(unknown)

[Alex Pavloff <apavloff@eason.com>]

Good (morning/afternoon/evening) folks,

I am writing Modbus RTU code for Linux 2.4.19.  While seeminly simple to do
in userspace, there's one big kicker that I can't handle there easily.
Modbus RTU is a binary serial protocol used in industrial automation.  First
used by the Modicon PLCs, it's found on a variety of devices from a variety
of manufacturers.

The specification is at www.modbus.org, under "Modbus Standard Library", and
its the Modbus Serial Protocol Reference Guide.

Here's the part that I don't think I can resolve in userspace:
-----
RTU Framing
In RTU mode, messages start with a silent interval of at least 3.5 character
times. This is most easily implemented as a multiple of character times at
the baud rate that is being used on the network (shown as T1-T2-T3-T4 in the
figure below). The first field then transmitted is the device address.

The allowable characters transmitted for all fields are hexadecimal 0-9,
A-F. Networked devices monitor the network bus continuously, including
during the 'silent' intervals. When the first field (the address field) is
received, each device decodes it to find out if it is the addressed device.

Following the last transmitted character, a similar interval of at least 3.5
character times marks the end of the message. A new message can begin after
this interval.

The entire message frame must be transmitted as a continuous stream. If a
silent interval of more than 1.5 character times occurs before completion of
the frame, the receiving device flushes the incomplete message and assumes
that the next byte will be the address field of a new message.
-----

3.5 character times at high baud rates is not something I can pull off with
termios.  To get the Modbus RTU frames without resorting to some really
strange code in userspace, should I write a line-discipline module whose
sole purpose is to deal with the character timing?

Any comments (yay/nay/huh?) appreciated.

Alex Pavloff
Software Engineer
Eason Technology