Dev.ser missing serial data

Problem Description
Data loss on the serial lines.

Connection details

|------------------------------------------ |------|
| | CTI
| RS 485 Line Monitor.
| QNX 4.24 Computer | R |
| Dev.ser –t 14 –N cti –b 38400 280,10 | S

288,10 290,10 298,10 2A0,10 2A8, 10
2B0,10 2B8,10… &
Dev.ser (version 4.23I)
Serial App.

IO controller transmits a message (80 bytes len) every 10 msec.
SerialApp receives this message. SerialApp also keeps transmitting
messages to IOC. Over a period of time SerialApp finds loss of data in
the messgae. Most of the time the data lost is the start byte of the
message and few bytes after that.

Data on the 485 Line monitor is good. So it is not the IO controller,
sent the bad data.

I logged the data in Serial App right after the dev_read(fd, &buf,
sizeof(buf)-1, 0, 0, 0, proxy, &armed) fn. which reads the serial data
from Dev.ser. I find the data bytes are missing in the buf itself.
So I modified Dev.ser (ver 4.23X) source to log the received data right
after it does a inp(dev->iobase). I expected every single byte collected
after dev_read() in SerialApp, to be present in the Dev.ser’s log also.
But what I found is, there is a message collected by Serial App with few
bytes missing in that but the coplete message is not found in the
Dev.ser’s log. It’s fine if Dev.ser lost the same byte. But it lost the
complete message.

How could the application receive data bypassing the Dev.ser. The data
seen by SerialApp is exactly what I see on the 485 line monitor. It is
not junk .