[time-nuts] RE : Re: FASTRAX GPS

[jim s]

On 7/2/2014 1:29 PM, Jean-Louis Oneto wrote:
> Hello,
> AFAIK, the differential variant of RS-232 is RS-485. I'm not sure about the levels.
> Best regards,
>
>
> Jean-Louis Oneto
You have to control the direction  of RS485.  You don't have to with 
RS232.  One of the problems with support of RS485 in Linux OS kernels is 
that the kernel can't respond to the turning of the line quickly enough 
in real time.  I had an application with 115k requirement, and it had to 
turn the line within 1/10 of a bit time. Pretty quick for a kernel.

It is differential, and it does send data, but is a different beast in 
many ways.

I would have liked to have had an external processor to handle the 
RS485, and some chips do that, but we had the engineer just attach two 
serial ports on an Arm SOC to the usual serial uarts, and it was fun 
figuring out how to make it work.

BTW this was for communicating with an airframe, and the avionics 
dictated the rate we had to turn the lines.  It was for passenger 
signals, etc., so was simple, but you basically ran all the cabin 
lighting and attendant signalling thru one line for the entire aircraft.