renamed _reset to _start, added comments

SoftRS485.cpp

@@ -14,30 +14,26 @@ int _RO, _RE, _DE, _DI;
 
 const long _pulse_len = 616;
 const long _break = _pulse_len * 20;
-boolean _line = LOW;
-long _last_flip = 0;
-long _diff = 0;
-long _now = 0;
-long _count = 0;
-boolean _reset = false;
+boolean _line = LOW;     // current state of the transmission line
+long _last_flip = 0;     // timestamp since last flip of line state
+boolean _start = false;  // indicates whether a start bit shall be ignored
 
 // Receive vars
-byte _recv_msg[MSG_LEN];
-boolean _avail = false;
-int _pos = 0;
-int _idx = 0;
-
-
-
-
-
+byte _recv_msg[MSG_LEN]; // message buffer
+boolean _avail = false;  // indicates, wheter a message has been received
+int _pos = 0;            // character position within the buffer
+int _idx = 0;            // bit position within the current character
 
+// variables, that could be local, but are stored globally for speed optimization
+long _diff = 0;          // time difference between last flips
+long _now = 0;           // current time
+long _count = 0;         // helper for bit iteration
 
 void interrupt485(){
     _line = !digitalRead(_RO); // HIGH level = logical zero and vice versa
-    _now = micros();
-    _diff = _now - _last_flip;
-    _last_flip = _now;
+    _now = micros();           // get current time
+    _diff = _now - _last_flip; // duration of the _previous_ bit
+    _last_flip = _now;         // store time of the current flip
 
 #ifdef RECV
     Serial.print(!_line);
@@ -48,36 +44,37 @@ void interrupt485(){
     Serial.print("   ");
 #endif
 
-    if (_diff > _break){
-        _pos = 0;
+    if (_diff > _break){    // long break means we are starting with a new transmission
+        _start = true;      // first bit will be ignored on the next edge
+        _pos = 0;           // start writing at the beginning of the message buffer
         _idx = 0;
-        _reset = true;
 #ifdef RECV
         Serial.println("==RESET==");
 #endif
     } else {
-        _count = _pulse_len>>1;
-        while (_count < _diff){
+        _count = _pulse_len>>1;    // bit lengths may vary in certain bounds. this makes sure we always are in the bounds
+        while (_count < _diff){    // iterate though each bit
             _count += _pulse_len;
-            if (_idx){
-                _recv_msg[_pos] |= !_line << _idx;
-            } else {
-                _recv_msg[_pos] = !_line;
+            if (_idx){                             // not the first bit
+                _recv_msg[_pos] |= !_line << _idx; // set the bit of the current character
+            } else {                               // first bit
+                _recv_msg[_pos] = !_line;          // reset all bits, set the first bit of the current character
             }
-            if (_reset){
-                _reset = false;
-            } else {
-                _idx++;
+            if (_start){                           // current bit is start bit
+                _start = false;                    // ignore by not increasing the bit index
+            } else {                               // current bit is not a start bit
+                _idx++;                            // advance to the next bit
             }
 #ifdef RECV
             Serial.print(_line);
 #endif
-            if (_idx == 8){
-                if (_recv_msg[_pos] == 0x00){
-                    _avail = true;
+            if (_idx == 8){                        // full byte received
+                if (_recv_msg[_pos] == 0x00){      // received byte is string terminator
+                    _avail = true;                 // notify about complete transmission
+// TODO: prevent writing out of buffer boundaries!                    
                 }
-                _pos++;
-                _idx=0;
+                _pos++;                            // advance to nect character of buffer
+                _idx=0;                            // start with first bit of that character
             }
         }
 #ifdef RECV
@@ -90,13 +87,12 @@ void interrupt485(){
 
 
 void init485(int RO, int nRE, int DE, int DI){
-    _RO=RO;
+    _RO=RO;  // save the pin numbers
     _RE=nRE;
     _DE=DE;
     _DI=DI;
     _avail = false;
 
-
 #if defined SEND || defined RECV
     Serial.begin(115200);
     Serial.print(F("RO = "));
@@ -116,7 +112,7 @@ void init485(int RO, int nRE, int DE, int DI){
     Serial.println("SEND enabled");
 #endif
 
-    pinMode(_RO,INPUT);
+    pinMode(_RO,INPUT);    // we are reading on line _RO
 
     pinMode(_RE,OUTPUT);
     digitalWrite(_RE,LOW); // enable reading
@@ -144,11 +140,11 @@ boolean writeBit(boolean bit){
     digitalWrite(_DI,!bit);
     delayMicroseconds(5);
     if (_line != bit){        // collision!
-       digitalWrite(_DE,LOW);
+       digitalWrite(_DE,LOW); // abort transmission immediately
 #ifdef SEND
        Serial.println(F("Collision!"));
 #endif
-       return false;
+       return false;          // notify caller
     }
     delayMicroseconds(_pulse_len-5);
     return true;
@@ -176,8 +172,10 @@ boolean send485(char message[]){
     Serial.print(F(" / micros = "));
     Serial.println(micros());
 #endif
-    while (_line || (micros() - _last_flip) < (20 * _pulse_len)) {
-        delayMicroseconds(20*_pulse_len);
+    
+    // if the line is busy: wait for it to get free
+    while (_line || (micros() - _last_flip) < _break) {
+        delayMicroseconds(_break);
     }
 #ifdef SEND
     Serial.println(F("enabling driver…"));
@@ -187,7 +185,7 @@ boolean send485(char message[]){
     int pos = -1; // increment pos prior to sending char!
     boolean sync = writeBit(0);
     do {
-        pos++;
+        pos++; // incrementation at beginning of loop in order to allow post-loop condition!
         sync = sync && writeByte(message[pos]);
     } while (sync && message[pos]);
     digitalWrite(_DI,LOW);