What rs485 chip to use
Archiver
Posts: 46,084
What's a good RS/485 Transciever chip to use with BS2? What I mean by that
is, Minimal number of support components when used with the BS2. I don't
know what's out there. Is there a chip that can work directly from the
stamps serout?
C-
is, Minimal number of support components when used with the BS2. I don't
know what's out there. Is there a chip that can work directly from the
stamps serout?
C-
Comments
>What's a good RS/485 Transciever chip to use with BS2? What I mean by that
>is, Minimal number of support components when used with the BS2. I don't
>know what's out there. Is there a chip that can work directly from the
>stamps serout?
>
>C-
I don't remember the number of the chip, but search for RS-485 driver in
the interface section of the TI web site
[noparse][[/noparse] http://www.ti.com ]
Bruce
come in other packages. Also, this is a 1/1 unit load. Meaning that it
will support up to 32 nodes.
This chip works well for me and I recommend it.
Original Message
From: Chris Parasida [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=yh40V2I2AivS7XDRNntX21amrW6sr0rhF2OqcfhYiVvrS1XwRaCuaYo3sOy3-t1MY-qQduT1ryOyh7kFfy0ao0kDgw]parasida@r...[/url
Sent: Sunday, November 05, 2000 1:17 PM
To: basicstamps@egroups.com
Subject: [noparse][[/noparse]basicstamps] What rs485 chip to use
What's a good RS/485 Transciever chip to use with BS2? What I mean by that
is, Minimal number of support components when used with the BS2. I don't
know what's out there. Is there a chip that can work directly from the
stamps serout?
C-
>is, Minimal number of support components when used with the BS2. I don't
>know what's out there.
Just about any of the TTL/RS-485 converters will work with a Stamp, but I
like the Max3082, or any chip with a limited slew rate. Limited slew rate
means that the chip has a much lower maximum speed (115kbps). The low speed
is plenty fast for Stamps, and because the outputs switch at a slower rate,
in most cases you can treat them as "electrically short" lines. This means
that you don't need to use the parallel terminations recommended for other
RS-485 links. So you need fewer components and the link also draws much
less current.
>Is there a chip that can work directly from the
>stamps serout?
As you may know, Serin and Serout aren't like most other RS-232 interfaces.
Instead of having its own negative supply for the transmitter (via a MAX232
or other means), the Stamp's transmitter borrows the negative voltage
received from the remote end's transmitter. The remote transmitter must be
idle (negative) when the Stamp is transmitting. The circuits also invert
the signals. The Stamp manual shows the circuit.
With RS-485, (I'll assume you mean 2-wire, half-duplex), there is no
negative voltage and only one transmitter on the network can be active at a
time. You also need to be sure the polarity is correct, because you can't
set baudmode to invert the signals.
A typical RS-485 interface chip consists of a 2-wire, 2-way RS-485
interface, a TTL-compatible input and output, and transmit-enable and
receiver-enable inputs.
If you connect Serin and Serout to a 2-wire RS-485 chip, you need to be
sure that the receiver on the TTL side of the converter is close to 0V when
the Stamp is transmitting. At the receiver on an idle RS-485 line, line A
has a greater voltage than line B, and the receiver's TTL output is logic
high. Serin expects to see a negative voltage, or at least something near
0V. So you would need to add an extra inverter (any TTL-compatible inverter
will do) between the RS-485 converter's TTL output and the Stamp's Serin to
get the correct polarity. And do the same between Serout and the
converter's TTL input. I haven't tried it, but off the top of my head I
think it would work.
If you're using the Stamp's other port pins, you don't have to worry about
any of this. The Stamp's port bits can interface directly to an RS-485
converter.
(In a similar way, if you want to use a Max232 to extend the range of Serin
and Serout, you need to re-invert the signals. You can use the extra driver
and receiver in the Max232 for this.)
You'll also need to dedicate a port output pin on the Stamp for controlling
the RS-485 transceiver's transmit-enable line, unless you use a
hardware-only method of control.
I have more info about RS-485 on my website's Serial Ports page. For info
about hardware-only transmitter control, see my article "Designing RS-485
Circuits".
http://www.lvr.com/serport.htm
Jan Axelson
http://www.lvr.com
jan@l...