Atari ST MMU

Troubles with your machine? Just want to speak about the latest improvements? This is the place!

Moderators: Mug UK, Zorro 2, Greenious, spiny, Moderator Team

TeX38
Atari User
Atari User
Posts: 43
Joined: Fri Aug 19, 2016 11:49 am

Atari ST MMU

Postby TeX38 » Thu Feb 07, 2019 9:38 am

Hi,
My Atari has failed.
Checking around with my CRO shows the expected clocks etc, the Data is there on reset and there is activity on the address lines.
RAS is active but CAS is high, no activity at all.

When there is a problem with an ST, does the CAS signal get stopped or have I got a faulty MMU? (C025912)

Who is selling these MMU chips these days?

Thanks

Terry

User avatar
Greenious
Hardware Guru
Hardware Guru
Posts: 1448
Joined: Sat Apr 24, 2004 5:39 pm
Location: Sweden

Re: Atari ST MMU

Postby Greenious » Thu Feb 07, 2019 10:05 am

https://www.dev-docs.org/docs/ download the Mega ST service manual, look for a section called "troubleshooting a dead unit"

Atari IC's can be had from various places, this sticky post in the sales forum has a list of a few.
viewtopic.php?f=33&t=11092

In the sales forum there are also posts from people selling spares aswell, although some a buried deep down among the posts, but many still got IC's for sale.
Updated my guides as of june 28th, 2016. Check'em out and feedback!
viewtopic.php?t=5040

User avatar
Arne
Atari Super Hero
Atari Super Hero
Posts: 512
Joined: Thu Nov 01, 2007 10:01 am

Re: Atari ST MMU

Postby Arne » Thu Feb 07, 2019 10:24 am

TeX38 wrote:When there is a problem with an ST, does the CAS signal get stopped or have I got a faulty MMU? (C025912)

No /CAS on upper or lower bank? Just verified it and I see that there is /RAS activity at boot time on the upper bank but no /CAS activity. On lower bank /RAS and /CAS are active.
Image

TeX38
Atari User
Atari User
Posts: 43
Joined: Fri Aug 19, 2016 11:49 am

Re: Atari ST MMU

Postby TeX38 » Fri Feb 08, 2019 3:15 am

Thank you for ur replies..
I wen back and re checked..

RAS0 is active all the time.
CAS0 H/L = INTERMITTENT waveform
So i suggest the MMU is ok, maybe a ram problem.

TeX38
Atari User
Atari User
Posts: 43
Joined: Fri Aug 19, 2016 11:49 am

Re: Atari ST MMU

Postby TeX38 » Fri Feb 08, 2019 5:36 am

Thanks Greenious,
I have a STF(M) service manusl but not the mega one, I'll look i to it.
Will a faulty SHIFFTER also stop the ST from booting?
Problem with the ST is their being so INTERGRATED, makes is so much harder to troubleshoot.
I guess a diag cart would be helpful but i dont have 1.
My ST is not starting but i do get a screen, Black with white border.
Screen shows random colours then clears to black on reset.


Social Media

     

Return to “Hardware”

Who is online

Users browsing this forum: tcat and 4 guests