When one serial port is not enough
Published:  26 September 2017

Ahh, Serial ports. DTE, DCE, crossed over or not crossed over? In these days of USB, Bluetooth Low Energy devices and WiFi the serial port is dead technology, right? Well, not so fast.

Many industrial devices still use RS232 to provide local connectivity as this protocol is well recognised and is lightweight and cheap to implement on a microcontroller.

A single serve

A single RS232 port (UART) is brought out to a DB9 connector on the mangOH Green board, and the FX30S provides a single configurable RS232 or RS485/422 DB9 connector.

Or you could use the Renfell RS232 4 Wire or RS485 2 Wire IoT cards to plug into the IoT connector on either the mangOH Green/Red boards or the FX30/FX30S Airlink devices.

If you are using a mangOH Red, then you have no choice but to use an IoT card to provide the serial port.

Please, sir, I want some more

But what if you want to connect more than one serial device? If the devices you need to connect use the half-duplex RS485 physical standard, then you might be able to use the Renfell RS485 card and connect your devices in a multi-drop network. For more than one RS232 connection though you're out of luck (unless you're using the mangOH Green which has three IoT slots for three RS232 IoT cards).

Well, not quite.

Renfell has produced a USB Host IoT card to provide the FX30 family of devices with a USB Host connection like the mangOH Green and Red boards have. This USB Host connection can be used to expand the number of RS232 serial ports available using a powered USB hub and USB-RS232 adapters - or in a more elegant solution, the FTDI USB-COM-PLUS family of expansion modules.

FTDI provide 1,2 and 4 port modules that support RS232, RS485 or RS422 serial protocols. These modules are bus powered and use one of FTDI's well known USB to serial conversion devices.

But what about the driver?

The underlying Linux distribution for both the mangOH boards and FX30 devices already has the FTDI SIO (Serial Input Output) driver built into the kernel. Adding more serial ports is as simple as plugging the FTDI module into the USB host IoT card and waiting for the FTDI chip to enulerate as /dev/ttyUSBn serial devices.


Hardware sources

Renfell IoT Modules available from Linkwave Wireless Connectivity

FTDI USB-COM modules: http://www.ftdichip.com/Products/Modules/USBRSxxx.htm