Different devices on RS485 line?

As RS485 is multi-drop, one would assume that you can hang multiple
devices on the same line (meaning different devices of different
makes, like say two controllers from two different manufacturers). I
have two questions:
1. Is this at all possible, or should all devices on the 485 line be
the same type/make
2. If possible, what should I check about the 485 parameters/protocols
to ensure it works

The reason I have a doubt is because unlike TCPIP where it's just the
addressing that matters (no relation to what the device does, or
what's in the packet), I think 485 can have clashes in the above
scenario - but I have not worked with 485 so hence the question....

Thanks

"ElderUberGeek" <aribloch@gmail.com> wrote in message
news:1174029439.926590.59100@p15g2000hsd.googlegroups.com...
>
> As RS485 is multi-drop, one would assume that you can hang multiple
> devices on the same line (meaning different devices of different
> makes, like say two controllers from two different manufacturers). I
> have two questions:
> 1. Is this at all possible, or should all devices on the 485 line be
> the same type/make
> 2. If possible, what should I check about the 485 parameters/protocols
> to ensure it works
>
> The reason I have a doubt is because unlike TCPIP where it's just the
> addressing that matters (no relation to what the device does, or
> what's in the packet), I think 485 can have clashes in the above
> scenario - but I have not worked with 485 so hence the question....

Whereas TCP/IP over ethernet is a complete protocol and hardware definition,
RS-485 is not. RS-485 only specifies layer 1 of the OSI model, i.e. it
specifies the voltage levels and the differential characteristic of the
line. That's it. So an RS-485 capable device may just use any data protocol
on that line, completely incompatible with other protocols. One device may
just send ASCII packets with 8,N,1 characters while another device does HDLC
(8 bit synchronous with stuff bits). Having both together on one RS-485 is
impossible.

Meindert

On Mar 16, 12:17 am, "ElderUberGeek" <aribl...@gmail.com> wrote:
> As RS485 is multi-drop, one would assume that you can hang multiple
> devices on the same line (meaning different devices of different
> makes, like say two controllers from two different manufacturers). I
> have two questions:
> 1. Is this at all possible, or should all devices on the 485 line be
> the same type/make
> 2. If possible, what should I check about the 485 parameters/protocols
> to ensure it works

I'll answer the question I *Think* you're asking. The devices don't
need to be the same 'type/make' to play on the bus. Any RS485
compatible device will work provided, as Meindert has said, that they
all adopt the same protocol.

Bob Stephens