FT232R for RS-485 (DMX512) with isolation - TX optocoupler

I am trying to make a DMX controller board using the FTDI FT232R (FT232RL) chip for USB to UART and a MAX485 to output data over RS-485. I have a working prototype done, but for the finished product I am designing a PCB and want to add in isolation between the UART side and the RS-485 side.

The design I've come up with for the isolation is to use a DC to DC converter to isolate the VCCs from each other and an optocoupler (6N137) to isolate the FT232R TX pin from the MAX485's driver input.

So now for the question: For driving the optocoupler, can I use the TX pin with a 470 Ω resistor to sink the optocoupler's input cathode?

What I calculated is I need a minimum of 5 mA to make sure I exceed the LED threshold. In the case of the lowest voltage difference of 2.6 V:

$$\(VCC\rightarrow5~V) - (V_{F,max}\rightarrow1.8~V) - (V_{ol}=0.6~V)\$$

520 Ω (2.6 V / 5 mA) is the maximum resistance (specs below).

I can provide a schematic if that would help makes sense of all that I've said

(Note: The RX pin is not connected at all as I am doing DMX only, not DMX + RDM, so I have the MAX485 constantly driving)

6N137 Optocoupler

FT232RL FTDI USB to Serial

• Have you already seen the FTDI application notes for opto-isolated interfaces? ftdichip.com/Support/Documents/AppNotes/… Dec 19, 2019 at 18:10
• From where would the power come from to the isolated side? DMX512 transmitter is usually ground referenced so what would the isolation gain? Also are you absolutely sure that the PC software and USB drivers are able to perform their timing within DMX512 specs? Usually the PC would feed data to a microcontroller with non-realtime timing and the microcontroller would send DMX512 frames with realtime timing. Also, FTDI sells RS485 cables, so it would just need the XLR connector really.. Dec 19, 2019 at 19:01
• @Justme The power for the isolated side would be the output of a 5V to 5V Isolated DC/DC Converter (example) Isolation for DMX is recommended since RJ45 connectors are allowed connectors for DMX512-A and some applications using RJ-45 connectors (namely phone lines) have up to 48VDC on the line. Isolation is to limit what gets fried if one were to accidentaly connect a phone line to a RJ-45 DMX port Dec 19, 2019 at 21:57
• @Justme I am basically imitating the Entec OpenDMX Controller which is based on the FT232R chip. DMX512 only operates at 250kbaud which is well within the capabilities of of the FT232R (3Mbaud) and MAX485 (2.5Mbaud) You are correct that I could just use an FTDI RS485 cable, but I am going to use the a similar isolation design for some DMX lights I have designed (MAX485 and Arduino Nano/ATMega328P) Dec 19, 2019 at 22:03
• @Matthew Ok, isolated converter is good, but preferred transmitter is still ground referenced while isolated transmitter is only allowed. It is the receivers that have to be isolated. Also the RJ45 connectors are only allowed in fixed installations or locked patch bay areas with limited entry, so that nobody connects phone lines to them accidentally, as that could indeed fry the RS485 transmitter chip and everything else on the bus. But it could also be protected with suitable components. There are also all-in-one isolating tranceivers that don't need external isolated supply. Dec 19, 2019 at 22:14

Your question does not complete the required design margin stackup tolerance analysis to be certain if 470 is the correct resistance to ensure adequate margins for logic levels to be reliable.

The low level Tx driver impedance Rol= 300 Ohms (max) = Vol (0.6max) /Iol(2mA)

A far better design choice is the TLP116A Schmitt-Input Logic-level output OptoIsolator from Toshiba and much cheaper than the Broadcom Ltd (nee HP) parts.

But this does not address the larger system level communication issues suggested by your missing DMX512 interface specs.

• I don't understand what you are saying: 300 Ohms (max)? Read the TI app note. If more than 2mA of current flows into the pin then Vol could be higher than 0.6V. Less current, less voltage across the pin. (Consequently, a TI logic device will operate with a low-level output current that is above the recommended operating range (but below the absolute maximum rating), but TI does NOT represent that the device can sustain the specified VOL level or that the device will operate without any reliability concerns.) mouser.com/datasheet/2/405/szza036b-89603.pdf Feb 15 at 4:13
• Have a look at IOL in the table. As IOL goes up so does VOL. toshiba.semicon-storage.com/info/docget.jsp?did=63520 Feb 15 at 4:38
• Yes I am certain about what I said. Vol/Iol = Rol which is fixed for constant Vdd and Temp, so yes Vol goes up/down with Iol but ratio is due to RdsOn Feb 15 at 6:56
• I see, you're referring to the internal impedance of the pin. Feb 15 at 16:52