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Author Topic: Enterprise 64 with EXOS 2.4 reports random memory errors [Fixed!] (Read 973 times)

Offline Quickie

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Hi,

First of all let me thank you joining this community.
I am a happy owner of an Enterprise 128 which has been working bulletproof until now (fingers crossed).

As it didn't come with any software I decided to build a floppy controller based on the WD1772 + EXDOS 1.4 and give it a try with a Gotek device. It proved to work fine and for the first time I experienced what EP hardware was capable of :)

Having said this I decided to give this FDC to a colleague who has an EP64. He told me that on his EP64 the FDC didn't work as it caused crashes and green screen hanging when booting.

My colleague send me his EP64 and those are all the actions I tried to understand what was going on / fixing it:

- Updated with EXOS 2.4 eprom W27C512
- When the EP64 boots without the FDC it shows briefly the EXOS 2.4 screen and boots to the "ENTERPISE" screen
- When I performed an advanced memory test on EXOS 2.4, it reported random memory errors: BAD 0016Kb, BAD 0064Kb...
So I decided to change the 4164 RAM ICs on U10 and U17 for NOS memory of the same vendor and speed. No luck: the error messages kept appearing randomly on the same locations.
- I have made a 1536Kb RAM expansion. I installed it on the EP64 to check how it behaved, knowing in forehand that works fine. Well, with this computer it keeps giving me errors on different RAM memory locations with the Advanced test: BAD 0080Kb, BAD 0192Kb...

I gave a last try to check the 74F157 multiplexors. I removed them and tested on a MiniPro. They passed the test. Swapped them on the EP64 for new 74LS157 (just to test, I know the F series are faster) and behaved the same way, so I kept de F157s.

Finally I tried to start the EP64 with the FDC and the memory expansion and the computer got again stuck on the boot with some green screen, garbage on border (like a tape loading pattern) or some random "@" here and there.

I am expecting the worst (having a defective Dave custom IC), but before considering this worst case scenario I wanted some advice from more versed colleagues on the Enterprise.

Thanks
« Last Edit: 2022.May.25. 18:05:42 by Quickie »

Offline Zozosoft

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #1 on: 2022.May.25. 11:48:58 »
Try to add 4.7K Pull-up resistors (to +5V) on Data lines. Most easy install at U7.

Offline Quickie

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #2 on: 2022.May.25. 12:35:58 »
Hi Zozo,

Thanks a lot for your swift reply.
Ok, I will give it a try and made the pullups with some 4.7k resistors I do have.

Have you ever faced this problem in the past?

P.S.: I am going to give it a try with this hacky resistor pack:



Update: Ok, for the first time the EP64 is able to go further the EXOS 2.4 memory test with the FDC plugged in.
On the first try it quickly passed the test and marked all 1600Kb as good and afterwards I was presented with an EXOS 2.4 black screen with new memory tests. After a while an error appeared and the EXOS 2.4 text transformed to @POS 2.4:



I tried to boot more times by resetting the EP64, no luck. On the next memory tests random memory adresses arise:



After that, the usual green screen with garbage :(
« Last Edit: 2022.May.25. 14:03:46 by Quickie »

Offline Zozosoft

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #3 on: 2022.May.25. 14:22:21 »
Last hint: try another Z80, preferably new CMOS version.

Offline Quickie

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #4 on: 2022.May.25. 15:30:14 »
¡That was it! Even if there were no evidences from being manufactured in Singapore (@gflorez gave me the hint) I had previously checked the M1 line and was healthy...
However I have followed your advise and changed this Zilog Z80A for a Sharp Z80B...and for the first time ever the EP64 booted to IS-Basic, with the FDC controller connected and the memory expansion properly recognized!



Thanks a lot!!


Offline Zozosoft

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #5 on: 2022.May.25. 15:49:46 »
Can you make photos of the faulty Z80?

Offline Quickie

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #6 on: 2022.May.25. 16:08:50 »
Sure,

Here is the culprit Zilog Z80:


Offline Zozosoft

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Re: Enterprise 64 with EXOS 2.4 reports random memory errors
« Reply #7 on: 2022.May.25. 16:17:55 »
I have many similars in bad CPU bin...

29130-0
29128-1

Offline tofro

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The stickers to the left should be removed ;)

Offline Zozosoft

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The stickers to the left should be removed ;)
:smt043

Offline gflorez

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Funny!

I have one with the Q.C. sticker at the right.... The one from my first EP. It was problematic with a Microteam EXDOS card connected, hangs and random errors on Zozo's Ram test.

29132-0

The bottom one is also made in Philippines, removed from an Amstrad CPC. The chance was to put a finger over it, as it gets hot very fast and hangs.

The problem with these faulty Z80s is that they are not completely broken.

Offline Quickie

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Yeah, indeed.

They seem to present an erratic behavior on the data bus, or at least the unit I had to deal with. On a first thought I did check the M1 signal (being aware of problems with Zilog Z80s present on some Sinclair Spectrums), but this showed a healthy behavior.

Offline gflorez

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Sinclair saving money with defective components... but thanks to him a lot of people started with computers.