RS 232 Break Signal

Question

I got a RS232 signal capture device. and it working great.

I need some help making sense of the data. Basically we bought it because we are dealing a late 80's machine controller that uses serial communication. We had little luck despite knowing the port parameters.

From the data I dumped machine control is using the break signal as part of it's protocol. I am having trouble duplicating it using VB and the MSComm. I know to toggle the break signal and on and off. But I am not sure what I am supposed to be doing with it. I am supposed to leave it on for each byte of data I send. Or send a byte of data and then toggle.

Also I am confused how I supposed to receive any data from the controller. Do I toggle a flag when the break is turned on and then when it turned off read the input?

Answer 1

Michael Burr's description of the way break works is accurate. Often, "break" signals are sent for significantly longer than one character time.

These days, "Break" is infrequently used in serial comms, but the most common use is as a 'cheap' way of providing packet synchronization. "Break" may be sent before a packet starts, to alert the receiver that a new packet is on the way (and allow it to reset buffers, etc.) or at the end of a packet, to signal that no more data is expected. It's a kind of 'meta-character' in that it allows you to keep the full range of 8 or 7-bit values for packet contents, and not worry about how start or end of packet are delineated.

To send a break, typically you call SetCommBreak, wait an appropriate period (say, around 2 millseconds at 9600 baud) then call ClearCommBreak. During this time you can't be sending anything else, of course.

So, assuming that the protocol requires 'break' at the start of the packet, I'd do this (sorry for pseudocode):-

procedure SendPacket(CommPort port, Packet packet)
{
    SetCommBreak(port)
    Sleep(2);  // 2 milliseconds - assuming 9600 baud. Pro-rata for others
    ClearCommBreak(port)

    foreach(char in packet)
        SendChar(port, char)
}

Pseudocode for a receiver is more difficult, because you have to make a load of assumptions about the incoming packet format and the API calls used to receive breaks. I'll write in C this time, and assume the existence of an imaginary function. WaitCommEvent is probably the key to handling incoming Breaks.

bool ReadCharOrBreak(char *ch); // return TRUE if break, FALSE if ch contains received char

We'll also assume fixed-length 100 byte packets with "break" sent before each packet.

void ReadAndProcessPackets()
{
  char buff[100];
  int count;

  count = 0;

while (true)
{
  char ch;
  if (ReadcharOrBreak(ch))
    count = 0; // start of packet - reset count
  else 
  {
     if (count < 100)
     {
       buff[count++] = ch;
       if (count == 100)
         ProcessPacket(buff);
     }
     else 
       Error("too many bytes rx'd without break")
  } 
} 

WARNING - totally untested, but should give you the idea...

For an example of a protocol using Break, check out the DMX-512 stage lighting protocol.

The start of a packet is signified by a Break followed by a "mark" (a logical one) known as the "Mark After Break" (MAB). The break signals end of one packet and the start of the next. It causes the receivers to start reception. After the break up to 513 slots are sent.