Jameco Electronics Rabbit 3000 - Manual (Page 206)

User’s Manual 197
the receiving interrupt service routine to detect this gap, it is suggested that dummy char-
acters be transmitted to help detect the gap. This can be done in the following manner.
The transmitter starts transmitting dummy characters when the first character interrupt is
received. Each time there is an interrupt, either receiver data register full or transmitter
data register empty, a dummy character is transmitted if the transmitter data register is
empty. Although the transmitter and receiver operate at approximately the same baud rate,
there can be a difference of up to about 5% between their baud rates. Thus the receiver
full and transmitter empty interrupts will become out of phase with each other, assuming
that the remote station transmits without gaps between characters. A counter is zeroed
each time a character is received, and the counter is incremented each time a character is
transmitted. If this counter holds (n), this indicates that a gap has been detected in the
frame; the length of the gap is (n - 1) to (n) characters. The start of frame could be marked
by (n) reaching 3, indicating that the existence of a gap at least two characters long.