serial communication problems

["Deepak Gupta" <deepak@cse.iitk.ac.in>]

Hi,

I have a strange problem with the serial line communication in Linux
(RH 7.2, kernel 2.4.7-10).  We have some electronic locks that are
controlled by a PC.  The locks are connected by a RS485 twisted pair
(multidrop) and this line terminates in a 485-232 converter box which
is connected via the serial line to a PC.  The lock vendor has supplied
a Windows software to program the locks which works fine on this PC.
He has also given me the protocol that is followed between the PCs and
the locks.  I am trying to write my own Linux based software for
programming the locks.

The problem is that when the vendor software sends a command to a lock
it replies obediently but when my program writes exactly the same sequence
of bytes to the serial line, I don't get a response (the protocol
says that the lock will not respond unless it gets a valid command).
I know the locks use 9600 baud, 8 bits, no parity, 1 stop bit and I
am using the same.  I also tried with and without hardware flow control
(RTS/CTS) but no luck.  I know that the command I send is right since
I used another PC to listen to what the vendor software writes on the
serial line and it matches the protocol description that I have.

Incidentally I opened up the 232-485 converter box and saw that it
uses only the TX, RX, SG, RTS and CTS lines on the RS232 side.  So 
it cannot be a matter of DTR or some other signal.

Please help.

Deepak Gupta