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I have a planned 250 kbit/sec, single-pair, 36-metre RS-485 installation where it is advantageous for wiring purposes if the stub lengths can be a little longer than optimal. There is a single almost-always transmitter and three almost-always receivers. "Almost always" means during normal operation, but during monthly maintenance visits, the receivers will transmit for a few seconds. (It's DMX-512 lighting control with very occasional RDM.) It is controlling underwater lighting for architectural purposes, so the downside consequence of data corruption would be merely aesthetic.

RS-485 is clearly designed to be used with the shortest stub lengths practical. However, it's an engineering question about how the stub length affects signal quality and what to do if other matters require longer stubs.

The following is from Texas Instruments "AN-1057 Ten Ways to Bulletproof RS-485 Interfaces"

Although the discussion of configurations and the section on stubs advises minimizing stub length to avoid transmission-line problems, the application may not permit minimizing stub length. Another approach is to increase the driver's transition time to permit longer stubs without transmission-line effects. If you use the DS36C280, long stubs can branch off the main cable. This arrangement keeps the main cable short, whereas looping the cable back and forth to reach inconveniently located nodes would greatly increase the main cable length. Besides allowing longer stubs, the slower edge rates generate lower emissions. Thus, this transceiver is also useful for applications that severely limit emitted noise.

The cable is custom high-quality unshielded 0.34 mm2 conductor twisted pair (approx 22 AWG) with 1.0 mm2 24 VDC in the same sheath. The installation is about 1 metre under water.

What is the effect of a driver with 10 ns rise time versus one with 1.2 μs rise time?

My understanding is that rise/fall time is limited to 30% of bit time and that various rules-of-thumb put the stub length at the 10% to 25% propagation distance of the rise time. (Using various TI and Maxim application notes.)

Our desired driver is something like that Maxim 3430 which is slew-rate limited suitable for 250 kbit/sec (datasheet), but we might be forced to use equipment with Maxim 3485 (datasheet); alternatively Analog Devices 2582 with 15 ns when we would prefer 2587 with 200-1100 ns (datasheet).

What effect might this have? Should we do something special if we have the "wrong" driver?

My calculations

  • Data rate is 250 kbit/sec
  • Bit time therefore 4 μsec
  • Max rise/fall time therefore 1.2 μsec (30% of bit time)
  • At 0.6 c signal velocity this is 216 m
  • 25 % of that is 54 m

Simplified diagram:

            22m     14 m
    t-T-+--------+-------R-t
        | 6 m    | 6 m
        R        R

 T transmitter, R receiver, t termination, m is metres

Given my total length is 36 m and my stubs are 6 m, I believe this suggests it will work reliably. Am I misguided?

Many thanks for suggestions and advice.

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  • \$\begingroup\$ Why didn't you scope the signal during commissioning to verify signal integrity at the host? How much does the impedance lower in a dielectric medium of 80 vs air of 1? You should have used STP cable to minimize that effect. \$\endgroup\$
    – D.A.S.
    Commented Mar 7, 2019 at 12:25
  • \$\begingroup\$ Thanks for comment, I think we're likely to test it out first. Re shielding, following Perrin's article "Art and Science of RS-485" (Circuit Cellar 1999) we decided against: we have many sites with this cable, mostly much bigger (approx 200 m), typically 30 nodes, 500 mm stubs, and the signals look like a textbook. \$\endgroup\$
    – jonathanjo
    Commented Mar 7, 2019 at 12:37
  • \$\begingroup\$ From what I read in Perrin's article. Daisy Chain is best, not Stubs, host common ground to each target is best (but more $), but STP is better (!) and if your rise time is fast , you better have Daisy Chain otherwise it's a mess. \$\endgroup\$
    – D.A.S.
    Commented Mar 7, 2019 at 14:37
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    \$\begingroup\$ 1010101 i.sstatic.net/lMOSu.png \$\endgroup\$
    – D.A.S.
    Commented Mar 7, 2019 at 15:08
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    \$\begingroup\$ I have no actual experience on this so take this with a grain of salt, but the RS485 signalling itself is also quite robust against reflections. As long as they aren't strong and long enough to alter the bit state at the middle of the bit interval (when the receiver samples it), even big spikes won't cause actual problems. This is because only the start bit is edge sensitive, and it's preceded by a stop bit delay. An edge sensitive signal (such as some clock pulse) would be much more sensitive to spikes. \$\endgroup\$
    – jpa
    Commented Mar 7, 2019 at 19:21

2 Answers 2

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RS485 clearly specifies a network topology without stubs, well except ones few milimeters from connector to the transceiver IC. So it is up to you to decide.

I would like to suggest you to use additional 2x6m = 12m of cable and eliminate the stubs, afterall 250 kbps isn't a slow speed.

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    \$\begingroup\$ 250 kbps is a) quite slow and b) what matters more for the signal bandwidth which depends on the rise time of the transmitter. With tr=15ns your allowed stub length will shrink to tens of centimeters. The amount of additional cable is only 2x6m=12m BTW. \$\endgroup\$
    – Manu3l0us
    Commented Mar 7, 2019 at 11:49
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    \$\begingroup\$ @Manu3l0us b) you are correct, I have edited only 12m additional cable is needed. \$\endgroup\$
    – Marko Buršič
    Commented Mar 7, 2019 at 11:56
  • \$\begingroup\$ I don't disagree for a second it would be better electrically to run the extra cable. But this is engineering: my question is about what actually happens if we use the 10 Mbit/s driver instead of the 250 Kbit/s driver. Should we add some other kind of signal conditioning? I amended the question with quote from TI app note re longer stubs. \$\endgroup\$
    – jonathanjo
    Commented Mar 7, 2019 at 12:40
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    \$\begingroup\$ @jonathanjo Anyway, if you are constrained with those 6m stubs, then there might be a solition by altering/adding termination resistors. I have seen this on CAN networks, don't know if can be applied to rs485. \$\endgroup\$
    – Marko Buršič
    Commented Mar 7, 2019 at 13:35
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    \$\begingroup\$ I agree with Marko. A long time ago my "managers" wanted to hang a 50 foot stub on a 1553 bus. I did a time domain simulation and showed them the ugly consequences. But with a proper termination, reflections from the receiver were greatly reduced. Of course, these bus systems need high impedance receivers, so there may be a price to pay if you hang a load on the bus. \$\endgroup\$
    – richard1941
    Commented Mar 8, 2019 at 2:22
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Classically, if you can HIDE the reflections within the rise and fall times, then you have a robust system, IF you have some hysteresis.

With 4 microseconds SYMBOL (NRZ, at that) time, will you set up the link for 3 microseconds Rise and Fall times? and hysteresis at 25% and at 75% of the full scale swing?

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  • \$\begingroup\$ yes if 2x latency is < rise time but if 75% T risetime, ISI can increase from group delay not being so flat but that can be compensated sometimes. \$\endgroup\$
    – D.A.S.
    Commented Mar 7, 2019 at 16:24

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