RE: Re: [Project] Lights was Foundations are in!

["Keith Doxey" <ukha@xxxxxxx>]

Limitation is from the datasheets.

The drivers can supply a certain amount of current
Each receiver draws a certain amount of current. Above 32 receivers the
voltage levels can drop to indeterminate levels. Newer RS485 chips draw
less
current and you can have upto 256 on a bus.

If you want more you just add a repeater.

RS232, RS422, RS485 are ELECTRICAL standards. V24, V90 etc are Protocol
Standards. The fact that V24 is almost always sent over RS232 makes most
people think they are the same thing.

RS232 uses + and - voltages of between 3 and 12 volts to indicate "0
& 1"
Separate wires for TX & RX

RS485 uses two wires one at +5V the other 0V for a "0" and 0V and
+5V for a
"1"
Half Duplex - Send and Receiver on the same 2 wires.

RS422 as RS485 but two pairs One TX, One RX. Full Duplex

Keith



> -----Original Message-----
> From: patrickl@xxxxxxx [mailto:patrickl@xxxxxxx]
> Sent: 02 June 2001 22:39
> To: ukha_d@xxxxxxx
> Subject: [ukha_d] Re: [Project] Lights was Foundations are in!
>
>
> --- In ukha_d@y..., "Keith Doxey" <ukha@d...> wrote:
> > The reasons for RS485 are...
> > BiDirectional Comms over 2 wires
> > Balanced therefore 99.9% immune to interference
> > Multidrop - can support 32 devices on the bus
>
> I've never come across this limitation - what does it arise from?
>
> > Single +5V supply
>
> I thought RS485 was usually implemented over V24 style
> physical/electrical just like RS232?
>
> > the reasons against RS232...
> > BiDirectional comms requires 3 wires Tx,Rx,GND
> > Not immune to interference
> > Point to Point only so would need 1 RS232 port per attached
device
>
> It doesn't have to be - it can be driven open collector... but I get
> the point.
>
> Patrick
>
>
>
>
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