What's The Difference Between Open-Drain And Open-Collector?
Jack Portier
anyone tell me?
Jack Portier
"Jack Portier" <nom...@nergens.nl> wrote in message
news:nVU18.63$JK6...@pollux.casema.net...
> I see these terms often in datasheet but I'm not sure what it means. Can
> anyone tell me?
>
>
Open-Drain is either GND or floating and Open-Collector is either VCC or
floating, right? Or is it the other way around?
Filip Miletic
Jack Portier wrote:
> > I see these terms often in datasheet but I'm not sure what it means. Can
> Open-Drain is either GND or floating and Open-Collector is either VCC or
> floating, right? Or is it the other way around?
AFAIK, the terms are used interchangeably, and mean an output either
connected
to the drain of a MOS transistor (in CMOS technology, for instance)
or a collector of a bipolar transistor (in TTL, for instance).
Both perform a similar role. If active, they will short the pin in
question to
the negative supply (e.g. GND), and if inactive will act as if detached
from the
rest of the net.
f.
Rich Grise
Filip is accurate as to their function; the difference being that "open-
drain" refers to FET or CMOS logic (source, gate, drain) and "open-
collector" refers to bipolar logic (emitter, base, collector). Their
most common use that I know of is "wire-or" circuits, (which should
really be called "wire-and," but that's another topic) - you can
connect more than one "open-x" output together, and any or all
of them can pull the output line low, without damaging any of the
other devices on the same line. Keep in mind, however, that if you
want to get a logic high out of either of them, that you need a "pull-
up" resistor; i.e. when an "open-x" output turns off, there's no
current path at all, so it can float to anything.
--
HTH!
Rich
"We have met the enemy, and he is us!"
- Pogo Possum, ca. 1950's
John Popelish
>
> I see these terms often in datasheet but I'm not sure what it means. Can
> anyone tell me?
One refers to an output from a field effect transistor, and the other
refers to the output from a bipolar junction transistor.
--
John Popelish
Richard Steven Walz
Jack Portier <nom...@nergens.nl> wrote:
>I see these terms often in datasheet but I'm not sure what it means. Can
>anyone tell me?
Sure. Most logic gates either pull their output pin HI or LO, and this is
done by two different transistors, only one of them turned ON at a time.
The one that pulls the pin HI is connected to the Vcc or Vdd positive supply,
which is where it gets its "HI" from, and the other is connected to ground,
which is where it gets its "LO" from.
These transistors are just being used as switches. Imagine a pair of
switches, a HI-pulling and a LO-pulling connected to a common output pin.
Since these schematic symbols are drawn on on top the other in the diagram,
these two transistors kind of look like two faces one atop the other, so
this kind of output is called a "totem-pole" output.
The other two terms you will find used about these kinds of outputs is that
some are "tristated" which means they can have three states, HI "ON"/LO "OFF",
HI "OFF"/LO "ON", or BOTH "OFF" which is like being entirely disconnected!
Now if BOTH were on that would connect the output pin to HI and LO at the
same time, which is like shorting out the power supply, and it would catch
fire!
Now these 4 states for two transistors might at first seem like all you
could do with these, but it's not so. You could ALSO have it HALF tristated
and HALF pulled up or down!! How? By having the LO-pulling transistor turn
ON and OFF, but leave the HI-pulling transistor OFF!! That way the two
states are either tristated or LO!!
Why is that good?
Imagine a transmission wire in space is pulled up to the positive supply
rail, Vcc or Vdd, by means of a resistor outside the chip output which is
connected to the wire. It is always HI unless somebody pulls it LO!!
ANYBODY ANYWHERE along its length can pull it LO!!
This is very useful and is called an open-collector or open-drain
transmission line. Why? Because it is precisely the collector of a bipolar
(NPN) transistor or the drain of a MOSFET that is the only thing driving
that kind of LO-pulling output.
The current to pull the wire LO goes in the output through the collector
or drain to ground out its emitter or source, respectively, whenever that
LO-pulling transistor is turned ON.
-Steve
--
-Steve Walz rst...@armory.com ftp://ftp.armory.com/pub/user/rstevew
Electronics Site!! 1000's of Files and Dirs!! With Schematics Galore!!
http://www.armory.com/~rstevew or http://www.armory.com/~rstevew/Public
Richard Steven Walz
Neither. Both are LO or floating (tristate). A drain is used the same way
as a collector is in switching transistors, bipolar vs MOSFET.
These names are just to distinguish a LO-pulling output made with
bipolars vs MOSFETs.
Rich Grise
>
> In article <j%U18.65$JK6...@pollux.casema.net>,
> Jack Portier <nom...@nergens.nl> wrote:
> >
> >"Jack Portier" <nom...@nergens.nl> wrote in message
> >news:nVU18.63$JK6...@pollux.casema.net...
> >> I see these terms often in datasheet but I'm not sure what it means. Can
> >> anyone tell me?
> >
> >
> >Open-Drain is either GND or floating and Open-Collector is either VCC or
> >floating, right? Or is it the other way around?
> ------------------
> Neither. Both are LO or floating (tristate). A drain is used the same way
> as a collector is in switching transistors, bipolar vs MOSFET.
>
> These names are just to distinguish a LO-pulling output made with
> bipolars vs MOSFETs.
> -Steve
> --
> -Steve Walz rst...@armory.com ftp://ftp.armory.com/pub/user/rstevew
Well, not exactly "tristate," this term is usually used for a totem-pole
output when both of the output transistors are turned off. The third
state, get it? :-) Open-collector (bipolar) and open-drain (FET) are
just simply on (active device pulls output to Vee or Vss) or off,
floating.
--
Cheers!
Richard Steven Walz
In the case of the "HI" output of the open-collector or open-drain the output
is floating, which is the same as tristated. I meant to equate floating to
tristate so that people get used to thinking of tristate properly, because
it is the same. Tristate is not being pulled LO or HI at all.
Rich Grise
...
> In the case of the "HI" output of the open-collector or open-drain the output
> is floating, which is the same as tristated. I meant to equate floating to
> tristate so that people get used to thinking of tristate properly, because
> it is the same. Tristate is not being pulled LO or HI at all.
> -Steve
> --
> -Steve Walz rst...@armory.com ftp://ftp.armory.com/pub/user/rstevew
I don't want to start a fight, but let's not confuse the newbies.
By convention, "open-collector" or "open-drain" refer to a different
circuit topology than "tri-state." Yes, it is true, that an
open-collector
output, when off, is floating, and so is indistinguishable to the
outside
world from a tristate output that's in its tristate mode, where both
of the totem-pole transistors are off. But an OC or OD only has two
states: "on" (low) or floating. A tristate will provide source current
in the "high" state. (as long as the output is enabled.) Albeit, not
a whole hell of a lot, but the top totem-pole transistor does the
pulling-up. So, essentially, it has three states: low, high, and
floating.
At least, these are the definitions of those terms that I learned
in school. (actually, learned them on my own, such that school
was mainly review.) :-)