[ukvac] [Tech] Z80 / The Pit
Bear with me on this, it might be a little long! (F*ck it, if you don't
want the reasoning behind the question, cut to near the end!)...
I wonder if one of the guys that has knowledge of Z80's can give me a
little patience here to help me out. I've been sitting at my "Pit" board
for quite a few hours now and have systematically gone through it trying
to establish why I have no sound.
There is no output from the 8910. All the data lines are active as you
would expect, and the processor is not held in reset. There is a clock
signal going in, but the two input's "BCI" and "BDIR" are stuck (high if
I remember rightly).
So, tracing this through on the schematics these two are fed from a
LS153, on which one of the inputs is labelled "sound" (pin2). This input
is fed through an LS138, and one of the inputs to this is tied to the
IOREQ of the Z80 (we're getting there!).
Now, on checking out the Z80, the IOREQ is stuck high. This didn't sound
too right to me. The output from the IOREQ is or'd with the M1, and
becomes an input to a Dtype (7474), Pre and D are high inputs, there is
a pulsing CLK, and the not Q (try that in text!) is (I think) tied low,
which as it is the INT for the Z80, again doesn't sound right.
Thing is this, I read up the passage in the MCRII (Tron) manual which
has a description of the Z80, and maybe because it's late, I doesn't
follow it too well. What I don't know is, if the IOREQ (held high) is an
input to the INT (held low), what breaks this chain of events? Should I
suspect the 7474 even though according to the spec's truth tables it
appears to be working happily?
Where would be my next thing to look at?
The NMI, WAIT and BUSRQ, are all held high (therefore not active) as
they are fed directly from +5v via a resistor. I assume this is
something to do with why the game continues independently of the sound
not working, and the processor's reset is high, so the Z80 isn't in a
state of reset.
I know the answer to this is unlikely to be simple, but I feel like I've
made major steps forward in understanding this hardware tonight, so
bring it on :O)
Any and all help as ever MUCH appreciated. I could have jumped onto IRC
for this, but TBH, I imagine the answers (or even a short description of
how the Z80 functions) would be extremely useful to more than just me.
If you read this far - thanks!
Guddler.
-------------------------------------
Martin White
http://www.guddler.com
Arcade search site with PCB info too...
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/
Bear with me on this, it might be a little long! (F*ck it, if you don't
want the reasoning behind the question, cut to near the end!)...
I wonder if one of the guys that has knowledge of Z80's can give me a
little patience here to help me out. I've been sitting at my "Pit" board
for quite a few hours now and have systematically gone through it trying
to establish why I have no sound.
There is no output from the 8910. All the data lines are active as you
would expect, and the processor is not held in reset. There is a clock
signal going in, but the two input's "BCI" and "BDIR" are stuck (high if
I remember rightly).
So, tracing this through on the schematics these two are fed from a
LS153, on which one of the inputs is labelled "sound" (pin2). This input
is fed through an LS138, and one of the inputs to this is tied to the
IOREQ of the Z80 (we're getting there!).
Now, on checking out the Z80, the IOREQ is stuck high. This didn't sound
too right to me. The output from the IOREQ is or'd with the M1, and
becomes an input to a Dtype (7474), Pre and D are high inputs, there is
a pulsing CLK, and the not Q (try that in text!) is (I think) tied low,
which as it is the INT for the Z80, again doesn't sound right.
Thing is this, I read up the passage in the MCRII (Tron) manual which
has a description of the Z80, and maybe because it's late, I doesn't
follow it too well. What I don't know is, if the IOREQ (held high) is an
input to the INT (held low), what breaks this chain of events? Should I
suspect the 7474 even though according to the spec's truth tables it
appears to be working happily?
Where would be my next thing to look at?
The NMI, WAIT and BUSRQ, are all held high (therefore not active) as
they are fed directly from +5v via a resistor. I assume this is
something to do with why the game continues independently of the sound
not working, and the processor's reset is high, so the Z80 isn't in a
state of reset.
I know the answer to this is unlikely to be simple, but I feel like I've
made major steps forward in understanding this hardware tonight, so
bring it on :O)
Any and all help as ever MUCH appreciated. I could have jumped onto IRC
for this, but TBH, I imagine the answers (or even a short description of
how the Z80 functions) would be extremely useful to more than just me.
If you read this far - thanks!
Guddler.
-------------------------------------
Martin White
http://www.guddler.com
Arcade search site with PCB info too...
Your use of Yahoo! Groups is subject to http://docs.yahoo.com/info/terms/