[lug] Latency for serial port hardware handshake?

[Dan Julio]

Hey all,

Does anyone know how Linux processes de-assertion of
CTS on a typical PC-based serial port?  I need to
understand how many more characters may be sent from
the Linux box (DTE) to a DCE device when the DCE
device de-asserts CTS. 

Specifically I am running on a Dell 4100/RH7.3.  The
built-in port uses the 16550A UART chip.  I am talking
to an embedded controller with a small buffer.  This
buffer holds 4 up-to-16 byte packet entries.  The
controller de-asserts CTS when 3 buffer entries are
full allowing for transmission of part-of or a
complete 4th packet.  

I have enabled my linux control software to look at
CTS (options.c_cflag |= (CRTSCTS);)   I verified this
works by manually de-asserting CTS (the computer won't
send anything until it is re-asserted).  However it
looks like there is some latency between the serial
driver acting on CTS when there is data in some
internal FIFO.  It looks like I get another packet+
sent to the device after it de-asserts CTS and, of
course, it loses the packet after #4.  

And...then...if any of you wizards know how this all
works I have another question.  Can I expect this to
be different on different hardware (probably yes). 
The eventual target for the control software is
another embedded system running on a StrongARM
(built-in UART and probably different drivers).

Thanks, Dan


 
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