23 Aug
2014
23 Aug
'14
6:44 p.m.
On 2014-Aug-23, at 5:20 PM, drlegendre . wrote:
Ok good call gents, thanks for pointing this out..
yes, the LM309 only
provides power for the termination network (marked VT+). Here's what I see
in the network:
- Some lines have no connection to the network at all
- Most lines have a 330R pull-up connected to the VT+
- A number of lines are biased at +roughly+ 1/2 VT+ by being placed at the
junction of a 330R and 470R -> Gnd resistor.
And for some reason, all of the data lines - with the exception of D2 - are
in the latter category. They're biased at the 330 / 470 junctions. So my
current questions are..
Why did they skip D2 of all things?
Will any of this have any effect on the operation of the machine, once I
plug in an original CPU board & static RAM board?
> I began asking this, as when I powered-up the
boards for first smoke test,
> I noticed that all of the Data LEDs are on.. +except+ for D2. Investigation
> found that all data lines (except D2) are sitting at 1/2 Vcc causing the
> connected 7404 inverter outputs to go low, which turns on the LEDs (as the
> other leg of the LEDs is tied to Vcc). So when the inverter output remains
> high, both sides of the LED see Vcc and hence no current - inverters go
> low, and they sink current and turn-on the LEDs.
>
> It was that D2 being off that drew my attention. And upon further reading,
> I don't think +any+ of those data LEDs should be on, when no other boards
> are installed.
The absence of termination resistors on D2 just sounds like a mistake.
The open-circuit state for a TTL input is high and the S100 data lines are active-high, so
with no boards on the bus one could expect all the Data LEDs to be on, with or without
termination resistors. With the D2 LED off it suggests something is pulling the line low.
(Are all the data/address front panel switches in the same position?)
You might try pulling the appropriate D2 line (DI2) high, through a 220 to 330 ohm
resistor for a little safety, to see if the D2 LED will come on.