> On Oct 24, 2016, at 10:37 AM, allison <ajp...@verizon.net> wrote: > > On 10/23/16 2:59 PM, Al Kossow wrote: >> >> On 10/23/16 11:50 AM, shadoooo wrote: >> >>> The problem is that there aren't open drain bus transceivers, but the >>> problem could be solved simply using input-only and output-only components, >>> connecting two in parallel but opposite direction on bidirectional pins. >>> >> The reason for using the old parts is the logic thresholds are unique to >> the Unibus to handle worst-case bus loading and the termination voltage they >> used. >> >> > The voltages are based on TTL levels. What are the unique voltages? > > The key was limited leakage current and input current to not load the bus by > inserting or removing > current from a node (there is a specified maximum in per node and total > nodes). That cover input > to card devices and bus driver leakage. > > Logic low voltage is typical of TTL and the driver device has to sink that > current and meet that value. > Logic High was set by the terminator devices at 3.36V but the threshold is > lower based on the bus > receivers. > > By late 1970 it was an easy spec to meet, When first used (pdp8e) it was new > and the ICs > were not so great with leakage current and output device saturation current. > > Every time this comes up the world is supposed to stop if not met. The LSI-11 > bus (qbus) > was actually harder as it was 120 ohm terminated and HeathKit did it with > common TTL > and the CPU was DEC standard LSI-11 and it worked out to 18 slot backplanes. > >
The biggest concern is when interfacing to UNIBUS. In the PDP-11 UNIBUS Design Description document on Bitsavers, page 4-1 indicates what the Unibus interface chips are and what are *not* recommended (8640, 8641 and 8881 are the only ones recommended). There are a number of rules that must be adhered to when building out a Unibus system. These include: Maximum cable length must be < 50’ Maximum DC loading < 20 Maximum lumped loading < 20 There are rules where cable lengths must be *increased* to avoid reflections. A single Unibus can be divided into multiple segments. Each segment must adhere to the above rules, so you can see that a Unibus can be quite large. For example, my PDP-11/40 resides in 2 BA11-F boxes (23” tall) and are fully populated with Unibus backplanes (5 9 slot backplanes each) with a BA11-15 (15’ cable) connecting the two. My point here is that the Unibus has a very different electrical environment than Q-bus or Omnibus and what may work for them will probably have troubles on a Unibus. TTFN - Guy
