Re: PDP-8/I Negative-bus termination
> > Date: Wed, 5 May 2021 00:03:05 -0700 > From: Josh Dersch > Subject: PDP-8/I Negative-bus termination > > Hey all -- > > 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 > The DEC Field Service Technical Manual has some notes on bus termination. It says: Termination is required on I/O cables longer than 20 ft., and may be desirable on shorter cables. For negative bus, use 220 Ohm shunt resistors to ground on IOP 1, IOP 2, IOP 4, BTS 1, BTS 3 and Initialize. No special termination module exists for the negative bus. -- Michael Thompson
Re: PDP-8/I Negative-bus termination
On Wed, May 5, 2021 at 12:27 AM Vincent Slyngstad via cctalk < cctalk@classiccmp.org> wrote: > 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. > 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 > Vince >
Re: PDP-8/I Negative-bus termination
On 5/5/2021 12:27 AM, Vincent Slyngstad via cctalk wrote: 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 is Vince's site, (I think) http://svn.so-much-stuff.com/svn/trunk/Eagle/projects/DEC/Gxxx/G717/ 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
Re: PDP-8/I Negative-bus termination
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
Re: PDP-8/I Negative-bus termination
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
