ROCKPro64 Serial Buffer Circuit
Connecting to the ROCKPro64 single-board computer serial interface can sometimes be tricky, as the lack of buffering can cause the Uboot-SPL to stop functioning due to transient voltage spikes during startup. This simple circuit allows a user to hot-plug their serial connection at any time, or can be left attached full time.
You may have some or all of these in your junk/parts drawer. They're all common value, and easily recovered from scrap circuit-boards.
- 3"x4" perf board, will be used for construction of the circuit
- misc breakout board wires, or any random copper wire that works, I used ribbon cable from an old floppy harness.
- misc jumper wire with headers, a three-pin header, and a one-pin header, since you'll be connecting to the RP64 mainboard pin headers in different places
- Transistors, 1 (one) NPN-type, 2n2222 or equivalent; and 1 (one) PNP-type, 2n2907 or equivalent
- Resistors, 3 (three) 4.7kΩ 1/4-watt; and 2 (two) 10kΩ 1/4-watt
- Serial interface jack, 3-pin TRS audio jack works well, as does RJ45 or DB9. You only need, and use, 3 pins though.
- Double-sided sticky tape, at least 1"x1", to mount the perf board to your case.
- A soldering iron, e.g. the Pinecil, and some
- solder, fine gauge, anything suitable for electronics
- Wire strippers, suitable for the thickness of wire you chose
- Diagonal cutters, to trim excess leads from components and wires
- Start by creating "vias" using a stretch of bare wire for both ground and +3.3v along the long edges of the perf board; ground on one side, +3.3v on the opposite
- At one end of the perf board, start laying out the parts for the NPN transistor portion of the circuit, then solder down.
- At the opposite end, lay out the PNP transistor portion, and solder down.
- Connect up the serial interface jack, using enough wire to reach your desired mounting point on your case, from the mounting place for this board
- Solder the jumper wires to the perf board for the serial jack
- Solder the header jumpers to the perf board, making sure they're long enough to reach the RP64 headers with a bit of slack.
- Let cool, affix the double-sided tape, and mount to your case.
- Mount your serial port
- Serial RX <-> DCE RX (in)
- Serial TX <-> DCE TX (out)
- attach the header jumpers to their respectively labeled points on the various headers
- RP64 Ground <-> pi2bus-6
- RP64 RX (in) <-> pi2bus-8
- RP64 TX (out) <-> pi2bus-10
- RP64 3.3v <-> pi2bus-1