Ideas for a simple, but somewhat extendable computer bus

Rob Doyle radioengr at
Tue Nov 21 01:45:01 CST 2017

On 11/20/2017 6:50 PM, Noel Chiappa via cctalk wrote:
>      > From: Charles Anthony
>      > a hybrid PDP-11 (16 bits) / PDP-15 (18 bits) on a shared bus (UNIBUS?)
> That's a UNICHANNEL-15: it allowed devices on the -11 to do DMA directly into
> the PDP-15's memory through the MX15-B Memory Multiplexer.
> Odd factoid: this UNIBUS could run in 18-bit mode (!!), where the UNIBUS' two
> parity lines were recycled into 2 extra data lines. Some DMA interfaces (e.g.
> the RK11) could support this; in this particular case, it allowed the PDP-15
> to use RK05 drives.
> 	Noel

More than just that -

The DEC KS10 (PDP-10) used the UNIBUS and Massbus peripherals (RP06
disk, etc) in 18-bit mode. There was a special version of the RH11
(RH11C?) that had extra buffering and could be configured in "Fast
Transfer Mode" where it used a pair of 18-bit UNIBUS transfers to do
36-bit DMA.

I've never decided if this was really clever engineering, or just
'messed up' ... I guess it saved having 36-bit peripherals and
peripheral buses.


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