Differences between CMOS and TTL EPROMs

Boris Gjenero

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Sep 6, 1996, 3:00:00 AM9/6/96

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Hello,

I have an EPROM programmer that can read and program TTL EPROMs, but is
totally unreliable and useless at reading or programming CMOS EPROMs.

(It's the MOD-EPROM unit from JDR Microdevices. They certainly ripped
me off with that thing. Not only does it have this problem, but it also
requires actual IBM-AT speed to work properly. Fortunately, I can solve
the speed problem with a program I wrote that puts the CPU into single
step mode.)

Anyways, now I need to use the thing on CMOS EPROMs. I need to know
what are the significant electrical differences between TTL EPROMs such
as the 27128 and CMOS EPROMs such as 27C128. What do I need to do to
make the CMOS ones behave more like TTL? I'm thinking I probably need
pull up resistors or something like that.

--
| Boris Gjenero <bgje...@undergrad.math.uwaterloo.ca> |
| Home page: http://www.undergrad.math.uwaterloo.ca/~bgjenero/ |
| "Luke, you're going to find that many of the truths we cling to |
| depend greatly on our own point of view." - Obi-Wan Kenobi, ROTJ |

Ed Paolo

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Sep 7, 1996, 3:00:00 AM9/7/96

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Boris Gjenero (bgje...@mnsi.net) wrote:
: Hello,

: I have an EPROM programmer that can read and program TTL EPROMs, but is
: totally unreliable and useless at reading or programming CMOS EPROMs.

: (It's the MOD-EPROM unit from JDR Microdevices. They certainly ripped
: me off with that thing. Not only does it have this problem, but it also
: requires actual IBM-AT speed to work properly. Fortunately, I can solve
: the speed problem with a program I wrote that puts the CPU into single
: step mode.)

: Anyways, now I need to use the thing on CMOS EPROMs. I need to know
: what are the significant electrical differences between TTL EPROMs such
: as the 27128 and CMOS EPROMs such as 27C128. What do I need to do to
: make the CMOS ones behave more like TTL? I'm thinking I probably need
: pull up resistors or something like that.

The only problem I have ever seen in reading eproms is that there
are the totem-pole and open collector eproms. The totem-pole have internal
pull-ups and the open collector have external pull-ups {from the circuit}.
You would then have to add pull-up resistors {about 1000 ohms}, on the
data and address lines.

EdDataFix
edp...@intac.com

BECMan

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Sep 11, 1996, 3:00:00 AM9/11/96

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Ed Paolo wrote:

> major snipThe cmos eproms do not have the same output drive capability as the ttl. Also
I think the programming voltage is 12.5V instead of 27V.

Francisco A. Shi

Bas van Rossem

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Sep 13, 1996, 3:00:00 AM9/13/96

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Boris Gjenero <bgje...@mnsi.net> wrote:

>Hello,

>I have an EPROM programmer that can read and program TTL EPROMs, but is
>totally unreliable and useless at reading or programming CMOS EPROMs.

<snip>

>Anyways, now I need to use the thing on CMOS EPROMs. I need to know
>what are the significant electrical differences between TTL EPROMs such
>as the 27128 and CMOS EPROMs such as 27C128. What do I need to do to
>make the CMOS ones behave more like TTL? I'm thinking I probably need
>pull up resistors or something like that.

The NMOS types mostly use 27V or 21V, most CMOS types use 12.5V (some
use 21V).

Regards,

Bas

Mel Tsai

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Sep 17, 1996, 3:00:00 AM9/17/96

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On Fri, 13 Sep 1996 08:42:35 GMT, kcs...@icns.nl (Bas van Rossem)
wrote:

It could also be that the programmer doesn't pull high enough for the
CMOS EPROM to read. A TTL input requires 2.7 volts (or something like
that, I can't remember exactly) for it to recognize a logic "1". A
CMOS input, however, requires about 3.4 volts for it to recognize a
logic "1". You would therefore need to put some in-line pull up
resistors on each input so that the programmer outputs sufficient
voltages. Get a digital electronics book and read the chapter on
"interfacing logic families" or something like that.

Of course, I could be totally wrong but that's just what I believe!

-Mel