17 Dec
2016
17 Dec
'16
2:02 p.m.
On 16/12/2016 18:25, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
On Fri, Dec 16, 2016 at 6:19 PM, Peter Coghlan
<cctalk at beyondthepale.ie>
wrote:
And IIRC pins 6 and 7 are reset _inputs_ on the 74x90 (I don't have the
pinouts to hand, what about the 74x92 or more likely 74x93?). But active
high, not active low. So it's possible the reset signal comes from elsewhere,
resets these counters (as some kind of clock divider chain) and is inverted
by that '14 to feed the 8085 reset input.
I doubt it's a uA733. I can think of no logical reason to have one of those
in this sort of circuit.
Thanks all, the pinouts are matching the LS9x counters so I just need to
trace more lines to hopefully narrow it down. Pins 6 and 7 are definitely
inputs so you're right Tony, the reset must come from elsewhere. One of the
outputs is confusing though since it appears to come FROM 5V via a resistor,
I can't see anything in the datasheets that hints it may also be used as an
input or that pulling it high affects behaviour. Maybe I mistraced in
tiredness the other night.
Back to the DMM then!
Cheers folks :D
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
They aren't 'standard' 14-pin DIPs in that they don't follow the
GND-on-pin-7-Vcc-on-pin-14 layout. Pins 6 and 7 on both are wired together
(not to GND) and form the RESET signal for the 8085 via the 7414 at 10A,
source for this signal is unknown currently. Pin 5 on both appears to be Vcc
and pin 10 is GND or at least are pulled high and low respectively.
Could they be 74(LS)90? Those have Vcc on pin 5 and GND on pin 10.