Ive put an ESP-12E on the V1.2 serial card with a little mod & it works, my pocket386 is now acessing my WiFi network!
OK, so the resistors on IO0, IO2 & IO15 set the ESP-12 boot mode and should be left alone.
I changed R5 to 1k8 & put a 3k3 resistor from the ESP12 side of it to ground (scraped off a little resist on the ground plane to solder to) as in the pic (honest the soldering looks worse in the pic than it did under my mircoscope!) , this forms a potential divider on the serial tx line from the 16c550 to drop the 5V signal down to about 3v3. I was wrong in my post above, with a 5V supply the 16C550 can supply 4mA from this pin, we are way under that with these resistors. A quick guestimate says the rise fall times should be OK with these resistors for the speeds we are looking at, but I havent put a 'scope probe on it yet to check... You might get away without doing something like this, but I wouldnt risk it myself.
R4 can be left @100R, though the hardware guide for the ESP8266 does suggest a 500Ohm resistor here to reduce RF emmisions - there should be one in the module really, close to the screening can, but the datasheet on mine says there isnt one.... So you change it 500Ohms or so if you like (might help with speaker clicks below)
A quick look at the 3v3 signal levels from the esp to the input on the 16c550 looks there is a decent enough noise margin without level translation in that direction.
Talking of RF inteference, I did notice some clicks from the speakers, in particular the right one, though oddly that seems to have dropped in level now (perhaps now its connected its dropped its RF output level??), this should be no great suprise - Ive just put and RF module & aerial right over the audio circuits!! If it gets annoying I may look at shielding the audio ccts somehow, but the level its dropped to doesnt seem tooo bad, given we arent talking quality audio anyway..
I programmed the modue with the binaries straight from:
https://github.com/martin-ger/esp_slip_router
soldered it in place...
I used EtherSL, & mTCP library
All works a treat so far!
but ive only got as far as pinging computers on my home network so far....