A simple PCB to interface an FTDI FT232-RL IC with an Avago (now Broadcom) HFBR-x4xx fibre optic transmitter & receiver pair. The board has some simple protections and supporting circuitry as well as Tx, Rx, and power LEDs. A USB mini-B connector was used to talk to the host machine.
Fibre Optics
The choice of Tx/Rx pair was dictated by the fibres with which I would need to interface. The HFBR-x4xx family encompasses transmitters and receivers that work on the 820nm scale. They are available with ST, SMA, FC, and SC connectors and are compatible with various multimode plastic-clad-silica (PCS) fibres, including OM1 (62.5/125um), OM2 (50/125), 100/140, and 200um. For this iteration of the build, I used the HFBR1412Z transmitter and HFBR-2412Z receiver. These are ST connector parts, with plastic connectors. This choice was again determined by the fibre I had to interface with.
Protection
As a minimum, I decided to use:
- Overcurrent protection on the +5V input from USB
- A TVS on the USB +5V line
- ESD protection diodes on the USB lines
I'm hoping that this will give me a minimum level of protection against spikes, ESD, and general miss-treatment. Most of the protection was sourced from a Littelfuse app-note (which I don't seem to be able to find right now!).
For the USB Vbus protection I used a V5.5MLA0603 TVS diode, between Vbus and the shield. This is a 5V varistor diode recommended for this particular application.
For the ESD protection on the USB D+, D- lines I also turned to the Littelfuse datasheet, and selected 2x PGB10603NR low capacitance ESD protectors. Two discreet devices were selected, rather than one SOT-23, as this somewhat simplified PCB layout.
Supporting Circuitry
tbc...
Hi!
Interesting! What maximum transfer speed can you get? Did you manage to use the D2XX driver or is it an UART VCP interface?
Thanks