> On Oct 24, 2016, at 1:30 PM, Paul Koning <[email protected]> wrote: > > >> On Oct 24, 2016, at 3:55 PM, David Bridgham <[email protected]> wrote: >> >> On 10/24/2016 12:01 PM, Paul Koning wrote: >> >>> I don't know about the receiver part, but I'd expect that the drivers could >>> very easily be done with a simple transistor circuit. >> >> Agreed. However ... >> >>> As for slew rates, unless you have antique transistors, that's not going to >>> be an issue given that you meet the current sink spec; the slew rate of an >>> OC circuit is determined by the system capacitance and the sink current of >>> the driver. >> >> I think you read this part backwards. The slew rate requirement is not >> a minimum slew rate but a maximum one. That is, any modern transistor >> (probably ancient ones too) will be way too fast. You have to do >> something to slow it down. Still, I think this one is easily met as >> it's just a series resistor on the gate of the driver MOSFET working >> against the gate capacitance. Some FPGAs have current limiting on their >> output which may obviate the need for the resistor even. > > I don't see any max slew rate spec in the driver specs in the peripherals > handbook. >
OK, I guess my last email didn’t make it. It appears to me that the rise time is set at 25ns. You need to look at the PDP-11 UNIBUS Design Description document on Bitsavers. Firstly, in section 4-1, it specifies which chips to use and recommends not using a whole list of other chips. The only recommended chips are: 8640, 8641 and 8881. 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
