The Arduino Mega behind the LCD panel communicates serially through the
Serial1 hardware, leaving the USB connection available for its usual console + program loading functions. The cable also carries +7 VDC for the Mega’s on-board regulator, plus a few bits that might prove useful, and enough grounds to be meaningful.
The pinout on the DE-9 female back-panel connector:
- TX (Mini -> Mega)
- RX (Mini <- Mega)
- Current sense amp
- D4 Enable ATX
- +7V regulator
Which looks like this:
One could argue that I should use insulation-displacement connectors and pin headers, but there’s something to be said for a bit of meditative hand-soldering.
The 7 V supply drops about 90 mV through its slightly too thin wire. With current around 100 mA, that works out to 900 mΩ, including all the connectors and gimcrackery along the way. Close enough.
More cogently, one could argue that I should have used a DE-9 male connector, so as to remove the possibility of swapping the cables. So it goes. The pinout attempts to minimize damage, but ya never know.
The green jumper on the Mini’s serial pins reminds me to unplug the UI cable, lest I plug the USB adapter into it and put the serial drivers in parallel.
The 7 V regulator stands over on the left, powering both the Arduino Pro Mini and the Mega + LCD panel. My thumb tells me that piddly little heatsink isn’t quite up to its new responsibilities, unlike the now vastly overqualified heatsink on the ET227. On the other paw, that’s why I used a pre-regulator: so that same heat isn’t burning up the SMD regulators on the Arduino PCBs. Time to rummage in the Big Box o’ Heatsinks again.