RS-485 can mean several things.
It always represents a different electrical specification. In RS-232 there
is a transmit wire, a receive wire and ground. TX and RX is always paired
with ground. In RS-485 there is a TX+ and TX- and a RC+ and a RX-. This is
called a balanced line driver. It’s advantage is that you can run RS-485
lines much greater distances at hight speeds with less risk of interference.
RS-485 can mean that there is one master station that is always permitted to
transmit and a number of slave stations that may only transmit when given
permission by the master station, ala polling. But many users of RS-485
aren’t using this attribute of RS-485.
If you do have multiple slave devices then each slave device must turn off
it’s transmitter when it is not permitted to transmit. This can usually be
done by turning off RTS in the control signals. Other cards may have a
If you are programming for the master device, or if there is only one slave
device you don’t have to worry about any of this. You can just use the QNX
If you are programming for multiple slaves then you can either et your
application control the transmitter. Or better yet, write a driver that
sits on top of the QNX serial driver that turns on the transmitter when it
needs to transmit and then turns it off again.
Bill Caroselli – 1(626) 824-7983
“zhz_zhang” <email@example.com> wrote in message
You mean,If I define the dual-port card all as rs485 ,I must program the
“Mario Charest” <> firstname.lastname@example.org> > wrote in message
news:a2976a$bat$> email@example.com> …
“zhz_zhang” <> firstname.lastname@example.org> > wrote in message
news:a28q3b$2l6$> email@example.com> …
I have a dual-port rs422/rs485 interface card.But the company have no
in QNX6 system.
In the card,there are two 16550 chips.
I have seen doc about devc_ser8250 utilities,it seems that this card
drive through devc_ser8250 utilities.
I don’t know whether it really can drive the rs422/485 card.
Yes it can, that’s transpararent.
However using 485 takes some special (and not so simple precaution).
With 485 you can only have one computer transmitting at one time,
the driver will not deal with this. You have to.
Some people have modified the source of the serial driver
to handle this