Conference hydra::amiga_v1

2119.0. "Technical Question" by DPDMAI::ANDERSONA () Mon Jan 16 1989 17:43

    I got some questions for one of you guys that has dug deep into the 
    Amiga.  What type of signals should I see on the 86 pin expansion bus?
    
    I have been having a problem with my Hard disk that up till now I 
    assumed was in the Disk drive and or the Adaptec.  I have proven that 
    it is not the drive and am in the process or ordering a new Adaptec.  
    But while messing around I took a scope to the 86 pin bus and I found 
    one signal that does not look right.  Most of the bus was at TTL levels 
    with peak to peak signals of 4 to 4.5 volts.  This one signal is on pin 
    68 the PRW* signal and only rises to 2.5 volts peak to peak.  I was 
    able to trace it back to Fat Agnus pin 22.  It also goes to one of the 
    PALS on the C.Ltd SCSI500.  If I remove this PAL from its socket then 
    the signal returns to the normal 4.5 volts peak to peak. I have called 
    C. Ltd and their technical people didn't know what their equipment was 
    using this signal for or why their PAL would be clamping it down to 2.5 
    volts.  They were not sure if this was normal or not.  In short they 
    were no help at all.
    
    My questions to you guys is what is this signal used for?  My schematic 
    that came with the Amiga is blurred and half unreadable.  So is this 
    signal used elsewhere in the Amiga?  If the signal is used elsewhere 
    then I would assume the C.Ltd PAL is loading the signal down.  But if 
    the signal goes straight from  Fat Agnus to the 86 pin bus then it is 
    possible that Fat Agnus is sick.  I tried the reseat trick it did not 
    affect it.  I also scoped the signal on Fat Agnus's pin directly.  I 
    can always tack together a Schottky buffer between pin 68 and the PAL a 
    74LS241 should do it.  This would be an attempt to isolate Fat Agnus's 
    pin 22 from the PAL pin 11.  I am assuming Fat Agnus is the source of 
    the signal am I right in this?  Or is this signal normally at 2.5 volts 
    with a device on the expansion bus and I am worried about nothing.
    
    Other than the problem I am having with my hard disk which only shows 
    up after Ami has been off for a few hours.  Everything else has been 
    running normally.  Just the normal Guru's that can be attributed to a 
    loose nut on the keyboard or running out of resources etc etc.
    
    
    Thanks in advance for any insight you can bring.
    
    Alan
    

    Sorry it took so long to respond but I haven't read notes in two
    weeks!
    
    Anyway, PRW* is the R_W signal shared by the fat lady, little gary
    and the 68000. Actually this signal is gated by a transiever buffer.
    R_W as you can imagine is a read/write signal. Control of this signal
    is determined by who is taking over the buss. So, if you take out
    the pal and the level goes back to normal it can only be the pal
    thats loading it down. You can replace it by getting a good one
    from someone else (out of the same controller and rev), take it
    to ACTIVE Electronics (or some other place that programs pals).
    They will read in the good one and blast a new one with the same
    pattern.
    
    Hope this helps!
    
    			Tom
    

    Is it gated just for the expansion bus?  I scoped it at Fat Agnus
    pin 22 and the expansion bus.  It was 2.5V at both places.  Any
    ways my C.Ltd controller is still under warenty for another few
    months.  So I will try and call them again.  The new Adaptec fixed
    my problem.  But I am worried that this degraded signal will prevent
    me from adding on to the expanaion bus in the future.
    
    Do you know of anybody that sell prints that are readable?  I am
    going snowblind with the ones that came with the Amiga.
                                                              
    Thanks for your help
    Alan