diff and Z_comm precisely matched = terminated.
Note that conceptually the termination should happen at the end of the TML (there are two of them) and is in no way correlated to the actual location of the fieldbus protocol master (or any other bus node for that matter). There can even be a section of transmission line with no further nodes attached, just a terminator at the free end. In my case, it just so happend that the RS485 master was located as a "terminal node" on the bus, and it was convenient to take +5V from the master device (rather than set up a separate power supply).

4.

## three-wire termination, common-mode AC-coupled by capacitors, DC-biased ##

The result was somewhat surprising: the wobble has actually increased a bit, compared to the previously unbiased delta terminator. Perhaps the matching got slightly worse.
But the biasing resistors are not about termination - they serve a different purpose, which is also very important: they keep the line at the recommended voltage levels in inactive state, thus keeping the receivers happy all the time, while at the same time reducing the common-mode "kicks" upon TX_on and TX_off. The biasing also improves noise immunity by gently pushing the line slightly further upwards from the decisive treshold for log.0 (-200 mV differential).

I really started my final experiment with CAT7 S/STP by checking the common mode impedance of the signal pair against its shielding. The scope was set for 1 GSps (highest resolution available), the reflectometer probe (pulse generator) was jumpered for 50 Ω output impedance (lowest available), the signal pair and shield wired for common mode measurement, and the line was experimentally terminated by 33 Ω (common mode, single resistor). You can see the resulting waveform below. The section of cable was about 25m long (judging by the scope screenshot) and initially was coiled on the table (coil diameter about 30 cm).

## reflectoprobe plot, CAT7 STP, common mode, terminated by 33 Ω ##

Clearly the termination resistor of 33 Ω is still a tad too high, there is a positive reflection at the far end. It seems that the common-mode impedance for the 100 Ω cable (differential impedance of each pair) is around 25 Ω. That would seem to match a train of thought saying that if the differential-mode impedance of a shielded twisted pair is 100 Ω, the impedance of a single wire (from the pair) against the common ground is half that (= 50 Ω), and thus the two wires forming a twisted pair, if hooked up in parallel and measured against the common shield ground, exhibit half their individual impedance, i.e. a quarter of the nominal differential impedance of the shielded twisted pair. (Oops. Is it really that simple? The chapter on odd+even impedance modes at microwaves101 suggest a math model that's yet more complex. Mine may be a good enough approximation.)
The one thing in the graph that I don't completely understand is the local-end overshooting at the pulse edges. I used to think they were a sign of inter-turn coupling when the cable was coiled on the lab table, but the glitches are there even if I unwind the cable and spread it throughout the room. My current hypothesis is that the pulse could be a sign of local-end impedance mismatch. The glitch, which corresponds to maybe 10 ns (i.e. approx. 1 m of cable), indicates after what time / propagation distance the cable's own impedance really kicks in. And, it may be related to the cable's characteristic available bandwidth (cutoff frequency).

5.

## quality STP cable, three-wire termination, common-mode AC-coupled by capacitors ##

Note that due to the quality shielding, which makes the CAT7 STP impedances act precisely according to the serial/parallel connection math, the 3wire terminator is reduced to 2 resistors (+1 capacitor for common-mode AC-coupling). In this case, each of the two resistors is calculated as
R = Z_diff / 2 = Z_comm * 2
Z_comm = Z_diff / 4

• Draka Comteq FB-02YS(ST+C)H FC 1x2xAWG22/1 VE2L (Z_nom. = 150 Ω, R_{DC loop} = 110 Ω/km, loss = 10 dB/km @ 1 MHz. "For RS485 up to 10 Mbps")
• Draka Comteq FB-02YS(ST+C)H FC 1x2xAWG22/1 VE2R (Z_nom. = 150 Ω, R_{DC loop} = 110 Ω/km, loss = 10 dB/km @ 1 MHz. "For RS485 up to 10 Mbps")
• Belden portfolio of EIA-485 cables (various models)
• Draka Comteq S-2Y(St)Y 1 x 2 x 0.5/1.5 (Z_nom. = 120 Ω, R_{DC loop} = 187 Ω/km, loss = 20 dB/km @ 1 MHz. Originally intended for E1/T1 at 2 or 1.5 Mbps)
• Draka Comteq S-2Y(St)Y 1 x 2 x 0.4/1.2 (Z_nom. = 120 Ω, R_{DC loop} = 149 (?) Ω/km, loss = 30 dB/km @ 1 MHz. Originally intended for E1/T1 at 2 or 1.5 Mbps)
• Draka Comteq KLVMAM (Z_nom. = 120 Ω, R_{DC loop} = 300 Ω/km, loss = 45 dB/km @ 1 MHz. Originally intended for E1/T1 at 2 or 1.5 Mbps)
• Draka Comteq Li-09YS(St)C11Y 2 x 0.35 mm² LSZH (Z_nom. = 120 Ω, R_{DC loop} = 54 Ω/km. CAN bus cable)
• Helukabel 81286 CAN-bus 1x2x0.22mm² (Z_nom. = 120 Ω, R_{DC loop} = 87 Ω/km. CAN bus cable)
• Belden 9860 Twinax (Z_nom. = 124 Ω, R_{DC loop} = 14 (28?) Ω/km, loss = 6.5 dB/km @ 1 MHz) = "Computer and instrumentation cable", but it looks more like an RF cable...
• NKT FLR31YB11Y-A (2+1)x0.5mm² (RF properties unknown) = vehicle cable. Special feature: wide temperature range (-40 to +110 ^oC)
• NKT TBVFV (RF properties unknown / depending on wire gauge desired) = resilient interconnect cable: inner conductor insulation and outer sheath made of FEP Fluoroplastic, wide temperature range (-60 to +200 ^oC), flame retardant, resistant to most organic solvents, oil, as well as aggressive inorganic chemicals, resistant to UV, ozone and weather conditions in general, mold-resistant, mechanically resistant against wear, creep, cracking under stress - owing to both the insulation material and the fine conductor stranding.
• Draka Comteq JAMAK (Z_nom. = 70 Ω, R_{DC loop} = 80 Ω/km, loss = 45 dB/km @ 1 MHz). This is a general-purpose industrial instrumentation cable. For RS485, the impedance of just 70 Ω may prove a tad too low for longer distances. There is a whole family of JAMAK cables, e.g. JAMAK ARM is armored by a steel tape. There is a similar family of NOMAK cables which has only a common screen for all the pairs (individual pairs in the cable are unshielded).

## balanced pair cabling impedance formula ##

## explicit signal reference ground, GND loop avoided by isolation - click for a hi-res picture ##

There are small devices (I/O modules) that have the I/O side isolated from the rest, so that the RS485 port shares ground with the DC PSU input (the terminal labeled "24V DC PSU GND" can be used as signal reference ground). This is (almost) no problem as long as the power is distributed along with the bus signals (note that it's inappropriate for other reasons to daisy-chain power distribution cabling just like the RS485 signals). It may become a problem when you need to power the IO gadget from a local source of power, that is not earth-isolated.

## IO module = RS485 node, with IO terminals isolated ##

Devices with a properly galv.isolated RS485 port (isolated from PSU input) theoretically don't need a reference ground, and can work with the bus using just some weak internal biasing resistors. There are devices that actually have such a properly isolated RS485 transceiver and DO NOT have a signal ground terminal. (Which may bring compatibility problems with other devices on the bus, if you don't provide external strong bias - read on for details.) In such cases, the choice not to provide a reference ground terminal (and rely on internal resistive biasing) is a matter of how much you trust your "miniature solid-state DC/DC converter" that it doesn't have parasitic capacitive coupling from the primary to secondary side (note that the primary is switched at some RF frequency). It's always better to have a reference ground terminal. It may even be a good idea to hack your device to add one :-)

## RS485 node with isolated RS485 port, no reference ground ##

There are devices boasting a so-called "tri-galvanic" isolation - such as async modems or RS232/485 converters. When you hear that buzzword for the fist time, you will likely imagine three neat isolation gaps, surrounding the three ports (local data port, the longer-haul line interface, and the power input). But actually there are only two gaps - two gaps are enough to make the three ports isolated from each other.

## Tri-galvanic isolation - the basic idea ##

And, the gaps tend to follow the essential premise of "least effort = least cost", conveniently making use of components that "have to be there anyway".
The modem's PSU is typically a switch-mode module, with a relatively high voltage at the primary side, so it already tends to contain a transformer of some sort. You only need to make sure that the feedback from the secondary to the primary side goes through an opto-coupler (or is derived by an additional winding on the trafo, which is perhaps less precise). So that's the first gap - easy money.
Next, PSTN/LL modems tend to have a signal transformer (or two) as part of the line interface - to adapt the old-style 48V/600Ω telephone circuitry to the modern semiconductors running at or below 5V. If the modem allows for PSTN operation, it also needs a relay contact that serves as a "phone hook switch" - so that the line can be cut away from the signal trafo when on-hook (central office detects a high loop impedance = phone hung up on the line) and also so that the incoming ringing voltage of ~200V AC doesn't reach the signal transformer. The hot side of the line interface doesn't even need a power supply.
That way, the local data inteface (232 or 485) ends up being directly connected to the "brain" of the modem - the processor chip(s).

## Tri-galvanic isolation - in a bit more detail (PSTN+LL modem example) ##

In other types of communication gadgets, say a 232-to-485 converter, both the local (232) part and the long-reach (485) part may have to be powered somehow, usually by a miniature monolithic DC-DC converter. And the signal tends to be transferred by a necessary number of opto-couplers. The local 232 port will likely remain un-isolated from the converter's MCU, if there is any.

Wiring as suggested by the standard

The Circuit Cellar has some excellent articles on the topic:
http://www.embeddedsys.com/subpages/resources/images/documents/microsys_art_RS485.pdf
http://www.circuitcellar.com/library/ccofeature/perrin0799/index.asp

Even and odd mode impedances at Microwaves101.com:
http://www.microwaves101.com/encyclopedia/evenodd.cfm