Subject: Electronic nuts and bolts...

From: "Marc H" <[email-masked]>
Date: Sun, 08 Nov 2009 23:21:24 -0000

I have a couple of questions tonight...

#1..open collector theory....  Does the open collector trait have the ability to cause problems in a circuit upstream from the offending gate?  I have a 245 that is changing states on one gate only and I can't figure it out for the life of me except I have a chip with open inputs and outputs downstream.  It's a bit of a hassle grounding these when I plan to use them later so if I need to look elsewhere I'd sure be happy ;-)

#2..I see the need for a device that can emulate buss signals such as address lines, data lines and some misc signals.  I have a plan to build one composed of DIP switches.  My questions are...

#2A.. can I use 5 volts as my high signal or is that too much...

#2B.. I assume I will need to add pull-down resistors to the output side of the switch to ensure a proper low signal (and avoid the open collector pitfall.)  How can I determine what value I need to use or is there a rule of thumb ?

Thanks hardware peep's

Marcus

---

From: "Tursi" <[email-masked]>
Date: Sun, 8 Nov 2009 18:32:54 -0500

> #1..open collector theory....  Does the open collector trait have the 
> ability to cause problems in a circuit upstream from the offending 
> gate?  I have a 245 that is changing states on one gate only and I 
> can't figure it out for the life of me except I have a chip with open 
> inputs and outputs downstream.  It's a bit of a hassle grounding these 
> when I plan to use them later so if I need to look elsewhere I'd sure 
> be happy ;-)
>

Floating outputs are not a problem. Floating inputs should always be 
treated as suspicious at best.

I've seen the term "open collector" before and even understood it once, 
but it escapes me right now. Without experience to back it up I'll defer 
that question to others. :) I do know it's talking about transistors.

According to Wikipedia:  http://en.wikipedia.org/wiki/Open_collector

"Open collector is a type of output on many integrated circuits (IC). 
Instead of outputting a signal of a specific voltage or current, the 
output signal is applied to the base of an internal NPN transistor whose 
collector is externalized (open) on a pin of the IC. The emitter of the 
transistor is connected internally to the ground pin."

Which means, basically, that the output can output LOW, or open, never 
HIGH. You can see a schematic at the page.

> #2A.. can I use 5 volts as my high signal or is that too much...

5v is high on 5v circuits. It's too much if you are using 3.3v parts, 
but if you are using 3.3v parts you'll need some work to adapt them to 
the TI, which uses 5v. :)

> #2B.. I assume I will need to add pull-down resistors to the output 
> side of the switch to ensure a proper low signal (and >avoid the open 
> collector pitfall.)  How can I determine what value I need to use or 
> is there a rule of thumb ?

I think you are misusing the term "open collector"...? It's generally 
only used to refer to transistorized connections.

But pull down and pull up resistors are used to get a constant reference 
in the case of no other current flow. Thus, when they are active, they 
are drawing current. The lower the value of the resistor, the higher the 
current flow will be, and the harder it is to override it. I'm not sure 
if there are any hard and fast rules, offhand.. I see 5k and 10k used a 
lot (I use 10k but I don't have a good reason why).

There are some very good pages online that discuss the theory behind 
pullup/pulldown resistors far better than I can. Here's a start: 
http://everything2.com/title/pull-up+resistor

---

From: Insane Multitasker <[email-masked]>
Date: Sun, 8 Nov 2009 17:38:34 -0600

Marc, I might recommend Tony Lewis's "Interface Standards & Design"  for the TI.  I haven't looked at mine in some time but it gives an overview from the ground up, including signals, common chips, and DSR.  I just ran across mine in the dungeon; not sure if anyone has ever posted a scan to WHT.  
 
> To: [email-masked]
> From: [email-masked]
> Date: Sun, 8 Nov 2009 23:21:24 +0000
> Subject: [TI994A] Electronic nuts and bolts...
> 
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---

From: Marc Hull <[email-masked]>
Date: Sun, 8 Nov 2009 17:31:41 -0800 (PST)

<I think you are misusing the term "open collector".. .? It's generally 

only used to refer to transistorized connections.>

Your exactly right.  I should have used the term open input.  

<5v is high on 5v circuits. It's too much if you are using 3.3v parts, 

the TI, which uses 5v. :)>

I ask because I have yet to see an output over 3.5V on the TTL chips....Hmmmm??

--- On Sun, 11/8/09, Tursi <[email-masked]> wrote:

From: Tursi <[email-masked]>
Subject: Re: [TI994A] Electronic nuts and bolts...
To: [email-masked]
Date: Sunday, November 8, 2009, 5:32 PM

  

According to Wikipedia:  http://en.wikipedia .org/wiki/ Open_collector

> is there a rule of thumb ?

I think you are misusing the term "open collector".. .? It's generally 

http://everything2. com/title/ pull-up+resistor

---

From: Marc Hull <[email-masked]>
Date: Sun, 8 Nov 2009 17:32:53 -0800 (PST)

<not sure if anyone has ever posted a scan to WHT. >

YET ??? ;-)

--- On Sun, 11/8/09, Insane Multitasker <[email-masked]> wrote:

From: Insane Multitasker <[email-masked]>
Subject: RE: [TI994A] Electronic nuts and bolts...
To: [email-masked]
Date: Sunday, November 8, 2009, 5:38 PM

  

      

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---

From: mark wills <[email-masked]>
Date: Sun, 8 Nov 2009 20:20:39 -0800 (PST)

OPen collector can be driven low, but must be TIED high.

They are used when multiple devices communicate on the same bus.

When the device needs to output a low, it can 'overcome the pull' of the pull-up resistor.
When it is NOT outputting a low, it tri-states the output (fancy word for dis-connects it). Then the pull-up resistor takes over and pulls it high.

Viola.

That's how I remember it anyway... It was 1993... Gulp ;-)

________________________________
From: Marc H <[email-masked]>
To: [email-masked]
Sent: Mon, 9 November, 2009 0:21:24
Subject: [TI994A] Electronic nuts and bolts...

Thanks hardware peep's

Marcus

---

From: "Tursi" <[email-masked]>
Date: Mon, 9 Nov 2009 00:41:00 -0500

>I ask because I have yet to see an output over 3.5V on the TTL 

Is the part under load? If anything is sinking current from the output 
you may well read a lower voltage. Also check that your input voltage is 
really 5v.

---

From: "sconner3uk" <[email-masked]>
Date: Mon, 09 Nov 2009 09:50:55 -0000

Open-collector outputs can also be used to control components which use a different supply voltage. For example, you could use one to energise a relay which is powered from a +12V supply. You can use them for components on the same supply rail as well, for example to switch on/off an LED connected to the +5V supply (via a current limiting resistor).

Stuart.

--- In [email-masked], mark wills <markrobertwills@...> wrote:
> From: Marc H <marc.hull@...>