5 May
2021
5 May
'21
12:03 a.m.
Hey all --
I cabled up the RF08 to my 8/I this evening and it's showing some very
faint signs of life -- a DIML instruction appears to do the right thing.
That's about it.
Until this point I've never had any peripherals for my negibus systems
(apart from teletypes), and it occurs to me that I have no idea if the bus
needs to be terminated (and if so, with what). There are 6 slots in the
RF08 backplane (D01-D06) for daisy-chaining to the next device, which is
where I assume they'd go; the RF08 manual does not make it clear what this
looks like or if it's actually required, and I've gone through the
available PDP-8/I docs and I'm still at a loss.
Can anyone with negibus experience point me in the right direction?
Thanks,
Josh
5 May
5 May
12:27 a.m.
On 5/5/2021 12:03 AM, Josh Dersch via cctalk wrote:
Until this point I've never had any peripherals
for my negibus systems
(apart from teletypes), and it occurs to me that I have no idea if the bus
needs to be terminated (and if so, with what). There are 6 slots in the
RF08 backplane (D01-D06) for daisy-chaining to the next device, which is
where I assume they'd go; the RF08 manual does not make it clear what this
looks like or if it's actually required, and I've gone through the
available PDP-8/I docs and I'm still at a loss.
Can anyone with negibus experience point me in the right direction?
I don't have any actual Negibus gear except a TC01 (which I've yet to be
able to cable to anything), so I'm likely not the best expert.
Regardless, my understanding is that it is terminated in basically the
same way as the positive bus, which is to say you stick a G717 in the
BAC connector that has the IOP and TS signals on it (at the far end of
the bus, of course).
This has the effect of terminating the signals that need to be cleanest
with 100 ohm resistors to ground.
Not sure where I read that though, so I'm not able to easily double
check my memory.
Vince
2:05 a.m.
On 5/5/2021 12:27 AM, Vincent Slyngstad via cctalk wrote:
On 5/5/2021 12:03 AM, Josh Dersch via cctalk wrote:
this is Vince's site, (I think)
http://svn.so-much-stuff.com/svn/trunk/Eagle/projects/DEC/Gxxx/G717/
Until this point I've never had any
peripherals for my negibus systems
(apart from teletypes), and it occurs to me that I have no idea if
the bus
needs to be terminated (and if so, with what).? There are 6 slots in the
RF08 backplane (D01-D06) for daisy-chaining to the next device, which is
where I assume they'd go; the RF08 manual does not make it clear what
this
looks like or if it's actually required, and I've gone through the
available PDP-8/I docs and I'm still at a loss.
Can anyone with negibus experience point me in the right direction?
I don't have any actual Negibus gear except a TC01 (which I've yet to
be able to cable to anything), so I'm likely not the best expert.
Regardless, my understanding is that it is terminated in basically the
same way as the positive bus, which is to say you stick a G717 in the
BAC connector that has the IOP and TS signals on it (at the far end of
the bus, of course).
This has the effect of terminating the signals that
need to be
cleanest with 100 ohm resistors to ground.
Not sure where I read that though, so I'm not able to easily double
check my memory.
????Vince
Thanks
Jim
10:41 a.m.
On Wed, May 5, 2021 at 12:27 AM Vincent Slyngstad via cctalk <
cctalk at classiccmp.org> wrote:
On 5/5/2021 12:03 AM, Josh Dersch via cctalk wrote:
Thanks! Looking at the engineering drawings and other docs for the RF08 (
http://bitsavers.org/pdf/dec/pdp8/disc/DEC-08-HIEA-DA_RF08_Jun70.pdf):
Section 2.2.5 of the manual states "Figure 2-5 shows proper cable
connections between the RF08 Disk Control and the central processor and
proper termination techniques at the time of installation." In figure 2-5
(p. 25) there's the cabling diagram and no explicit termination
instructions called out for slots D01-D06; however, looking closer at the
IOP signals, the lines coming out of the block terminate in a circle, as do
the T1, PCL, and B RUN signals. Is this circle DEC shorthand for
"terminate this?"
Additionally, on p. 88, there are a series of asterisk-denoted 100 ohm
resistors (connected to the IOP1/2/4 signals) which I assume are meant to
be on the backplane. On other schematic pages there are other 100 ohm
resistors (with asterisks) for T1, PCL, etc. These don't appear to be
present on mine; there's a note on p. 88:
"* 100 OHM TERMINATING RESISTORS ARE USED ONLY ON THE PDP-8 NOT ON THE
PDP-8I"
which would likely explain it. Based on this, I /think/ I'm OK termination
wise with my system. Wonder why there's a difference between the two
systems...
- Josh
Until this point I've never had any
peripherals for my negibus systems
(apart from teletypes), and it occurs to me that I have no idea if the
bus
needs to be terminated (and if so, with what).
There are 6 slots in the
RF08 backplane (D01-D06) for daisy-chaining to the next device, which is
where I assume they'd go; the RF08 manual does not make it clear what
this
looks like or if it's actually required, and
I've gone through the
available PDP-8/I docs and I'm still at a loss.
Can anyone with negibus experience point me in the right direction?
I don't have any actual Negibus gear except a TC01 (which I've yet to be
able to cable to anything), so I'm likely not the best expert.
Regardless, my understanding is that it is terminated in basically the
same way as the positive bus, which is to say you stick a G717 in the
BAC connector that has the IOP and TS signals on it (at the far end of
the bus, of course).
This has the effect of terminating the signals that need to be cleanest
with 100 ohm resistors to ground.
Not sure where I read that though, so I'm not able to easily double
check my memory.
Vince
1:23 a.m.
On 5/5/2021 12:03 AM, Josh Dersch via cctalk wrote:
Can anyone with negibus experience point me in the
right direction?
As you and Ethan helped me with on FB about my 8/I, I don't
know which
bus I had.? And worse it
went to a second rack with about 10 of the expansion racks built out
with decoders (W103s IIRC)
and the like.? I couldn't afford or justify the custom stuff in that
rack because other than the manuals
for the 8/I it was all undocumented.? I bought a bunch of what I recall
were 1949 racks and some
cards from the broken up system and the 8/I. I didn't think to ask for
termination, but it had to be
at the end of a bunch of cables that they pulled apart.
I think there were 5 or 6 sets of 6 (?) or so cables daisychaining the
peripherals all together.
Obviously you have checked whether the cards Vince mentioned (or the
termination) in your
rack.? If it was the main external peripheral one would hope they were
there.
Otherwise a scamble to find some or make some.? If you do the latter,
let me know as I'm sure I'll be
interested and would be glad to buy some PCBs to make it cheaper to get
a run made.
thanks
jim
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derschjo＠gmail.com -
jwsmail＠jwsss.com -
vincent.slyngstad＠gmail.com