CPC 6128 - white screen, black border

[mmldrm]

I'm understanding... just about... 

[McArti0]

https://www.cpcwiki.eu/forum/hardware-related/cpc-6128-white-screen-black-border/msg238062/#msg238062

Pin7 to pin16 or pin17. Pin20 5v to empty 16 or 17.

[mmldrm]

https://www.cpcwiki.eu/forum/hardware-related/cpc-6128-white-screen-black-border/msg238062/#msg238062

Pin7 to pin16 or pin17. Pin20 5v to empty 16 or 17.

With this setup, both R160 and R161 are hi.

(Makes me want to double check the previous readings...)

[McArti0]

But only D2 and D14 . You should see #89 1000 1001

[mmldrm]

Ok I got 1000 1001, on pin2 and pin14, both RAM banks (IC127-134, and IC119-126). 

Just confirming - this is if I read from bottom to top on the board. i.e. IC134->IC127, and IC126->IC119. (I guess that is correct - lowest bit is on lowest IC number.)

[McArti0]

https://www.cpcwiki.eu/imgs/4/4a/CPC6128_Schematic.png


Now when you check pin14 , you quick grounded by diode (catode to ground) pin2. Multimeter should show low on pin14.

[mmldrm]

Now when you check pin14 , you quick grounded by diode (catode to ground) pin2. Multimeter should show low on pin14.

Every time I try, I still get 10001001 on pin2 and pin14. Is that unexpected?

I even tried pin2 D0-D7, pin14 D0-D7, and then checked pin2 D0-D7 again... thinking maybe checking/grounding pin14 could affect values. But still 10001001.

Maybe Friday at 5pm is a bad time for me to understand why multimeter should be lo on pin14.

[McArti0]

244 is like eight pipes.

When Pin1 244 is Lo. pipes are open.

When you grounded RAMs pin2, this ground flow by open 244 to Pin14 RAM.

If You are checking Pin14 by multimeter and grounding Pin2 by second hand then you are seeing this ground on multimeter.

[mmldrm]

244 is like eight pipes.

When Pin1 244 is Lo. pipes are open.

When you grounded RAMs pin2, this ground flow by open 244 to Pin14 RAM.

If You are checking Pin14 by multimeter and grounding Pin2 by second hand then you are seeing this ground on multimeter.

Thanks, now I understand how checking pin2 first could ground pin14.

Although I did only ever test one pin at a time. Black multimeter probe on ground, red probe on pinX. No second hand, unless you mean the red and black multimeter probes.

Silly question... what should I do next?

[McArti0]

Black multimeter probe on ground,

By crocodile, not in hand.

Now.
If You are checking Pin14 by multimeter and grounding Pin2 by second hand then you are seeing this ground on multimeter.

[mmldrm]

Now.
If You are checking Pin14 by multimeter and grounding Pin2 by second hand then you are seeing this ground on multimeter.

Ok, I grounded pin2 and checked pin14 on IC127-IC134.

All pin14s were lo (~0.05V) except IC127 which remained hi (~4.95V). Have we found a bad RAM chip?

[McArti0]

@mmldrm
To sum up:
Pin1 244: Lo

CAS0,CAS1 (R160,R161) Hi

D0 Pin2 IC127 Lo (Grounded)  and Pin14 Hi ? ? ? ?

This would mean that 244 is damaged.

Check it again the same thing in another place: Pin2 244 grounded and Pin18 244 multimeter

[SerErris]

mmldrm
To sum up:
Pin1 244: Lo

CAS0,CAS1 (R160,R161) Hi

D0 Pin2 IC127 Lo (Grounded)  and Pin14 Hi ? ? ? ?


This would mean that 244 is damaged.

Or IC127 is driving Pin2 so much (shorted to +5V) that it is overwriting Pin18 of 244, which will sink the current.

If IC127 is the culprit, you should see a permanent high signal if you let the computer run normally.

The problem is, that you cannot really decide which on of both it is (I would still bet on the IC127) as both can drive the line.

If you measure your situation above, then you should see the same high signal on IC127, IC119 (both Pin14) and IC115 Pin18 as well as IC114 PIN3 and GA Pin35. All those ICs are directly connected and should have the same voltage. If they do not have the same voltage, you are measuring something wrong.

you could test if the RAM is the culprit, if you bring pin 1 of IC115 to high. Then the output of LS244 is high impedance (actually no output) and whatever the RAM chips are doing is then what you will see on Pin14 of IC127.

But still it cannot be that you only see high on a single of all those chips. It is all chips high or any other voltage, but it is not IC127 high and IC119 low.

[McArti0]

Or IC127 is driving Pin2 so much (shorted to +5V) that it is overwriting Pin18 of 244, which will sink the current.

But in another dynamic scenario where GA works, pin14 is 2.5v so it is often zero there.

Look here...
 https://www.cpcwiki.eu/forum/hardware-related/cpc-6128-white-screen-black-border/msg238645/#msg238645

[McArti0]

Ok we have test hardware. Test.
Pin5 LS02 to internal D0- Pin14 Ram ic 127.
Pin6 LS02 to pin1 244.
Turn ON cpc.
If cpc stoped then check D0 lo, cas0 hi ,cas1 hi, iorq lo, pin 1 244 lo, address pin 5-1,40-30 z80.

[mmldrm]

@mmldrm
To sum up:
Pin1 244: Lo

CAS0,CAS1 (R160,R161) Hi

D0 Pin2 IC127 Lo (Grounded)  and Pin14 Hi ? ? ? ?

This would mean that 244 is damaged.

Check it again the same thing in another place: Pin2 244 grounded and Pin18 244 multimeter

When Pin2 LS244 is grounded, Pin18 LS244 is lo

[mmldrm]

If you measure your situation above, then you should see the same high signal on IC127, IC119 (both Pin14) and IC115 Pin18 as well as IC114 PIN3 and GA Pin35. All those ICs are directly connected and should have the same voltage. If they do not have the same voltage, you are measuring something wrong.

Thanks. I have checked all of these, with pin2 of IC127 grounded, and can confirm that they are all high.

IC127 pin14
IC119 pin14
IC115 pin18
IC114 pin3
GA pin35

[mmldrm]

Ok we have test hardware. Test.
Pin5 LS02 to internal D0- Pin14 Ram ic 127.
Pin6 LS02 to pin1 244.
Turn ON cpc.
If cpc stoped then check D0 lo, cas0 hi ,cas1 hi, iorq lo, pin 1 244 lo, address pin 5-1,40-30 z80.

With LS02 pin5+pin6 connected to IC127/LS244, the CPC does not stop. I tried 3-4 times.

LS02 pin5 was already connected to pin1@LS244
LS02 pin6 was already connected to pin15@AY3 (/CPU)

I have attached the diagram of how the LS02 is set up right now. I just want to make sure everything is correct, before I try the new connections you suggested.

[McArti0]

When Pin2 LS244 is grounded, Pin18 LS244 is lo

There is no problem. This is your measurement error.
Pin18 244 is the some wire as pin14 ram ic 127.
Pin2 244 is the some wire as pin2 ram ic 127.

[McArti0]

Ok I got 1000 1001, on pin2 and pin14, both RAM banks (IC127-134, and IC119-126).

Just confirming - this is if I read from bottom to top on the board. i.e. IC134->IC127, and IC126->IC119. (I guess that is correct - lowest bit is on lowest IC number.)

How do You do that?

[McArti0]

How set 0 color to purple.
All ground on z80 side. (Pin2 RAM)
A) Ready/Wait.  -----|>|------  IORQ and keep (Connect by diode, catode at IORQ)
B) start cpc or reset.
C) ground D0 and keep , next ground D3 and keep, quick short ground D7 and remove all ground
D) ground D0 and keep, Hi on D6 (best way by serial 4x diodes ) and keep, quick short ground D7 and remove all ground and Hi set.

Background screen should be purple.
But for this to work, the 244 must be operational.

[mmldrm]

Ok I got 1000 1001, on pin2 and pin14, both RAM banks (IC127-134, and IC119-126).

Just confirming - this is if I read from bottom to top on the board. i.e. IC134->IC127, and IC126->IC119. (I guess that is correct - lowest bit is on lowest IC number.)

How do You do that?

I finally went back to retest this. Steps were :

When CPC stops on address lines A15-A0 you will have address #7F89
Z80 Pins 5,4,3,2,1,40,39... is 30

Next on Pins 
0,1,1,1,  1,
1,1,1,  1,0,0,0, 1,0,0,1
RD Hi, WR Lo, IORQ Lo, nCPU Lo, Pin1 244 Lo.

You can tests D0-D7 Pin2-Pin14 RAM chips

But I must have messed up reporting the results. Maybe I recorded D7-D0. So I tried again today.

These are the results on pin2 and pin14 (and tested twice):

Code Select Expand

IC127  D0  1
IC128  D1  0
IC129  D2  0
IC130  D3  1

IC131  D4  0
IC132  D5  0
IC133  D6  0
IC134  D7  1

[mmldrm]

How set 0 color to purple.
All ground on z80 side. (Pin2 RAM)
A) Ready/Wait.  -----|>|------  IORQ and keep (Connect by diode, catode at IORQ)
B) start cpc or reset.
C) ground D0 and keep , next ground D3 and keep, quick short ground D7 and remove all ground
D) ground D0 and keep, Hi on D6 (best way by serial 4x diodes ) and keep, quick short ground D7 and remove all ground and Hi set.

Background screen should be purple.
But for this to work, the 244 must be operational.

Do you mean I need to ground pin2 on all RAM chips (IC127-IC134)?

If I need 8 pin clips to ground 8 RAM chips, then I will need to take apart some of the other things we set up, like the LS02. (I only have 10 pin clips. Very cheap to get more, but takes time.)

This sounds like a great test but also a very difficult one to get right. 😅

[McArti0]

When CPC stops on address lines A15-A0 you will have address #7F89

Once you write that you are testing the data, the second time you write that CPC does not stop at nCPU and Lo on pin1 244. We want to test the Data when the CPC stops.

[McArti0]

Do you mean I need to ground pin2 on all RAM chips (IC127-IC
134)?

NO,NO,NO.

https://www.cpcwiki.eu/imgs/4/4a/CPC6128_Schematic.png