ON 20010312@2:28:39 PM at page:
http://www.piclist.com/techref/logic/555s.htm
JMN-EFP-786 James Newton edited the page

ON 20010312@2:30:17 PM at page:
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JMN-EFP-786 James Newton edited the page

ON 20010314@3:11:25 PM at page:
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JMN-EFP-786 James Newton edited the page

ON 20010314@6:35:38 PM at page:
http://www.piclist.com/techref/logic/discreet.htm
JMN-EFP-786 James Newton added 'See also:
http://www.piclist.com/techref/logic/opencollector.htm
Open Collector'

ON 20010314@6:37:07 PM at page:
http://www.piclist.com/techref/logic/opencollector.htm
JMN-EFP-786 James Newton added 'Code:

Q is the value in the internal register corresponding to the output pin.
Usual CMOS output: When Q is high, P is off and N is on. When Q is low, P is
on and N is off.
(TTL is the same except P is PNP and N is NPN)

Vcc-------+
          |
      ---[ P channel FET
     |    |
Q ---|    +----------Out
     |    |
      ---[ N channel FET
          |
Vss-------+

Open drain output: When Q is high, N is on, output sinks current. When Q is
low, N is off, output is high-impedance.

Vcc-------+ (a pullup resistor between Vcc and Out gives you a logic 1 when
N is off)



          +--------- Out
          |
     Q ---[ N channel fet
          |
Vss-------+
'

ON 20010314@6:39:30 PM at page:
http://www.piclist.com/techref/logic/opencollector.htm
JMN-EFP-786 James Newton edited the page

ON 20010314@6:40:15 PM at page:
http://www.piclist.com/techref/logic/opencollector.htm
JMN-EFP-786 James Newton edited the page

ON 20010317@7:32:42 AM at page:
http://techref.massmind.org/techref/logic/opencollector.htm
JMN-EFP-786 James Newton added 'Says

This configuration is
often used in what is called  'wired OR'.  Any number of open-drain
devices can sit on the same bus, with an external pull-up resistor.
The un-asserted state is high (negative logic).  Any one of the
devices on the bus can pull the signal low to its asserted state,
hence creating an OR function on the bus.
<P>
The open-collector is often used on interrupt lines.  A number of
devices sit on the one interrupt signal, and any one of the devices
can assert the signal by driving it low.  You can add and remove
devices from the bus as necessary.  This wouldn't be possible with
normal logic outputs, because one device would be driving the bus high
while another would be driving it low.  But an open drain can't drive
a signal high, hence the need for an external pull-up.  It can only
pull a signal low.

'