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to pull or not to pull up

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nero_xxx

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i am doing an eprom(2732 only) programmer for the primitive 8085 kit.Interfacing is being done thru PPI 8255.Port B and aan 8 bit latch are being used to establish the address sent from 8085 kit by the software.Port c is being is used for controlling the signal....latch...oe/vpp....latch....ce..etc.Port a is used for reading data from eprom and is set to i/p mode for that.it is also used for supplying data to the eprom ,port a is set to O/p mode for that.
I rigged up the circuit......... tested signal.....latching addressing.etc .......all that is working fine .the only glitch is my eprom reads random data for same memory location....so i am not sure of the writing either as i have no clue if what i have written is rite or rong.I have'nt used any pullups for the data lines ......could that be the problem....or something else.I have often seen pullups on data lines of eproms . could some one tell me if i need them too. and also i was wondering if i have 10k pullups will it affect the logic present at a certain pin .i have a multisim file .....i could pass the file..if u would like to help.

thanx
 

nero_xxx

New Member
nobody knows something about pullup or pulldowns?

c'mon.......there's got to be someone here knows something about pullups.
i have heard that most memories or open collector ,since it allows them to be wire ored.Also i have a doubt if it'll hurt to have pullup of 10k.......will it effect the logic present on the bus.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: nobody knows something about pullup or pulldowns?

nero_xxx said:
c'mon.......there's got to be someone here knows something about pullups.

It's you who is designing the circuit, you who knows what chips are connected to what chips - how can we know if you need pullups or not?.

Simple answer - you need pullups if you are using open-collector chips, if not, you don't!.

As far as I'm aware, EPROM's are not open-collector, open-collector logic chips are clearly marked as such in the specs.
 

nero_xxx

New Member
some parallel port interfaces require pullups

thanks for that piece of info.I have heard that most parallel interfaces require either pullup or pulldown regardless of what they are attached to.
my question is: will adding pullups on data bus affect the logic that is already existing,considering the use of 10k values for pullups.
 

motion

New Member
Go ahead and try to put pull-up resistors.

Also try removing the eprom and check if you are still getting random readings. Are they really random or some bits are being affected and not all.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: some parallel port interfaces require pullups

nero_xxx said:
thanks for that piece of info.I have heard that most parallel interfaces require either pullup or pulldown regardless of what they are attached to.
my question is: will adding pullups on data bus affect the logic that is already existing,considering the use of 10k values for pullups.

Adding pullups when not needed won't cause any problems, it will just waste current and components.

I don't know where you've heard that most ports require resistors?, try looking at PIC programmer designs (there's some on my website at https://www.winpicprog.co.uk), or any other designs using the parallel port. You will notice that the P16PRO40 doesn't use pullups, except for the line feeding back into the port (Data In), this requires a pullup because the gates used (74LS07) are open collector.
 

nero_xxx

New Member
pics usually have internal pullups

Thanx for the info.usually pics have their own intenal pullups..as for the eprom i could find much info ,except the info that most eprom programmers have either pullups or pulldowns .As a matter of fact i came across a design which makes use of pulldowns on data..the author justifies by explain some problem with parallel interface;he also adds that use some brands of eproms may require pullups in place of pulldowns.
wonder if u have come across any eprom pgmr design with pullups or pulldowns.take a look at attached file.
 

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Nigel Goodwin

Super Moderator
Most Helpful Member
Re: pics usually have internal pullups

nero_xxx said:
Thanx for the info.usually pics have their own intenal pullups

But the PIC doesn't connect to the port! - the 7407 does!.

Also, PIC's only have weak pullups (software activated, not present unless the PIC is running and they are activated) on certain pins, usually just PortB. So it's not relevent at all.

It's no good chasing round trying to find a circuit that conforms to what you want, as you quite rightly say - some people add pullups, some add pull downs, some add nothing. I've previously listed the only pins which require pullups, anything else is just down to the personal preference of whoever designed it.

If you're building your own programmer, and using a PCB, leave room for either pullups and pull downs if you wish, then you can try it with or without them.
 

nero_xxx

New Member
wonder if its just a preference

thanx.But no chasing around work............i said that i came across a few circuit,and not just one ,a couple of them.
if it were an option or prefernce; the author of that circuit would not have asked us to switch from pulldown to pullup.If that were a matter of just preference.....the author would not want us to do that extra bit of work just for preference.there's goto to be more to it.since data is being read randomly in my case ......i think i should try the pullups.what say u?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: wonder if its just a preference

nero_xxx said:
thanx.But no chasing around work............i said that i came across a few circuit,and not just one ,a couple of them.
if it were an option or prefernce; the author of that circuit would not have asked us to switch from pulldown to pullup.If that were a matter of just preference.....the author would not want us to do that extra bit of work just for preference.there's goto to be more to it.since data is being read randomly in my case ......i think i should try the pullups.what say u?

Try them and see, it won't do any harm!.
 
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