pdp11 bus - how long can it go?

[Paul Anderson]

We had an 11/45 ( as I recall) at the U of Illinois that was said to be the
longest unibus in the world. It had 2 DB11 repeaters on it, and there was
talk or putting on a third. I can't remember how many bus loads were on
there, but this sys was loaded.

Paul

On Tue, Feb 2, 2010 at 12:36 PM, Jack Rubin <jack.rubin at ameritech.net>wrote:

> As I slowly dig into "my" new 11/34A, I'm amazed by the bus configuration.
> Starting from the CPU, the main BA11 chassis has a double (9-slot) system
> unit, joined to a single (4-slot) SU by an M9202 jumper. From the last slot
> in the chassis, a Unibus cable runs to a junction block on the cabinet back
> door. A second cable runs from there to a remote BA11, also with 2 SUs. A
> third cable runs back to the rack door/junction block (labled "bus station",
> BTW) and finally a fourth cable runs to a small rack mounted I/O box with a
> single SU and an M9302 terminator in the last slot. Cable runs must be about
> 20-25 feet total.
>
> At first, I was worried that the length was too long (though it was a
> working system in use for several years) but after a quick websearch (
> www.psych.usyd.edu/pdp-11/unibus.html), I see that the bus can be up to
> FIFTY FEET before a repeater is needed. My short term plan is to pull all
> the cables and move the 9302 into the system box for testing. Now I'm
> worried that the bus might be too short - do I need to look for one of the
> "new" jumpers with two feet of cable or should it work with my old M9202
> jumper block?
>
> Jack
>