Re: [Boost-users] Boost asio serial and rs-485

Subject: Re: [Boost-users] Boost asio serial and rs-485
From: Michael Caisse (boost_at_[hidden])
Date: 2010-02-15 14:19:05

Christian Auby wrote:
> Is it possible to use Boost asio serial with rs485? If yes, is there
> anything special you need to be aware of?
>
> regards,
>
> Christian
>   

Hi Christian -

The answer will depend on your rs485 hardware. RS485 tri-state
is typically handled one of two ways:

   1. the hardware detects the transmitter is active and
      asserts the transmitter signals onto the bus. The hardware
      has a "timeout" that continues to assert the transmitter
      after each byte (or bit) sent. After the last byte and the
      timeout period, the hardware will tri-state the bus again.

   2. Complete software control. There is a signal that
      controls the state of the bus. Depending on the state it
      is either in tri-state or the transmitter drivers are active.
      This signal is often tied to RTS.

If you have option 1... you are set. Just use asio as normal with
serial and it will all just work.

If you have option 2... life may be bad. The trick is asserting
RTS at the right time. You need to know when the last character
has actually left the transmit buffer of the hardware. This is
the tricky part. If you have a 2-wire 485 you can monitor
for your last character. If you have a 4-wire 485 then you are
going to need some way to determine when the last character has
left. Either way, it is the process is the same. Set the RTS mode
to enable transmitters when you want to transmit and then
set the mode for tri-state when done.

ASIO doesn't have direct control of RTS AFAICT. I have recently
answered another thread on this subject ... to quote myself:

You can use get_option and set_option to get/set various bits in the
windows DCB or posix termios. These pins are not currently described as
far as I can tell.

Options implement the SetabbleSerialPortOption and GettableSerialPortOption
type requirements.

Take a look at serial_port_base.hpp for the existing options (such as
baud_rate). You will find the implementation in impl/serial_port_base.ipp.

It should be fairly easy to use those as a guide to make your own
options that support your platform. For windows you will modify the DCB
structure and for posix the termios.

HTH
michael

-- 
----------------------------------
Michael Caisse
Object Modeling Designs
www.objectmodelingdesigns.com