The DRSSTC fibre link, and the fail-safe rule

The interrupter and the coil are joined by an optical fibre, and not for convenience. loneoceans gives both reasons for it on the UD2.7 page: physical isolation and noise immunity, over a link that carries one bit. flyingperson23 puts the first of them in the terms that matter, in a thread from somebody who wanted to run coax instead (HVF 3035): a metal cable is a conductor running into a machine full of lethal voltage, and a secondary arc landing on it ends badly for the cable, for the interrupter and for whoever is holding the other end.

Which leaves one rule, and it is a requirement rather than a preference.

A broken fibre must turn the coil OFF

Fibres get pulled out, broken, and shaken loose. Transmitters lose power. Controllers hang. In every one of those cases the bridge has to go quiet rather than stick on.

The convention on the UD2.7 is that light on means drive on, so the rule and the convention agree: dark is off.

Test it physically. Pull the fibre out of a running machine at low power and confirm that it stops. Looking at the schematic is not the test.

What the receiver settles, and what it does not

A logic receiver is not an analogue one. The IF-D95T and IF-D95OC carry a photodiode, an amplifier and a Schmitt trigger inside the package (Industrial Fiber Optics, IF D95), so the dark state is a defined logic level and not a floating one. On an active-high part, dark is low, low is off, and the receiver gets the rule right on its own.

What it cannot do is get the rule right with no supply. And there is a documented way for that to end with the coil enabled rather than quiet.

Both fibre inputs on a UD2.7 carry a pull-up, and the reason is the part rather than the safety case: the IF-D95OC has an open-collector output and its datasheet says it needs a pull-up resistor, so the board is laid out to take either variant. loneoceans names the one on the IF input as R11, which is a designator and therefore worth tracing rather than trusting. On profdc9's through-hole redraw of the UD2.7C the value is 1 k on both inputs (schematic), and when somebody asked on the forum whether the open-collector part could be substituted for the push-pull one, alexhanyuan's answer was that either works and that the 1 k is there for the open-collector variant (HVF 2271).

With the receiver fitted and powered, the receiver wins that node and dark is off. With no receiver behind it, the resistor is the only thing on the node.

That is the shape of the whole problem, and it is worth stating plainly because it is not what a schematic looks like it says. The receiver enforces the rule while it has power. The state the input sits in when the receiver has no power is set by whatever else is on that node.

One live page here says it the wrong way round. Overcurrent protection lists the fail-safe on the fibre as "a divider that pulls the receiver's input to off rather than leaving it undefined". Neither half survives the datasheet. The output is not undefined in the dark, and the resistor on the reference board pulls that input to on, not off. The line on that page needs replacing with the paragraph above.

Active-high and active-low are not interchangeable

They are also not all the same way round on one board, which is the part that catches people. loneoceans is explicit twice over on the UD2.7 page: the ST receivers, OPF-2412T and HFBR-2412T, are active low, and their output is inverted once on the board to match; the Industrial Fiber Optics parts are active high, and he tells you not to buy the active-low kinds. His bill of materials names those two as IFD95T and IFD95C; the datasheet calls the second one IF-D95OC, and it also notes that active-low outputs are available as a special order. So the trap has three doors: the wrong family, the wrong suffix, and a special order.

One more from the same page, and it is not about polarity: the two fibre inputs are ORed together, so only one of them may be driven at a time.

What people use instead, and the catch

• An optocoupler in place of the fibre. LoFoTroFo did exactly this on a handheld build, dropping the HFBR2412 for a TLP2345-E with a transistor added, and later a TLP2362 on the driver input, because he wanted it cheaper and without an optical cable (HVF 3455). What you give up is the kind of isolation, not the amount: a fibre has no conductor across the gap at all, and an optocoupler is a component with a rating.
• Bluetooth. paulsimik runs two HC-05 modules, one master and one slave, paired at a fixed address in AT mode, and sends only the potentiometer positions; the PWM ramp and the bridge switching are computed on the coil's own board, and an ATmega328 on an Arduino Nano is enough for it (HVF 3524). He says MIDI over the same link is worse, because the data has to be processed far faster. LoFoTroFo built one too and reports that it needs shielding very well.

What is not established

• Whether these receivers impose a minimum pulse width. A figure of 6 µs travels with the IF-D95T as an asymmetry between turn-on and turn-off, and the datasheet does not carry one: it prints a single typical propagation delay of 6 µs against both tPLH and tPHL, with no minimum and no maximum beside it. One number for both directions is not a difference between them, and with no maximum the spread between parts is not specified there either. What the same page does bound is the data rate, up to 125 kbps, and the output rise and fall, no more than 70 ns at ten TTL loads.
• Whether IF-D92 is the faster alternative. It is the slower one as specified. It is a bare phototransistor with no logic in the package, rated to 20 kbps, with typical switching times of 25 µs at a 1 k load (Industrial Fiber Optics, IF D92). Loading it far harder and supplying your own threshold is a real technique and it would change those figures, but no worked circuit for it has been read here, so this page cannot tell you where it lands.

The symptom, if you ever see it

Pulled the fibre and the machine kept running. Three things do that, and all three are on this page: the receiver is the wrong polarity, or the receiver is absent or unpowered while its pull-up is still ORed into the enable, or the break is somewhere the fibre is not, in an interrupter that carries on transmitting after its own logic has stopped deciding anything. Stop everything and find which. There is no version of this fault that is acceptable to work around.

What the transmit end costs

The range is set by how hard the transmitter drives its LED, which is one resistor. sndnvaps publishes a DRSSTC interrupter on OSHWHub and records changing that resistor from 470 Ω to 140 Ω "以增加光纤传输距离", to increase the fibre's transmission distance. Worth knowing before blaming a long run or a bad connector: a link that works on the bench and not across the room may be a drive current problem rather than a fibre problem.

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What the interrupter is doing on the other end of that fibre is bangs per second and ontime. What is on the other end of the cabinet wall is DRSSTC safety, and it is the reason this link is plastic fibre and not copper.