DEC bus transceivers

allison ajp166 at
Tue Oct 25 14:40:37 CDT 2016

On 10/25/16 12:10 PM, Guy Sotomayor Jr wrote:
>> On Oct 25, 2016, at 8:38 AM, allison <ajp166 at> wrote:
>> On 10/25/16 10:02 AM, Guy Sotomayor Jr wrote:
>>>> On Oct 24, 2016, at 11:35 PM, ben <bfranchuk at> wrote:
>>>> On 10/24/2016 2:18 PM, David Bridgham wrote:
>>>>> On 10/24/2016 01:37 PM, allison wrote:
>>>>>> The voltages are based on TTL levels.  What are the unique voltages?
>>>>> The QBUS spec from the 1979 Bus Handbook (the Unibus levels are the same):
>>>>> Input low voltage (maximum): 1.3 V
>>>>> Input high voltage (minimum): 1.7 V
>>>>> And from the TI datasheet for the 74LS74:
>>>>> Vil - low-level input voltage 0.8 V (maximum)
>>>>> Vih - high-level input voltage 2 V (minimum)
>>>>> So no, the DEC bus voltage levels are not TTL levels.  Yeah, TTL might
>>>>> work on a smaller system but you can see that if you push it out to its
>>>>> limits, TTL could start getting flaky.  That's the kind of bug I'm happy
>>>>> to have DEC's engineers figure out and not have to track down myself.
>>>> But who has the big systems now days? The days of 4K core is long gone.
>>>> Use TTL and try to keep the systems small.
>>> I just posted what one of my systems is (11/40).  It has 128KW of core which
>>> takes 4 9-slot backplanes.  I have a fair amount of I/O on that system so I
>>> have 2 BA11F chassis *full* of backplanes (each BA11F holds 5 9 slot backplanes).
>>> My other large system is an 11/70 with a BA11K as an expansion box. The whole
>>> point of Unibus was to allow for large configurations.
>>> And I can guarantee you that TTL will *not* work in those systems.
>> Maybe if care is not taken.   Again there are TTL part and those designed for bus use.
>> Bad choice, bad results.   I've seen a few like that that broke even with DEC parts.
> That’s why there’s the UNIBUS spec that DEC published.  In big systems, you must
> follow the guidelines even with the proper transceivers.  Sometimes it requires that
> things be split up and cable length added.
> Also, I think in a previous email you mentioned that the UNIBUS is 240ohm.  It’s not.
> It’s 120ohm.
My book says no. Qbus is for sure 120.

>> Then again My 11/73 is BA11S and Ba11N both with 18 slot 4mb ram (two cards), DELQA,
>> two RQDX3 (one for RX floppies, second for RD disks), RLV21 (in box2), CMS scsi,
>> RXV21V-RX02, DZH11V and Parallel IO (ttl) in box 2 and Gigilo card (DEC sound
>> card for Qbus) in Box 1.   Likely a few IO I forgot.
> OK, it’s still a small system by UNIBUS standards.  ;-)  I can’t check either my 11/40
> or 11/70 but (from memory) the configs are:
> 11/40:
> BA11F(1): CPU with all options (including FIS), 2 32KW core memory, additional backplanes to fill it up
> BA11F(2): 2 32KW core memory, 1 RK11D (4 slot backplane), additional backplanes to fill it up
> The “standard” Unibus devices as I recall are (there are more as most of the backplanes are full):
> 4 DL11s
> DZ11
> RL11
> At least one tape drive interface of some sort
> bootstrap/terminator
> BC11A-15 to connect the two BA11Fs together.
> 11/70:
> BA11F(1): CPU with all options, Hypercache (4MW option that looks like a 4MW cache to the CPU), 2 SMD controllers that plug into the slots for the massbus controllers (4 slots each).  2 Massbus controllers (4 slots each), 1 DL11
> BA11K: RK11D 4 slot backplane, 2 RL11 controllers, TS11 controller, TU81 controller, 2 DZ11s, 3 DL11s, 1 ethernet controller
> There’s more on this system, I just don’t recall it all at the moment.  Again, this system is *full*.  I’m at the point where if I want to add more stuff I need to add another BA11K.
> bootstrap/terminator
> BC11A-15 to connect the BA11F to the BA11K
>> By all DEC standards its unsupported config.  Bus signals still look good.
Mine has many LS24x part and 7438s as there are more than a few IO cards 
that are non DEC (mine).
Example IO a PIO IDE interface test dog wire wrapped (usually the worst) 
alone with A/D and D/A.

By Qbus standards its a monster and it was rare to see more than two 
boxes and usually the cages were
not the 18 AB slot but the 9 slot ABCD configuration.  Runs unix well 
and usually that breaks machines
that are flakey.  Also Qbus uses 120 ohm termination so the drivers have 
to sink more current.

Never could fake the DEC interbox cables, those are well done to work right.

The BA123 uVax also has a few odd cards, doesn't seem to care. THose 
however were not fast
on the bus as they used PMI for Memory.

Another example is the 11/23 with all heathkit cards for memory, IO and 
a few unique homebrews.
I've never seen a bus level issue die to devices used.

>> I've seen my share of BIG VAXen with many unibus crates too.
> Yes, and they all used the proper UNIBUS transceivers and not TTL parts.
mostly....  A few of the systems were not production and engineering did 

Bottom line is someday there will be no DEC parts and what then?  I 
reserve DEC parts
for repairing defunct boards for new and unique build it would be a 
waste of scarce
material.   The vector interrupt chip DC-series is one that is hard to 
fake without a
larger number of chips.

For preservation reasons its very important, for hacking an old system 
not as much.
In my collection I have both.

> TTFN - Guy

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