Electronic – Effect of “wrong” driver with slightly long RS-485 stubs

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.