A serial interposer board for NABU PCs with a FTDI USB-serial adapter and hardware flow control
This is a simple serial interposer board for NABU PCs that intercepts the Tx and Rx signals from the TR1863 UART and routes them to an FTDI USB-serial adapter rather than to the RS422 line drivers on the NABU main board.
In addition to intercepting the Tx and Rx signals, it also provides two enhancements:
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The option to use a 1.8432MHz oscillator to generate a 115.2K baud rate on the UART, rather than the stock 111.8K baud that the NABU's clock generates. Two different oscillator footprints are provided, for your convenience. A jumper is provided to select the clock source.
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The option to enable hardware flow control. This is done by connecting the TR1863's DR signal (Data Received) to the CTS input on the FTDI. Because DR is active-high and CTS is active-low, a pending receive interrupt effectively de-asserts CTS, which will cause the sender to pause transmitting until the interrupt has been serviced.
To build this board you're oging to need an FTDI USB -> serial interface chip mounted on some sort of break-out board. The board provides 2 footprints to provide some flexibility.
The first footprint is for a 6-pin right-angle pin header that is meant for an Adafruit 6-pin female FTDI connector. Such a connector can be found on the Adafruit FTDI Friend, the Adafruit FTDI Serial TTL-232 Cable, or the Adafruit FTDI Serial TTL-232 USB Type C 5V Cable. There are other cables out there with a compatible 6-pin connector, but I specifically referenced the pin configuration of the Adafruit products.
The other option is one of the FTDI FT232RL boards that are all over Ali Express. I ordered this one, but there are lots of identical boards floating around.
This board is 99.9% the work of Kris Sekula (MyGeekyHobby.com). All I did was add the hardware flow control modification. Kudos to Kris for publishing his idea here.