Paul Keinanen wrote:
On 6 Jul 2003 19:46:48 -0700, (R C) wrote:
In ordinary RS-232 asynchronous communication the start bit is "0"
SPACE, interrupting the idle MARK state, followed by the data bits
with LSB sent first. The stop bit is "1" or MARK, which then transfers
to a MARK idle state if no more characters are to be transmitted.
That part I figured out from APRS packets recorded off the air. Start
is definately 1200 hz, stop is 2200 hz.
How did you determine this ?
APRS is definitively AX.25, which is a version of the X.25 protocol,
which are all members of the HDLC protocol family. These protocols
are _synchronous_ protocols, thus, they do not contain any start or
stop bits. The AX.25 protocol sends 8 bit octets (bytes) packed
immediately side by side without any bits in between them forming a
multiple octet frame. The whole frame starts with on octet with the
"01111110" bit pattern (the flag pattern) and it also ends with the
same pattern (or a single flag is used as a _frame_ separators, if
multiple frames are sent at once). In order to avoid false flag
detection within the data part of the frame, the 0 bit is inserted
after five consecutive 1 bits.
Well, don't forget that HDLC is NRZI encoded - the absolute level
(mark vs. space) is meaningless. A transition indicates a 0 and
the lack of a transition indicates a 1. Sending a series of 0s
results in a square wave of 1/2 the baud rate (since each baud
is the opposite of the previous baud). Sending a series of 1s
results in a series of unchanged bauds.
So, if the initial state of the NRZI encoder is '1' and the
'01111110' flag sequence is sent, the resulting output is:
initial state 1 : 0 0 0 0 0 0 0 1 and the next flag looks like
0 0 0 0 0 0 0 1 and this pattern repeats as long
as flags are being sent.
If the initial state is 0 you get:
initial state 0 : 1 1 1 1 1 1 1 0 and this pattern repeats.
So, you can't tell anything about the date being sent
from the absolute levels.
Dana