前から、興味があった透過型のUARTーWiFIブリッジ的に使う方法をあれこれ試していました。

結論から言えば、出来ました。

こんな感じで、電源はRasPi2 から取っていますので蓋の中に収められそうです。

ESP-12 に収めるプログラムは、なんとサンプルスケッチの中にありました。WiFiTelnetToSerial です。

/*
WiFiTelnetToSerial - Example Transparent UART to Telnet Server for esp8266
Copyright (c) 2015 Hristo Gochkov. All rights reserved.
This file is part of the ESP8266WiFi library for Arduino environment.
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
*/
    

        もくじ                    
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                    1
                include <ESP8266WiFi.h>
    
    
    
                    2
                define MAX_SRV_CLIENTS 1
    
    


include <ESP8266WiFi.h>
//how many clients should be able to telnet to this ESP8266
define MAX_SRV_CLIENTS 1
const char ssid = "yyyyy";
const char password = "xxxxx";
WiFiServer server(23);
WiFiClient serverClients[MAX_SRV_CLIENTS];
void setup() {
Serial1.begin(115200);
WiFi.begin(ssid, password);
Serial1.print("\nConnecting to "); Serial1.println(ssid);
uint8_t i = 0;
while (WiFi.status() != WL_CONNECTED && i++ < 20) delay(500);
if(i == 21){
Serial1.print("Could not connect to"); Serial1.println(ssid);
while(1) delay(500);
}
//start UART and the server
Serial.begin(115200);
server.begin();
server.setNoDelay(true);
Serial1.print("Ready! Use 'telnet ");
Serial1.print(WiFi.localIP());
Serial1.println(" 23' to connect");
}
void loop() {
uint8_t i;
//check if there are any new clients
if (server.hasClient()){
for(i = 0; i < MAX_SRV_CLIENTS; i++){
//find free/disconnected spot
if (!serverClients[i] || !serverClients[i].connected()){
if(serverClients[i]) serverClients[i].stop();
serverClients[i] = server.available();
Serial1.print("New client: "); Serial1.print(i);
continue;
}
}
//no free/disconnected spot so reject
WiFiClient serverClient = server.available();
serverClient.stop();
}
//check clients for data
for(i = 0; i < MAX_SRV_CLIENTS; i++){
if (serverClients[i] && serverClients[i].connected()){
if(serverClients[i].available()){
//get data from the telnet client and push it to the UART
while(serverClients[i].available()) Serial.write(serverClients[i].read());
}
}
}
//check UART for data
if(Serial.available()){
size_t len = Serial.available();
uint8_t sbuf[len];
Serial.readBytes(sbuf, len);
//push UART data to all connected telnet clients
for(i = 0; i < MAX_SRV_CLIENTS; i++){
if (serverClients[i] && serverClients[i].connected()){
serverClients[i].write(sbuf, len);
delay(1);
}
}
}
}

ArduinoIDE へ、ESP8266 Arduino Core の最新（ver. 1.6.5-1044-g170995a, built on Aug 10, 2015）を収めました。

SDK1.3 が収まったものは、以下のStaging versionから入れました。