PDP-11 Memory

[Jerry Weiss]

On 1/12/19 11:31 AM, Bill Gunshannon via cctalk wrote:
> On 1/12/19 9:20 AM, Bill Gunshannon via cctalk wrote:
>>
>> Well, the problem is definitely bus grant as with just two memory
>> cards in the machine I put the disk controller in and it does not
>> show up in the Map.  The only problem with this is the question
>> Why?  I  have examined the memory cards.  The C-D (actually, C)
>> does have the bus grant lines connected.  I even looked under a
>> magnifier to be sure the trace hadn't been cut and it is intact.
>> So, what else cold cause this?
>>
>> (Another question, however, some cards have the ability to open
>> this.  Why would you ever do that as it would break pretty much
>> any card that followed?)
> Problem fixed.  Note, fixed, not solved.
>
> I dug out another MicroPDP BA23 box.  Stuck the CPU in slot 1.
> Started adding the memory cards one at a time and turning the
> machine on  waiting for the self-test and then doing a MAP.
> Worked perfectly.  The self test got slower as each memory
> card was added and the map showed the block of memory and the
> assigned CSR.
Congrats on getting this part of the problem resolved.
> So, what could go bad on a backplane to cause this problem?
> Or, could it be a power supply that is no longer delivering
> the rated power?

If you haven't already, check the backplane slots for debris or corrosion on the contacts.

Next with a voltmeter check the +5V and +12V first to see if the actual voltages are within spec.  If you have an oscilloscope, checking for excess voltage ripple.  If the power supplies have are using the original capacitors they might not be providing clean power under significant load.

   Jerry