Non booting BBC/B Issue 7 - Continuous beep Mk 2

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Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 4:37 pm

Over in the BBC Hardware Department thread (viewtopic.php?f=3&t=17016) I attached that board to a BBC B that now no longer boots. Initially, I thought the board was the cause but now I suspect the Beeb was suffering anyway. Too long in the loft.

Anyway, I've worked on it for a couple of days or so now without joy so here I am again ....

Powering up, with Tricky's test rom gives;

Long single beeeeeep forever. CAPs lock is on. No display.
OR
Long single beeeeeep that ends after about 12 seconds. CAPs lock is on to start and then cassette motor on at end of beeeeeep. No display.

Usually the former. The OS rom fairs no better usually (always?) the former. No keyboard response in either case.

The link at S25 can be N, S, none or inverted, makes no odds.

All the socketed ICs hae been tried in another machine and all seem good. IC4 & IC5 are not socketed.

ICs 14, 72, 76, 29, and 34 have been replaced. IC32 is up next I guess.

Anybody any clues?

Editted to say sorry, wrong group, should be in 8-bit. Can somebody move it for me please? Bad day or two at the office. :)
Last edited by Sniffer on Fri May 03, 2019 4:40 pm, edited 1 time in total.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Kazzie » Fri May 03, 2019 5:17 pm

(Post reported to the admins for relocation.)

The fact that it sometimes activates the cassette after a while is interesting. I believe that's one of the first things that Tricky's test ROM does.

My first guess would be that something is (almost always) stopping the CPU from fetching and executing instructions correctly (or at the right speed). But sometimes instructions eventually get through.

The fact that it sometimes appears to eventually execute some instructions suggests that the CPU is getting a clock signal correctly, and the CPU hasn't accidentally been given an illegal opcode (which would halt the CPU) in the meantime. So if there is something affecting the data or address buses, it isn't crashing it (on some occasions), just slowing it down.

Has the machine had a disc interface or Econet fitted? These both use a Non-Maskable Interrupt to get the CPU's attention. The NMI input is fed by two gates on IC27 (7438), one for each of the above interfaces. If they're not fitted, links S2 and S9 should force output pins 3 and 8 of IC27 high, so the NMI input of the 6502 CPU (pin 6) is never triggered.

While I put my thinking cap on, what sort of testing equipment do you have access to? (Multimeter, logic probe, oscillisope, etc.) If you're able to look at the state of pin 6 of the CPU, that could tell us if this is an NMI problem. Additionally, pin 7 of the CPU (the Sync pin) should show activity each time the CPU is executing an instruction; if it's showing no activity, the CPU has stalled somehow.
Last edited by Kazzie on Fri May 03, 2019 5:17 pm, edited 1 time in total.
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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Fri May 03, 2019 5:29 pm

Thread moved to the 8-bit hardware section.

Mark

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 6:44 pm

Thread moved to the 8-bit hardware section.
Thanks Mark.
Has the machine had a disc interface or Econet fitted?
I pulled the machine out of the loft to sell it so to be honest, I don't know much about it. It doesn't have econet and looks like it never has had. It doesn't have a disk interface though probably did at some point as S9 was cut. I have added a link and jumper there.
While I put my thinking cap on, what sort of testing equipment do you have access to? (Multimeter, logic probe, oscillisope, etc.)
All of the above but I'm not proficient with a scope as yet.
If you're able to look at the state of pin 6 of the CPU, that could tell us if this is an NMI problem. Additionally, pin 7 of the CPU (the Sync pin) should show activity each time the CPU is executing an instruction; if it's showing no activity, the CPU has stalled somehow.
Pins 6&7 are high but it hasn't gone into 'no beep' mode for me to check the states at that point.

Pin 37 is also high.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Fri May 03, 2019 6:51 pm

When you ran it with the test ROM in, was this EPROM in the OS socket? Were all the other ROM chips removed?

You may also want to try running in in a minimal configuration.

Mark
Last edited by 1024MAK on Fri May 03, 2019 6:52 pm, edited 1 time in total.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 6:56 pm

When you ran it with the test ROM in, was this EPROM in the OS socket? Were all the other ROM chips removed?
Yes.
You may also want to try running in in a minimal configuration.
Thanks, I'll take a look at that now.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 7:03 pm

You may also want to try running in in a minimal configuration.
Yes, I have tried all of that, except IC4 is not socketed (though I did try a piggy back at some point).

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Fri May 03, 2019 7:51 pm

Have you tested both the +5V supply at each connection where the wiring connects to the main board (test on the board connection) AND at each of the 0V / GND connections where the wiring connects to the main board. To do the later, connect the red meter lead to a +5V point, then test using the black lead...

The voltage on each connection should be between 4.85V and 5.15V and should be very close to the voltages on the other two supply points.

If all this is okay, test the supply pins and the 0V / GND pins on all the major chips or the sockets where they fit.

Mark
Last edited by 1024MAK on Fri May 03, 2019 7:59 pm, edited 1 time in total.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 8:16 pm

Have you tested both the +5V supply at each connection where the wiring connects to the main board
+5V supply
VCC1=5.03v
VCC2=5.023v
VCC3=4.995v

0V/Gnd (using +5 by S38)
VCC1=5.001v
VCC2=5.001v
VCC3=4.990v

I'll get onto the ICs ......

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by marcusjambler » Fri May 03, 2019 8:22 pm

http://primrosebank.net/computers/bbc/d ... 0Micro.pdf
Invaluable resource for diagnosing Core elements... Especially if Tricky's test OS will not boot...

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Kazzie » Fri May 03, 2019 8:35 pm

Sniffer wrote:
Fri May 03, 2019 6:44 pm
Pins 6&7 are high but it hasn't gone into 'no beep' mode for me to check the states at that point.

Pin 37 is also high.
Pin 7 staying high tells us that the CPU isn't executing instructions.

Pin 37 (phi0 clock input) staying high gives us a probable reason for this.

You should have a (reasonably) square wave at pin 37 to drive the CPU's clock. Without this, the CPU won't do anything. When your machine activated the cassette light, it must have gotten some pulses on the clock line to execute a few instructions. Note that the old 6502 chips used in the Model A/B don't run reliably with a clock speed lower than a few kilohertz.

The Beeb has a 16MHz clock, which is divided down by the video processor (IC6), and then goes through some logic gates on its way to the CPU.

:arrow: Can you check what you find on pins 7-4 of IC6? You should find square waves at 8, 4, 2 and 1MHz. If you do, we can then start testing the logic between IC6 and IC1.

It'd also be worth checking the state of the CPU's reset line, pin 40 of IC1. It should be high except when you press BREAK on the keyboard.
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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Fri May 03, 2019 8:57 pm

Okay, assuming the remaining voltage tests are okay, can you test the various CPU pins using a logic probe or oscilloscope as follows:

Code: Select all

1  VSS
2  RDY
3  PH1 OUT
4  /IRQ
5  N/C (no connection)
6  /NMI
7  SYNC
8  VCC
9  A0
10 A1
11 A2
12 A3
13 A4

Code: Select all

14 A5
15 A6
16 A7
17 A8
18 A9
19 A10
20 A11
21 VSS
22 A12
23 A13
24 A14
25 A15
26 D7

Code: Select all

27 D6
28 D5
29 D4
30 D3
31 D2
32 D1
33 D0
34 R/W
35 N/C (no connection)
36 N/C (no connection)
37 PH0 IN
38 S0
39 PH2 OUT
40 RES
Image

Pin 49 40 should change state with the BREAK key.

Pins 3, 37 and 38 are the CPU clock pins and should be running at 1MHz or 2MHz depending on the address on the address lines (bus). If pin 37 is static (constantly high or constantly low, then there is a problem with the clock circuitry.

Mark
Last edited by 1024MAK on Fri May 03, 2019 9:50 pm, edited 1 time in total.
Reason: Typo, pin 49 corrected to pin 40

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 9:12 pm

Can you check what you find on pins 7-4 of IC6?
7, Nothing
6, High
5, Pulsing
4, Pulsing

Can't get the scope till the morning. Pin 40 of IC1 is high until break is pressed.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Kazzie » Fri May 03, 2019 9:22 pm

Sniffer wrote:
Fri May 03, 2019 9:12 pm
Can you check what you find on pins 7-4 of IC6?
7, Nothing
6, High
5, Pulsing
4, Pulsing

Can't get the scope till the morning. Pin 40 of IC1 is high until break is pressed.
Thanks. Your probe(?) is detecting the 2 and 1MHz clocks on pins 5 and 4, but not the 4 and 8MHz clocks they're generated from. Perhaps your probe isn't able to detect frequencies that high?

Either way, best check the CPU pins as Mark suggested. Then we'll see where to go after that.
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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 9:28 pm

Okay, assuming the remaining voltage tests are okay
They did.
can you test the various CPU pins using a logic probe or oscilloscope as follows:
Can't access scope till morning, so, cpu pins;

Code: Select all

1 low
2 high
3 low
4 high
5 nothing
6 high
7 high
8 high
9 low
10 high
11 low
12 high
13 nothing
14 high
15 low
16 low
17 low
18 low
19 low
20 low
21 low
22 low
23 low
24 low
25 low
26 high
27 pulsing
28 pulsing
29 pulsing
30 pulsing
31 pulsing
32 pulsing
33 pulsing
34 high
35 nothing
36 nothing
37 high
38 high
39 high
40 high
Pin 49 should change state with the BREAK key.
If you meant pin 39, it doesn't.
If pin 37 is static (constantly high or constantly low, then there is a problem with the clock circuitry.
I changed out ICs 34 & 29 so far.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 9:29 pm

Your probe(?) is detecting the 2 and 1MHz clocks on pins 5 and 4, but not the 4 and 8MHz clocks they're generated from. Perhaps your probe isn't able to detect frequencies that high?
It is only a cheapo Laser probe.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Fri May 03, 2019 9:49 pm

Sorry, typo, RES (Reset) is on pin 40...

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 9:57 pm

Sorry, typo, RES (Reset) is on pin 40...
I should have realised. Yes, pin 40 does change state on <BREAK>

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 10:21 pm

Got the scope out, not a square wave in sight, though admittedly, I could be doing something wrong :)

Editted to add image (cpu clock);
SDS00001.png

Not sure if this means anything, will learn more in the morning.
Last edited by Sniffer on Fri May 03, 2019 11:25 pm, edited 1 time in total.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Fri May 03, 2019 10:49 pm

High frequency square waves in a 1980s computer are not very square...

In addition, to get a better representation of the actual signal, you need to be using calibrated X10 oscilloscope probes. These have a switch (often marked X1/X10).

Which pins of which chips are you testing and some photos would be helpful...

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Fri May 03, 2019 11:28 pm

Uused 10X probe setting. Still learning with this (any) scope.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Kazzie » Sat May 04, 2019 7:16 am

Sniffer wrote:
Fri May 03, 2019 10:21 pm
Got the scope out, not a square wave in sight, though admittedly, I could be doing something wrong :)

Editted to add image (cpu clock);

SDS00001.png


Not sure if this means anything, will learn more in the morning.
If that scope output is from measuring one of pins 3, 37 or 39 (the clock pins) on the CPU, then that's a good enough square wave to drive the CPU clock.

If so, the fact that your probe detected those three pins as being in a fixed state leaves me suspecting that 2MHz is too fast for that probe, and the other results may be unreliable.

:arrow: Could you confirm each of those measurements with your scope? (No need to post pictures for each and every pin, if you feel a simple text description is enough. But do feel free to share any interesting results with us.)

But to give a short analysis of the (suspect) results you got with the logic probe:
  • Pins 35 and 36 are not connected on a regular 6502, so no reading is reasonable. Pin 13, however, is part of the address bus, and should give a reading.
  • You've shown pulsing activity on all but the highest bit of the data bus, so there's definitely something happening on the board...
  • Address lines A15 to A6 are all low, so the CPU is currently referencing the lower parts of the RAM, the zero-page area. Acorn's MOS uses zero-page RAM while booting, but Tricky's test ROM has been written to avoid depending on the RAM.
:arrow: Can you confirm which ROM was/is in the OS socket for these tests, Tricky's or Acorn's MOS?

Thanks.
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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Sat May 04, 2019 9:50 am

If that scope output is from measuring one of pins 3, 37 or 39 (the clock pins) on the CPU, then that's a good enough square wave to drive the CPU clock.
CPU pin 37.
Could you confirm each of those measurements with your scope?
Will do.
Can you confirm which ROM was/is in the OS socket for these tests, Tricky's or Acorn's MOS?
99.9% certain it was Acorns MOS at that point. I will put in and leave in Trickys ROM moving forward so as to avoid RAM?

Edited to say this morning I socketed a replacement IC32. No major change but it sounds like it's trying really hard :)
Last edited by Sniffer on Sat May 04, 2019 9:52 am, edited 1 time in total.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Sat May 04, 2019 10:38 am

Could you confirm each of those measurements with your scope?
Here's an image of pins 4 to 7 of IC6. No idea if it's what you wanted, as I freely admit, I'm not proficient with the scope.

IC6_P4-P7.png

If you were expecting something else, measurement wise, just say and I'l try and do it.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Sat May 04, 2019 12:19 pm

Sniffer wrote:
Sat May 04, 2019 10:38 am
Could you confirm each of those measurements with your scope?
Here's an image of pins 4 to 7 of IC6. No idea if it's what you wanted, as I freely admit, I'm not proficient with the scope.
Yep, they are all fine :D

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Kazzie » Sat May 04, 2019 4:45 pm

The scoping of IC6 looks all fine. Could you scope out the CPU's pins too, now that we're sure which ROM it's running from?

(If a pin is in a simple fixed high or fixed low state, there's no need to post an image.)
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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Sat May 04, 2019 6:36 pm

Could you scope out the CPU's pins too
Sure, pins 1 to 40 in order;

Code: Select all

 1 L
 2 H
 3 L
 4 H
 5 N
 6 H
 7 H
 8 H
 9 L
10 L
11 L
12 L
13 L
14 L
15 L
16 L
17 L
18 L
19 L
20 L
21 L
22 L
23 L
24 L
25 L
26 P
27 P
28 P
29 P
30 P
31 P
32 P
33 P
34 H
35 N
36 N
37 P
38 H
39 H
40 H
Would you like images of the pulsing pins?

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by 1024MAK » Sat May 04, 2019 7:03 pm

Code: Select all

1  VSS - L - normal
2  RDY - H - normal
3  PH1 OUT - L - *** not good - should be pulsing ***
4  /IRQ - H - normal 
5  N/C (no connection) [meaning, we and the CPU don’t care...]
6  /NMI - H - normal 
7  SYNC - H - *** should be pulsing ***
8  VCC - H - normal 
9  A0 - L
10 A1 - L
11 A2 - L
12 A3 - L
13 A4 - L

Code: Select all

14 A5 - L
15 A6 - L
16 A7 - L
17 A8 - L
18 A9 - L
19 A10 - L
20 A11 - L
21 VSS - L - normal 
22 A12 - L
23 A13 - L
24 A14 - L
25 A15 - L
26 D7 - P

Code: Select all

27 D6 - P
28 D5 - P
29 D4 - P
30 D3 - P
31 D2 - P
32 D1 - P
33 D0 - P
34 R/W - H - *** should be pulsing ***
35 N/C (no connection) [meaning, we and the CPU don’t care...]
36 N/C (no connection) [meaning, we and the CPU don’t care...]
37 PH0 IN - P *** waveform needed ***
38 S0 - H - normal
39 PH2 OUT - H *** should be pulsing ***
40 RES - H - normal 
Can you please post images of the waveform on the following pins: 37, 39 and 3.

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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Kazzie » Sat May 04, 2019 7:16 pm

Sniffer wrote:
Sat May 04, 2019 6:36 pm
Could you scope out the CPU's pins too
Sure, pins 1 to 40 in order;

Code: Select all

 1 L
 2 H
 3 L
 4 H
 5 N
 6 H
 7 H
 8 H
 9 L
10 L
11 L
12 L
13 L
14 L
15 L
16 L
17 L
18 L
19 L
20 L
21 L
22 L
23 L
24 L
25 L
26 P
27 P
28 P
29 P
30 P
31 P
32 P
33 P
34 H
35 N
36 N
37 P
38 H
39 H
40 H
Would you like images of the pulsing pins?
Thanks.
To summarise:
  • The Address bus (pins 9-20, 22-25) is all low, meaning it's stuck at &0000
  • There's no activity on the Sync pin (pin 7), so the CPU isn't decoding instructions
  • The Read/Write pin is high, meaning the CPU is always reading from the data bus, never writing
  • The Data bus (pins 26-33) is all pulsing
  • There's a clock signal being input at Phi0 (pin 37), but no clock coming out at Phi 1 (pin 3) or Phi 2 (pin 39). Phi 1 is required by a number of components on the board, as it's used to generate the 2MHZE clock signal.
This last point is what I find strangest. The CPU should be generating it's own Phi1 and Phi2 signals based on the input to Phi0, but it isn't. This leaves me suspecting a fault with the CPU.

As it stands, it seems the subsystems that run off the CPU-generated clock signals (such as 2MHZE) are stalled, those that run off clock signals derived from IC6 (such as 1MHZE) are able to run fine, but the CPU in the middle is doing nothing at all.

:arrow: When you said you'd swapped out all socketed components, did that include the CPU?

:arrow: Could you post images of the signals you see at pins 3, 37 and 39, just to check exactly what's going in and coming out? Thanks.

Edit: Great minds think alike Mark! :lol:
Last edited by Kazzie on Sat May 04, 2019 7:18 pm, edited 1 time in total.
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Re: Non booting BBC/B Issue 7 - Continuous beep Mk 2

Post by Sniffer » Sat May 04, 2019 7:32 pm

Can you please post images of the waveform on the following pins: 37, 39 and 3.
Sure. Inorder 3,37,39;

3-37-39.png
When you said you'd swapped out all socketed components, did that include the CPU?

Yes it did but I suspected a problem with this cpu so I bought one off Dave H, which is on it's way. However, I tried the cpu in another beeb and that beeb booted fine so I thought that actually, this one was good. I will replace it again with a known good one and re-take the measurements.

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