EMRFD Message Archive 7578

Message Date From Subject
7578 2012-07-12 12:57:20 Paul Anderson Substitution for 7400?
This is a little off-topic. I have an old computer trainer that is basically a rudimentary processor built from TTL chips. I'm trying to get it working, but it is missing some 7400 TTL chips.

There's one missing from the memory, one missing from the time pulse distributor, and one missing from the zero-detect logic on the accumulator. Another one was missing from the program counter logic, but I had a 7400 in my junk box. That at least got the PC register incrementing properly...

Right now, the machine doesn't seem to execute anything. There's an exec bit and a run bit. There are three clock options, a 250kHz fast clock, 2Hz slow clock, and manual single-stepping. Running on the slow clock, the run light turns on immediately but it takes several clock cycles before the PC increments from 0000 to 0001. There are to clock cycles involved in executing most instructions, a fetch and an execute cycle. Exec indicates that it is in the execute cycle. After the PC begins incrementing, the exec light will come on and stay on across several addresses in the PC. The instruction register seems to be displaying data at random. The first 16 bytes of memory(of a total 32) are toggle switches on the front panel. The setting of these seems to have no affect.

My main question right now is if anyone knows which other 74 series chips can be dropped in as replacements for the 7400, hopefully I will have a few in my junk box. Any thoughts are greatly appreciated.

Sent from my iPad
7579 2012-07-12 13:14:22 Johan Bodin Re: Substitution for 7400?
Paul,

the 74HCT series should do the job. 74HC and 74HCT differ in input
threshold voltage. The 74HC input threshold is about mid-supply while
74HCT input threshold is about 1.5V which is compatible with the outputs
of old TTL and LS TTL logic chips. 74HC and 74HCT both swing
rail-to-rail at their outputs and will easily drive old TTL/LSTTL inputs.

73
Johan SM6LKM

Sent from my $199 Samsung netbook

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7580 2012-07-12 14:38:00 Dave Re: Substitution for 7400?
Paul:
Jameco still lists the SN7400N.

Dave - WB6DHW
<http://wb6dhw.com>

7585 2012-07-13 19:01:40 Guy Re: Substitution for 7400?
Jameco has a replacement for ~1.29. See:
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_48979_-1
http://www.jameco.com/Jameco/Products/ProdDS/910901.pdf
This sounds like fun! I had an Altair 8080.. and many others... :-)

Guy (AI1G)

7586 2012-07-13 19:01:50 David Newkirk Re: Substitution for 7400?
New 7400s are available from Digi-Key (http://www.digikey.com) from
multiple manufacturers; here's the link to the TI listing:
http://www.digikey.com/product-detail/en/SN7400N/296-14641-5-ND/555975 DK's
service is excellent.

How many in the group know how Digi-Key got its name?

7587 2012-07-13 19:01:58 Russ Ramirez Re: Substitution for 7400?
Paul, the only one I could come up with was 7403, which is an
open-collector version of 7400, which should work with some pull-up
resistors. There are numerous other series which have quad NAND gates like
4011, but I think you're looking for a true plug replacement that came
along later with a higher number in the line. Where's that darn TTL
Databook...

Russ


[Non-text portions of this message have been removed]
7589 2012-07-14 02:40:27 Dan Mills Re: Substitution for 7400?
On Thu, 2012-07-12 at 20:07 -0500, Russ Ramirez wrote:
> Paul, the only one I could come up with was 7403, which is an
> open-collector version of 7400, which should work with some pull-up
> resistors.

How about going to the 74HCT00?

Same logic levels, (that is what the T means) but lower power
consumption and stiffer output drive.

73 Dan.
7590 2012-07-14 06:11:13 popmedkoh Re: Substitution for 7400?
7591 2012-07-14 08:08:19 Paul Anderson Re: Substitution for 7400?
Ideally. Push comes to shove, I'll have to wait for next month and order some.

Sent from my iPad

On 2012-07-12, at 9:07 PM, Russ Ramirez <russ.ramirez@gmail.com> wrote:

> Paul, the only one I could come up with was 7403, which is an
> open-collector version of 7400, which should work with some pull-up
> resistors. There are numerous other series which have quad NAND gates like
> 4011, but I think you're looking for a true plug replacement that came
> along later with a higher number in the line. Where's that darn TTL
> Databook...
>
> Russ
>
>
> [Non-text portions of this message have been removed]
>
>
>
> ------------------------------------
>
> Yahoo! Groups Links
>
>
>
7602 2012-07-15 06:18:22 kb1gmx Re: Substitution for 7400?
7613 2012-07-16 16:10:14 Paul Anderson Re: Substitution for 7400?
It's an NRI model 832. Here are some pictures of a different one of the same model:

http://www.wensy.com/detail.asp?id=8485631&bigpic=0#img

It has 32 bytes of memory, the first 16 are toggle switches.

Sent from my iPad

7614 2012-07-16 19:28:20 Paul Anderson Re: Substitution for 7400?
I made some progress. I finally found schematics, which you can see here:

http://www.robsons.org.uk/nri832/schematic.pdf

I managed to cannibalize IC32(on the C2 board), replacing the gate used to drive the PC counter with an '02 on a breadboard. From my reading, this should just disable skip instructions.

I moved IC32 to fill IC8's socket. IC8 is responsible for generating most of the timing pulses. At present, the PC increments, although the machine doesn't seem to be loading any instructions. Halt instructions are ignored.

Sent from my iPad

7617 2012-07-16 21:20:11 Russ Ramirez Re: Substitution for 7400?
Wow, that brings back memories. Very few people understand computers at
this level any more, which is fine I guess, but this was the level logic
designers at IBM (I was there 21 yrs) worked at still in the early 80's,
except that they expressed the logic as software really, using something
called VHDL. Glad you found the schematics and thanks for sharing.

Russ
K0WFS

7626 2012-07-22 16:57:56 w0gk Re: Substitution for 7400?
7627 2012-07-23 16:08:36 deag4rj@att.net Re: Substitution for 7400?
Hi Paul,
I don't know if this is the one your looking for but I have some DM7400N 14 pin DIP IC chips. I've got no idea what they are I haven't looked up the data sheets. I also have ITT7402J,74LS02N, MMC7475P, DN7474N, 74L500, 74L505, 74L502, 74L574, 74L520, 74L530, SN7401N, 74LS161AN, 74LS04N, US74H10A, AND MANY MORE i HAVEN'T LISTED. Let me know if one of these will help you or give me the exact number you are looking for and I'll see if I have it.
73,
Russ AG4RJ (good
7630 2012-07-23 16:31:44 Paul Anderson Re: Substitution for 7400?
Thanks everyone! The generosity of the group has been tremendous. I've got the chips I need. It's greatly appreciated.

Right now I have to deal with a ripple problem. I've got 0.4v p-p of ripple on the +5v supply. The oscillator responsible for the clock isn't running reliably either. It uses an emitter follower with a 5.6v Zener diode, full-wave rectifier(in the form of two diodes and a center tap on the transformer). A 4,000uF electrolytic cap provides filtering, I think it probably needs to be replaced.

Sent from my iPad

On 2012-07-23, at 9:59 AM, "deag4rj@att.net" <deag4rj@att.net> wrote:

> Hi Paul,
> I don't know if this is the one your looking for but I have some DM7400N 14 pin DIP IC chips. I've got no idea what they are I haven't looked up the data sheets. I also have ITT7402J,74LS02N, MMC7475P, DN7474N, 74L500, 74L505, 74L502, 74L574, 74L520, 74L530, SN7401N, 74LS161AN, 74LS04N, US74H10A, AND MANY MORE i HAVEN'T LISTED. Let me know if one of these will help you or give me the exact number you are looking for and I'll see if I have it.
> 73,
> Russ AG4RJ (good