[Info-vax] RB730 Integrated Disk Controller (R80/RL02) usable with VAX-11/750?

[Johnny Billquist]

On 2011-05-06 07.54, SeanOBanion wrote:
> On May 5, 9:57 am, Johnny Billquist<b... at softjar.se>  wrote:
>> On 2011-05-05 07:02, Bob Koehler wrote:
>>
>>> In article<ipt5am$d6... at Iltempo.Update.UU.SE>, Johnny Billquist<b... at softjar.se>    writes:
>>
>>>> That's what the Unibus Map is there for. The Unibus map sits between the
>>>> Unibus and the memory, and remaps DMA from the Unibus into the larger
>>>> address space of the memory bus. Same both on the 22-bit capable PDP-11s
>>>> and all VAXen. The exact details of the Unibus map differs between a
>>>> PDP-11 and a VAX, though, but the principles are exactly the same.
>>
>>>      Too me, that map is part of the memory subsystem.
>>
>> The Unibus map is the very important piece that maps Unibus addresses to
>> memory bus addresses for DMA. As such, is it a part of the memory
>> system, or the Unibus? It sits on both...
>>
>> Either way, it is absolutely necessary in order to have DMA from the
>> Unibus work on a machine where the memory does *not* sit on the Unibus.
>>
>> On lowly PDP-11 models, where the CPU only address 18 bits (or less),
>> the memory really sits on the Unibus, and DMA can be performed directly
>> there, and memory is responding to Unibus signals. This is (obviously)
>> very different from when you have machines with a Unibus map.
>>
>> Thus, on any VAX, the memory does not sit on the Unibus, and the Unibus
>> is only used as an I/O bus. As someone else mentioned, the MicroVAX I is
>> the only design where you really only have one bus, the Q-bus in that
>> case. The uVAX I therefor is limited to the physical address space of
>> the Q-bus (which is 22 bits), and DMA can be performed directly between
>> peripherials and memory, which both sits on the same bus.
>>
>> A short additional note. On PDP-11s the Unibus map is located in
>> different places depending on model. For the 11/44 it's in the MMU. The
>> 11/70 have it along the cache. The 11/84 and 11/94 have the Unibus map
>> on the Unibus adapter which is a separate card on those machines.
>> For all of them, DMA from the Unibus "speaks" (that is, do all the
>> Unibus handshaking and signalling) with whatever have the Unibus map.
>> And the Unibus map in turn then "speaks" with the actual memory.
>> When the CPU speaks to memory, the Unibus is totally unaware, and
>> remains quiescent.
>> (For a machine like the 11/70, and VAXen, when devices not on that
>> Unibus speaks to memory, the Unibus is also quiescent.)
>>
>> So no, the memory is *not* on the Unibus. Memory can exist on the
>> Unibus, and do so for 18-bit PDP-11 models, and that is obviously very
>> different.
>>
>>          Johnny
>>
>> --
>> Johnny Billquist                  || "I'm on a bus
>>                                     ||  on a psychedelic trip
>> email: b... at softjar.se             ||  Reading murder books
>> pdp is alive!                     ||  tryin' to stay hip" - B. Idol
>
> Yes, that does make sense, and fits what the doc's seem to say.
> So my recollection / understanding of what was done to the 730 in
> college must be wrong.
>
> For the UDA50 version of the 730, I have to wonder if the UDA50
> displaced some of the memory cards in the system box, because the
> memory limit went from 5MB for the IDC version down to 3 MB.
> Or at least did so in the factory version, but I can't tell if the
> UDA50 could go in the expansion box (H9642-DH)

The UDA50 should definitely be possible to place in the expansion box. 
Looking at the slot assignments in the CPU box, it would appear that 
only the last memory slot could be used by some Unibus cards instead, so 
I'm not sure why the memory limit would go down from 5MB to 3MB, since 
5MB seems to imply 5 1MB cards, and thus two cards would be removed.

However, I do know that the UDA50 draws a lot of power, and there might 
be power limitations that caused the restrictions on memory in this case.

Obviously, having the UDA50 in an expansion box would remove the power 
limitation reason as well.

	Johnny