15 Jan
2017
15 Jan
'17
5:02 a.m.
On 15/01/2017 03:04, "allison" <ajp166 at verizon.net> wrote:
I've thought of that which is why I'm chasing down details on the Viewdata
chip and the D8741A which I assume is being used as a keyboard controller.
There are also 3 modules on the phone side which I can't find anything
about, marked "NKT NMC1515", NMC1516 and NMC1517.
3 interrupts are in use:
TRAP is hardwired to GND.
INTR is pulled low via a 1kohm resistor.
RST7.5 is clock driven. If my disassembled code is to be believed the first
thing it does after PUSHing PSW, B, D and H onto the stack is a RIM
instruction which reads from SID, this is directly wired to one of the NKT
modules I mentioned, possibly checking for an incoming phone call.
RST6.5 is from the Teletext chip, wired to the video status outputs.
RST5.5 is from the D8741A, again from a pair of status outputs (IBF/OBF)
Ok, it never gets interrupted then.
So I found last night.
Indeed! I'm doing this with absolutely zero documentation other than what
I've drawn up myself and a stack of reference books/datasheets. I did manage
to find the designer of this machine, David Leevers, and at some point he's
going to see what docs he has left though at present he can only think of a
user guide.
If you guys weren't around to put up with my amateurish questions I'd have
never started work on it and it would've remained just another unloved bit
of kit on a table in a museum like the other 5 that are known to exist.
Cheers!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
On 01/14/2017 08:18 PM, dwight wrote:
> If it is not doing I/O and waiting for something, it is likely waiting for an
> interrupt.
>
> Dwight
Apologies for all the daft sounding questions (this goes to Dwight, Tony and
Chuck too) but historically I'm a high level programmer so getting right
down into silicon level is something I've only been doing for 18 months or
so. I can sit with a reference book and read assembler listings, draw up a
logic flow, watch activity/read voltages on a scope but then there's
applying that to what the hardware is doing too, a fun learning curve.
STC Executel
fun continues and I'm at the point where I'm fairly sure code
is running but it's stuck in a tight loop waiting for something to happen.
And
maybe waiting for a interrupt? That's what I di when I'm building simple 8085
that interrupt driven.
Let it loop
doing repetitive stuff if anything, an example is the old days
refreshing Dram.
Then use RSTx.5 to force it into a active do something mode and when done
back to the loop.
A current project for myself has the 8085 runnign a simple multitasking OS
and the loop runs through the task table and every task retruns to the loop.
Interrupts are real time events or priority events that need out of
sequence attention. The 8085 makes that fairly simple.
I'd bet the Executel is doing that, refershing display and scanning for
events
with some interrupt driven for out of sequence response.
That would make sense so I'm pondering why IO/m never goes high and why the
teletext chip (and video driver) never generates a video sync which it's
supposed to do.
The 8085A
reference tells me a non-memory I/O is signalled by IO/M going
high while it puts the port number on the address bus (0xE3 to 0xE8 in this
case), 8 bits on the data bus and sets S0/S1 to be WRITE. If that's the case
then this machine in its current state doesn't do any non-memory I/O - IO/M
is the only signal that stays low. It's not a failed CPU since I have 3
different ones and they all do this.
Correct. IO/m will only stay high for an IO
cycle for all memory cycles
it goes low (instruction fetch, execute plus memory read or writes ).
IO/m (there is a bar over the m) also goes high during a RST 5.5, 6.5,
7.5 and Trap if interrupts are enabled along with INTR. I've
pondered if one of the 3 chips the IO/M signal goes to is pulling it
down; presumably I can test this by lifting the IO/M pin out of its socket
then briefly touching a 1Kohm resistor connected to +5V to the 3 inputs and
watch what the outputs do while the machine is running?
It could be you have a
chip with a blown input (stuck low).. However
doing that test is rather inconclusive as there is so much other state
activity to track. Hummm... 4116 dram, that means you have external
refresh logic or they are
going cheap and doing refersh on a interrupt (or maybe) timed loop.
There's an MC2342A doing the refresh and that's looking OK now that I've
swapped it. The original chip had no working outputs.
I'm also
guessing things would be a lot easier if I had a memory map...
A real schematic is
likely a great aid too.