Tech Q for electronics experts

[John Bennett]

[ukvac] Tech Q for electronics experts
Can anyone answer these questions?

1) How to replace an open-collector chip with an
eqivalent tri-state chip.

2) How to replace a Tri-state chip with an
equivalent open-collector chip.

e.g. If I wanted to replace a bad open-collector 7489 RAM
and I only had tri-state 74189 RAMs available what would
I have to do? Would I need to remove the pull-up resistor
pack or can I leave it in place? etc.

Thanks,
Gary



DATA Imported from archives: originally posted by shepg2002 (lists@ntlworld.com)

 

[LukeWells]

Re: [ukvac] Tech Q for electronics experts
I cant offer an explaination to if and how this can be done, but a Tri-State
chip is able to discconnect itself from the databus (go open circuit on the
data lines) and an open collector chip is one that can drive a load such as
an LED blah blah blah.

As the circuit that uses tri-state ram expect the ram to be able to
disconnect from the databus when its not being used this might be an
important factor and why an OC ram couldnt be used in its place

Luke

> Can anyone answer these questions?
>
> 1) How to replace an open-collector chip with an
> eqivalent tri-state chip.
>
> 2) How to replace a Tri-state chip with an
> equivalent open-collector chip.
>
> e.g. If I wanted to replace a bad open-collector 7489 RAM
> and I only had tri-state 74189 RAMs available what would
> I have to do? Would I need to remove the pull-up resistor
> pack or can I leave it in place? etc.
>
> Thanks,
> Gary
>
>
>
>
>
> Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
>
>
>

 

[Macro]

RE: [ukvac] Tech Q for electronics experts
on something like the sprite ram on galaxian, you can usually use either -
but you may have to modify the pull up resistors to match - on various
boards I have had to remove them, fit them, or drastically change the value
of them!

Presumably, the chip can only detach itself from the bus when it is does not
have the output enable in the correct state - in many boards this is
hardwired anyway, so in these cases both should work (depending on
driveability again)

basically, bung it in and see if it works - it really does depend upon the
circuit around it and what it is driving.

AFAIK 74189's will work in the pacman circuit, I'm sure I had a bootleg with
some on!

 

[John Bennett]

Re: [ukvac] Tech Q for electronics experts
> 1) How to replace an open-collector chip with an
> eqivalent tri-state chip.

You should be able to drop one straight in. The open collector version floats it's output when either not selected or when enabled + logic 1. Enabled + Logic 0 results in the output being pulled low. The tri-state version will pull the output high so probably won't need the pullup resistor that is typically present on the output. The necessity for the pullup resistor depends on whether the circuit expects the output to be high if the chip is ever deselected.

> 2) How to replace a Tri-state chip with an
> equivalent open-collector chip.

If the chip doesn't have a permanently wired enable line and / or it's outputs are wired to a bus, you probably can't replace a tri-state with an open collector. If the chip is permanently enabled, you can normally use an o/c with a pullup on the outputs - in this case the third (floating) state of the tri-state chip is never used since the chip is permanently enabled.

I guess ultimately the answer is, it depends on the circuit.

Steve

__________________________________________________________________________
Join Freeserve http://www.freeserve.com/time/

Winner of the 2003 Internet Service Providers' Association awards for Best Unmetered ISP and Best Consumer Application.



DATA Imported from archives: originally posted by (steve@coule.freeserve.co.uk)

 

[John Bennett]

[ukvac] Re: Tech Q for electronics experts

Thanks for the replies. Very helpful.

Cheers,
Gary.



DATA Imported from archives: originally posted by shepg2002 (lists@ntlworld.com)

 

[eggwheatis]

Re: [ukvac] Tech Q for electronics experts
Hi, as far as the 7489 goes ive replaced em with 74189
and 74289 with no mods and found no problems.

Phil.

--- shepg2002 <lists@ntlworld.com> wrote:
> Can anyone answer these questions?
>
> 1) How to replace an open-collector chip with an
> eqivalent tri-state chip.
>
> 2) How to replace a Tri-state chip with an
> equivalent open-collector chip.
>
> e.g. If I wanted to replace a bad open-collector
> 7489 RAM
> and I only had tri-state 74189 RAMs available what
> would
> I have to do? Would I need to remove the pull-up
> resistor
> pack or can I leave it in place? etc.
>
> Thanks,
> Gary
>
>

__________________________________________________
Do you Yahoo!?
The New Yahoo! Search - Faster. Easier. Bingo
http://search.yahoo.com