Re: RS-485 arrangements
- From: "Steve" <sjburke1@xxxxxxxxxxx>
- Date: Mon, 17 Sep 2007 12:50:06 -0400
"Richard Henry" <pomerado@xxxxxxxxxxx> wrote in message
news:1190044006.664926.14450@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
I have long been familiar with RS-485 wired as a single differential
pair connected to all the devices and termination at the both ends,
however "end" may be defined. I have recently been looking into a 4-
wire "full-duplex" implementation. However, it seems to be full-
duplex only if there are only two devices attached. Otherwise, there
appears to be a need to designate one device as the master (the device
to whose receiver all the other drivers are connected) and the rest as
slaves.
Unless there is some clever arrangement of which I am ignorant. Any
comments?
Full duplex (FDX) RS 485 can be wired as point-to-point (typical use) or as
point-to-multipoint (unusual but possible). If you use the
point-to-multipoint configuration, you are correct that it looks effectively
like the half duplex configuration, because you still need a master and
slave(s) relationship. Replies from the various slaves have to be
non-overlapping, and their output transceivers need to be controllable just
like the 2 wire HDX configuration. So its hard to see any benefit to the
extra wire pair in point-to-multipoint.
A point-to-point FDX configuration is usually used when you need high
throughput, or when you don't have access to the 485 transmitter enable
signal. If you are sending command-reply type messages, FDX doesn't increase
throughput because the two devices are alternating their messages anyway.
Most FDX 485 configurations with which I am familiar are really FDX hardware
with HDX software implementations.
Steve
.
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