I have decided to begin the process of replacing my venerable AMD Athlon based server which has been running continually since 2006 (99.526% uptime). After a considerable amount of research I settled on the ROCKPro64 because:
- It comes with 4 GB RAM.
- Very unusually for a single board computer it has a PCIe slot allowing a SATA card to be added.
- I would like to attempt to use the Xen hypervisor and the chipset is alleged to be compatible.
With the arrival of the board (which is even more compact than I had imagined) the first step was to be able to access the serial console which, like the Raspberry Pi runs at 3.3V I could have bought a compatible USB to serial converter, but I decided that I would be able to figure out some way to connect the one I already own and which I mostly use for downloading data from my old GPS receiver.
A few quick measurements with my trusty oscilloscope showed that my converter produces RS232 signals, so they are ±10V and inverted (i.e. -10V is logic 1).
I therefore designed and built the small interface circuit in the photo below based around a 7404 hex inverter. The ROCKPro64 transmit signal is TTL compatible so the signal simply passes through the inverter. Fortunately the USB converter recognises the TTL inverter output.
The Rx direction uses a diode to remove the negative voltage and two potential dividers to reduce the voltage into and out of the inverter.
The circuit works perfectly, even at the loony 1.5 MBaud rate of the Rock.