Image Map Image Map
Page 13 of 14 FirstFirst ... 391011121314 LastLast
Results 121 to 130 of 135

Thread: Frustrating Problem

  1. #121
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    33,403
    Blog Entries
    18

    Default

    Do you combine DACK and TC before you feed it to the TC pin of the 8272?

  2. #122
    Join Date
    Dec 2013
    Location
    Near Milwaukee Wisconsin
    Posts
    1,242

    Default

    No.... The DMA DACK0 8237 Pin 25, goes directly to the 8272 Pin 15. AND the DMA EOP 8237 Pin 36 is inverted and applied to the 8272 Pin 16. Why would you need to combine these signals? Mike

  3. #123
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    33,403
    Blog Entries
    18

    Default

    Don't know, but IBM evidently thought it prudent to do so. (cf. 5160 Tech manual--the XT "ANDs" the two lines.

    cf. Techref V.2, page D-57. DACK2/ is inverted and ANDed with TC at U14 (74LS08), thence to pin 16 of the 765.

    My guess is that the thought was that more than one DMA channel can be active at the same time, so you don't want a TC intended for a different channel messing things up with your transfer.

    However, I don't know if you use DMA for anything else on your system. On the PC and XT, it's certainly used for the hard drive as well--and of course, DRAM refresh.
    Last edited by Chuck(G); January 15th, 2020 at 02:23 PM.

  4. #124
    Join Date
    Dec 2013
    Location
    Near Milwaukee Wisconsin
    Posts
    1,242

    Default

    No, the FDC is the only interrupt I have. And since I'm so much luck using them, I don't think I'll be converting any of my polled deviced anytime soon. Today, I want to try and clean the s-100 connectors on my mother board. Maybe there is some problem there. Mike

  5. #125

    Default

    Quote Originally Posted by Mike_Z View Post
    No, the FDC is the only interrupt I have. And since I'm so much luck using them, I don't think I'll be converting any of my polled deviced anytime soon. Today, I want to try and clean the s-100 connectors on my mother board. Maybe there is some problem there. Mike
    Check your regulators under full load. I had a N* that with no load, the 12V line looked fine. Once I had both drive starting, the 12V line would drop and kill the DRAM. They'd used two regulators in parallel but one was doing nothing and the other was current limiting before the rated value.
    Dwight

  6. #126
    Join Date
    Dec 2013
    Location
    Near Milwaukee Wisconsin
    Posts
    1,242

    Default

    I believe I looked at that a while back, but will check it again.Thanks, Dwight for the tip. Right now I a little concerned about the S-100 bus connectors on my mother board. I've cleaned them with DeOX. I used a 0.042" drill bit to check the spacing between the front and back contacts. There is drag on them. My boards are about 0.070" thick and the contacts are gold plated. Since my PIC is on a different board than the FDC, the INT signal has to take the bus. I wonder if that could be the problem or maybe another signal. So right now the mother board is out of the box for a good inspection. Mike

  7. #127

    Default

    If you find a connector pin that is bad, depending on the type, these can be replaced. If you have a donor connector, most will push out from the bottom. To put one back in is a little tricky. I put a credit card in the connector in place of the board. I then slide the pin into the socket. If you just try to put one in without the card, it won't go in properly.
    Have you tried running at different voltages? Some times it is a weak capacitor or a open diode. It will mostly work but as things get hot and draw a little more power, the rail dips to low to keep the regulators happy ( some of the older ones may even need more than 2V headroom. Use a variac to drop the line a little.
    Dwight

  8. #128
    Join Date
    Jan 2007
    Location
    Pacific Northwest, USA
    Posts
    33,403
    Blog Entries
    18

    Default

    Note that pins on some edge connectors are held in place by a little deformation or "tang" on the bottom, securing them in place. That needs to be flattened before you start pushing.

  9. #129

    Default

    Quote Originally Posted by Chuck(G) View Post
    Note that pins on some edge connectors are held in place by a little deformation or "tang" on the bottom, securing them in place. That needs to be flattened before you start pushing.
    Oops, I forgot about that.
    Dwight

  10. #130

    Default

    I have found that the edge connectors on the old S-100 cards and the vintage ones on IBM style ISA cards have been super reliable. One reason I think is because they are sliding contacts, it cleans the gold surfaces very well. So provided there is reasonable spring tension, the connections should be ok, mostly.

    The problem with gold-gold contacts I have found is when there are no sliding or scraping forces, just touching, especially when the voltages are low (5V systems or less) and the signal currents low. The gold acquires a near invisible surface insulating layer over time. It can look perfectly clean to the naked eye though.

    The worst cases occur in things like attenuators in Tek scopes or in many (not all) designs of old DIP switches. Many of the DIP switches in my SOL-20 and those on PT's memory cards were defective and open circuit when switched on. I replace them with a DIL machine pin socket and low profile high quality Omron DIP switches, so if required one day they can be replaced again easily.

    One of the worst cases of bad socket issues was in my 2004 vintage HP computer that runs an Athlon 64. The CPU socket is a ZIF type gold plated contacts, all very convenient. But they only touch (approximate) the gold plated CPU pins when the socket arm closes. All works fine when everything is new. It took me weeks to finally find that my problems were coming from poor connections here. The quickest way (after cleaning it ) that solved the problem, was to incompletely insert the CPU to within about a mm of it being seated, just partially close the ZIF socket's locking arm and push the IC home, which caused scraping of the contact surfaces between the socket pins & the IC pins and solved the problem.

    This is why some people find if they re-seat the CPU, a fault can be cleared. But it will come back unless a little more work is done.
    Last edited by Hugo Holden; January 17th, 2020 at 03:23 PM.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •