After using my last commercially made isolated TTL serial to USB converter for my last project, with no hopes of it being re-stocked in the near future. I decided to try building my own version of the device.
I actually ended up borrowing my Zeptobit “Optically Isolated USB-UART Adapter” from a previous project to complete the Programmable Voltage Reference V3.14 project. So I needed to at least replace the one that I borrowed, and make a few spares for future projects.
It seems that most of the USB to serial IC’s are very hard to find and out of stock as I began to work on this project, like so many other IC’s lately.
What I did find was a FTDI UMFT234XF development module with a micro‑B USB connector attached that utilizes the FTDI FT234XD IC. It is a nifty little module with data transfer rates of 300 baud to 3 Mbaud, a configurable CBUS I/O pin, and programmable memory for configuration.
I used the same type of 6N137 high speed optocoupler in a DIP‑8 package as the Zeptobit module, but switched to a Vishay Semiconductors manufactured IC. A RX/TX activity LED was added and connected to the programmable CB0 pin of the FTDI module. Other than that it is a somewhat simple board with a PTC fuse, bypass capacitors, LED current limit resistors, and data pullup resistors. Most devices are 0603 size except R1 which is an 0805 and F1 which is a 1206 device.
The board is slotted to maintain a creepage and clearance distance of >7 mm to support the 5.3 KV isolation voltage of the 6N137 optocoupler IC. I did end up using some high-quality Mill-Max 8‑pin DIP sockets, which also helped to stiffen up the board.
The module was programmed with FTDI’s FT Prog software, mostly with default settings. Changes were made to set the CB0 pin for TX&RXLED, which lights the green LED when transmitting or receiving data.
Both the CBUS and DBUS “I/O pins” drive was set to 12 mA, to support powering the CB0 LED and the LED in the optocoupler. The Max Bus Power was set to 94 mA.
So far I have tested the units up to 115200 baud with great results. The TTL serial side of the board was optimized for 5 volt power and data signals. There is a low power blue LED on each isolated side of the board to indicate that power is available.
The mounting holes were designed to use 2–56 Filister head nylon screws.
I am thinking of making a new version of this board to connect to the 6 pin header of the Arduino Pro Mini board sometime in the near-future.
EagleCAD V7.7 schematic and board ZIP file for the Iso TTL USB V2.2
OSH Park shared project page for the Isolated TTL to USB Ver. 2.2