PDP-11 Memory

[Pete Turnbull]

On 12/01/2019 17:39, Bill Gunshannon via cctalk wrote:

> One confusing part of this.  Everything I have read today
> seems to say that the only difference between the 11/73
> and the 11/83 in a MicroPDP-11 was the memory used.
> QBUS = 11/73
> PMI = 11/83
> 
> Is this correct or not?

No, not really, but sort of.  Originally, the J11 chip was available in 
15MHz and 18MHz versions.  The 15MHz chips were used for 11/73 systems 
and the 18MHz ones for 11/83 systesm.  To emphasise the difference, they 
had slightly different boot ROMs, and the 11/73 systems were shipped 
with QBus memory while the 11/83 systems were shipped with PMI memory, 
which is faster.  So a factory-fresh 11/73 was always 15MHZ + QBus 
memory, whereas a factory-fresh 11/83 was always 18MHz with PMI memory, 
and significantly faster.  In fact the PDP-11 and Hardware Help lists on 
DECUServe were full of people discussing the difference as many people 
didn't believe that you could use PMI memory with the processor from an 
11/73 (you can), while others insisted that if you changed the memory an 
11/73 became exactly an 11/83.  In fact the XXDP diagnostics and RT-11 
differentiate 11/73 from 11/83 purely on the basis of PMI memory detection.

The J11 chip used on those systems was supposed to run at 20MHz or so 
from day one, but it didn't quite make it.  Some, especially early on, 
would only run fully reliably at 15MHz, hence the above.  I've got a 
later one in an 11/83 running at 20MHz, though.  It doesn't gain an 
awful lot, but I changed the crystal, just because I could.

-- 
Pete
Pete Turnbull