Having dismantled my Duo here's the circuit board:
Serial PortLooking at the top edge of the circuit board you find the serial port connector close by the front of the Duo. With the drives out of the unit there is sufficient space for you to plug-in/remove a lead to this connector.
The pinout of the port is:
- +3.3V (Leave unconnected)
- Rx. (Connect to Tx on adapter)
- Tx. (Connect to Rx on adapter)
- GND. (Connect to GND on adapter)
The serial parameters are: Baud rate 115200, no parity, 1 stop bit.
These days many PCs have dropped the 9-pin RS232 serial connector as it's only legacy peripherals that are fitted with such an interface. Also note that the serial interface presented here is connected directly to pins on the on-board CPU and the Tx and Rx line signalling levels switch between ground and +3.3V, not the +/- 12V of the old RS232 spec.
Luckily we have an easy way of safely connecting this serial port to a PC at a low cost: The USB-to-Serial adapter. The one I've used cost ~£2.50 from eBay and is shown below. This one came fitted in a clear tube of heat-shrink sleeving - a nice touch to prevent it shorting out to anything.
This is based on the CH340G chip and drivers for it are already built into recent versions of Windows. There are various variations that you can buy, but the important thing to look for is one where you can set the 'serial' side of the interface to run using +3.3V or +5V. There are some where all an on-board jumper does is set whether the Vcc presented at the edge of the card is +5V or +3.3V. The Tx line is still switching between +5V and 0V and thus overdriving any input pin on a device that's running off +3.3V.
Here you can see that I've set the jumper so that the interface is using +3.3V levels.
Luckily I had to hand a plug that fitted the onboard serial socket. You find these advertised on eBay as 'Micro JST' plugs and they come with flying leads already fitted. They cost peanuts from China, and tend to be sold in batches of 10. The pin spacing is 2.54mm (0.1").
Here's my rapidly cobbled together lead.
For the serial console I have putty installed on my Windows 10 system. This is an SSH client that also handles serial connections.
A quick way to find out which serial port you need to connect to is to fire up Device Manager and click on the 'Ports' setting.
Here you can see the adapter has appeared as COM5.
The serial setup screen in Putty where set the baud rate, stop bits etc.
The 'launch' screen in Putty. Here I have saved my COM5 serial console settings under the imaginatively named entry: 'Serial Console COM5'