Help on interpreting a serial protocol.

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Renato Mendes

Nov 12, 1999, 3:00:00 AM11/12/99

I have a requirement for a system that needs to be interfaced to a PC using
serial lines. These serial lines has no standard as they were made for a
special purpose equipment. I'll try to describe how my system architecture
is and how the serial line and protocol works, followed by some questions.

My architecture is as follows:

SYSTEM1 --------<Serial Line 1>------------ SYSTEM2
|------ PC Connected

SYSTEM3 --------<Serial Line 2>------------ SYSTEM4
|------ PC Connected

As seem above, I have a serial line connecting SYSTEM1 to SYSTEM2 and
another connecting SYSTEM3 and SYSTEM4. I'll have to connect in parallel a
PC with a serial board just to acquire the data that's being exchanged
between both pair of systems for analisys. I don't need today to send
messages, but only to acquire and interpret what's going on in the serial

The pairs SYSTEM1/SYSTEM2 and SYSTEM3/SYSTEM4 do not interact with each
other, but they're not similar. Serial Line 1 is different from Serial Line

My electrical specifications are:

Serial Line 1: 20mA current loop
Serial Line 2: 0-5V (0V = MARK, 5V = SPACE)

The protocol that's exchanged between both systems is BIT ORIENTED, detailed

Baud Rate: 1200bps or 2400bps

The protocol, bit oriented, has 4 start bits (all "1"), followed by 2 words
of data then 2 CRC's, followed by 18 stop bits (all "0"). On every 8 bits
from the start bits its set to "1" for synchronism.

So, it looks like:


Where: ddddddd is data words
ccccccc are CRC words.
<idle> is the time where the line is idle

My purpose is to receive both data packets from the message in a continuous
mode (like an serial line analyzer), running on a PC equipped with some sort
of serial intelligent board.

I would like to know where may I find a serial board that will fully attend
this necessities ? The board must accept such configuration (4 start "1"
bits, 7 data bits mixed in the message, 18 stop "0" bits, etc).

Probably the board is programmable in C so that I can program my protocol
and download it to it. The operational system requirement is Windows-NT.

Thanks in advance for any kind of help.


Renato Mendes

Renato Mendes
Contraste Engenharia e Automação Ltda.

Jun 18, 2014, 3:28:23 PM6/18/14
I Digital/Digital board would make it happen.
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