Question about modems

[alan at alanlee.org]

Again.. I really think you are overthinking this.

MCU should set receive line coding to 8,N,1.  When in command mode, you 
don't care about parity as it is presumed the connection from the host 
machine to the modem is short and largely has integrity.  Every byte you 
receive in 8,N,1 command mode, zero the MSB before interpreting the 
character.  Also keep four binning counts of each byte received in 
command mode on whether E,O,M,S space passes a respective validity 
check.  Whichever bin over time has the highest count of successes vs 
errors is the parity scheme you use when encoding/stuffing the MSB of 
the 8,N,1 response bytes back to the host computer.

-Alan

On 2019-11-15 01:23, Jim Brain via cctalk wrote:
> If you look at the values received by an 8N1 connection from a sender
> using the different settings, you get:
> 
> 
> 	AT
> 	at
> 	At
> 	aT
> 7E1
> 	E174
> 	41D4
> 	E1D4
> 	4174
> 7O1
> 	61F4
> 	C154
> 	6154
> 	C1F4
> 7M1
> 	E1F4
> 	C1D4
> 	E1D4
> 	C1F4
> 7S1
> 	6174
> 	4154
> 	6154
> 	4174
> 8N1
> 	6174
> 	4154
> 	6154
> 	4174
> 
> Obviously, still trying to find the magic boolean logic equation to
> tease out the parity, but you could brute force it with these values
> and only aT would cause you issues requiring looking at CR (7E1 would
> send 8d, while 7S1/8N1 would send 0d.
> 
> Jim