I'm not sure you understood what my intent was and I would have expected it to emit an 'a' every six seconds when the display is not communicating as expected.
- wait up to 3 seconds for the initial
'c' but move on when not received
- send the
'a' to initiate conversation
- wait up to 3 seconds for the expected
'b' response
- when
'b' is received return true immediately otherwise false after 3 seconds
Once the display does respond with a 'b' the cycle should be broken as the while(!send_begin()) will be satisfied to leave by the returned true.
I admit I have not tested this but I wouldn't see why my code would behave any differently than that.
However, if you don't want the repeated attempt to connect to the display you would just not wrap the send_begin() call in that while() loop 
Update:
@peekay123, since I rather doubt myself than others and listen to honest criticism I went on to test my code now and lo and behold it does behave as I intended it to do
It only keeps sending
'a' every
six seconds not
three and once the display responds with a
'b' after receiving an
'a' the cycle is broken and the image is sent out.
I tested with this code on an Argon
#include "Particle.h"
SYSTEM_THREAD(ENABLED);
const unsigned char IMAGE_BLACK[] = { /* 0X00,0X01,0XC8,0X00,0XC8,0X00, */
0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF,
...
0XFF, 0XFF, 0XFF, 0XFF,
};
const unsigned char IMAGE_RED[] = { /* 0X00,0X01,0XC8,0X00,0XC8,0X00, */
0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF, 0XFF,
...
0XFF, 0XFF, 0XFF, 0XFF,
};
#define RECV_ERROR -1
#define CONTINUE_TRANS 0
#define RECV_DONE 1
//Send data to e-link board.
void serial_send_data(const uint8_t* data, uint32_t data_len) {
for (int i = 0; i < data_len; i++) {
Serial1.write(data[i]);
}
}
//Send image array
void write_image_picture(void) {
for (int i = 0; i < 13; i++) {
serial_send_data(&IMAGE_BLACK[0 + i * 212], 212);
delay(80);
}
delay(90);
for (int i = 0; i < 13; i++) {
serial_send_data(&IMAGE_RED[0 + i * 212], 212);
delay(80);
}
}
const uint32_t TIMEOUT = 3000; // 3 seconds timeout
bool send_begin() {
// spin the loop till broken or the timeout hits
for (uint32_t ms = millis(); millis() - ms < TIMEOUT; Particle.process()) {
if (Serial1.read() == 'c')
break;
}
// don't care whether we actually got a 'c' response or not
Serial1.write('a'); // send "listen for transfer" command
// spin the loop till broken or the timeout hits
for (uint32_t ms = millis(); millis() - ms < TIMEOUT; Particle.process()) {
if (Serial1.read() == 'b')
return true; // expected response from display received
}
return false; // second loop ran into the timeout, so no success
}
void setup() {
Serial1.begin(230400);
while (!send_begin())
Particle.publish("Status ePaper", "begin failed", PRIVATE);
Particle.publish("Status e-Paper", "testing", PRIVATE);
write_image_picture();
}
And a simple serial pass-through sketch on a Photon
SYSTEM_MODE(MANUAL)
void setup() {
Serial.begin();
Serial1.begin(230400);
while(Serial.read() >= 0); // flush Serial input buffer
while(Serial1.read() >= 0); // flush Serial1 input buffer
}
void loop() {
while(true) {
if (Serial.available()) Serial1.write(Serial.read());
if (Serial1.available()) Serial.write(Serial1.read());
}
}
