Search found 145 matches

by JannievanZyl
Tue Feb 06, 2018 9:36 am
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

vanekp wrote:I saw in a previous pic you had both VIA's out one needs to be in (IC3) for the system to run.


My understanding is that one should still get a display with both out, but I did plug IC3 back in and attached a keyboard. Still the continuous tone and no display.
by JannievanZyl
Mon Feb 05, 2018 7:10 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Great news! Is there any change in the LEDs' behaviour having replaced that IC? No, no change. But I now see continuous active on the bus as well as the /CS on the System ROM. So nothing is hanging anymore. But........I need to put this on hold till the 23rd. Flying out to the States in a few hours...
by JannievanZyl
Mon Feb 05, 2018 5:42 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Success!! :D

Now get clock on the CPU. Also get CS on the system ROM.

However, still the same continuous tone and no display error. :cry:
by JannievanZyl
Mon Feb 05, 2018 6:06 am
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Thanks all,

Will replace it tonight.
by JannievanZyl
Sun Feb 04, 2018 9:05 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

You said you had replaced IC34 and IC30 previously. Take a look at IC31. It should have 1MHz on pin 11, and with pins 10 and 13 high, pins 12 and 8 should not be equal. If they are, despite having a clock signal applied, IC31 is faulty. I may not be at the computer for much longer tonight, but I'll...
by JannievanZyl
Sun Feb 04, 2018 8:52 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

You said you had replaced IC34 and IC30 previously. Take a look at IC31. It should have 1MHz on pin 11, and with pins 10 and 13 high, pins 12 and 8 should not be equal. If they are, despite having a clock signal applied, IC31 is faulty. I may not be at the computer for much longer tonight, but I'll...
by JannievanZyl
Sun Feb 04, 2018 8:14 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Could you check IC34 pin 6 and IC30 pin 5 as well while you're at it? 34-6 - H (goes L on reset) 30-5 - C - 2MHz (stays same on reset) So, while 34-6 is H, the output of the OR gate (IC29) stays high and there is no clock on the CPU. But while reset is pressed, it goes low, allowing the 2MHz clock ...
by JannievanZyl
Sun Feb 04, 2018 8:12 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Kazzie wrote:
Could you check IC34 pin 6 and IC30 pin 5 as well while you're at it?

Give me two secs :)
by JannievanZyl
Sun Feb 04, 2018 8:12 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

From the results you posted, IC29 appears faulty. IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's cl...
by JannievanZyl
Sun Feb 04, 2018 8:00 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Kazzie wrote:From the results you posted, IC29 appears faulty.

IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low.

Let me recheck that.
by JannievanZyl
Sun Feb 04, 2018 7:59 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

From the results you posted, IC29 appears faulty. IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's cl...
by JannievanZyl
Sun Feb 04, 2018 7:43 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

From the results you posted, IC29 appears faulty. IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's cl...
by JannievanZyl
Sun Feb 04, 2018 7:40 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

From the results you posted, IC29 appears faulty. IC29 pin 8 is high, but that's the output of an OR gate, and the two inputs (IC33 pin 2 and IC31 pin 8 ) are low. This faulty high is gated through IC34 by the 2MHz clock signal, and then ORed with it by IC29 on pins 11-13, resulting in the CPU's cl...
by JannievanZyl
Sun Feb 04, 2018 7:35 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

In the datasheet extract that Dave posted, an important point is that it is a positive edge on the /RST (/RES) input that initiates the CPU reset sequence, not the input being low. Mark Also read that. So I suspect by this time the clock had fallen away. But I'll need to put a logic analyser on to ...
by JannievanZyl
Sun Feb 04, 2018 7:04 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

That would be cool, if possible Dave. OK, both these were captured with BREAK held down. The signals are the 16 address lines. About 50% of the time you get continuous activity like this, the other 50% of the time you get a static pattern. So I think it depends on what state the 6502 was in when re...
by JannievanZyl
Sun Feb 04, 2018 6:55 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

If IC23's output is static, the system should be stuck in either 1MHz or 2MHz mode. (Mark's suggestion of breaking S19 leaves pin 3 of IC23 floating. Assuming it floats down, it forces IC23 to request 1MHz mode continuously.) I suggested the middle pin of S19 (and therefore IC23 pin 3) be connected...
by JannievanZyl
Sun Feb 04, 2018 6:53 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Thinking about this a bit more. I wonder if the 6502 needs a valid clock input in order to correctly reset. The Z80 CPU does need a valid clock input in order to reset. If a Z80 has no clock signal, the address bus, control bus and data bus pins end up in undefined states (that may not confirm to t...
by JannievanZyl
Sun Feb 04, 2018 6:49 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Reading back through the thread, I don't think you've described which side of this fence the clock circuitry is sitting when it's locked up. Could you check what is on the following when you're not holding reset/break, when you get the chance? (High, Low, or Clock.) I'll check if it seems in a work...
by JannievanZyl
Sun Feb 04, 2018 6:40 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

In my recent bout of 6502 bus protocol analysis, I've been doing lots of captures that start with reset held down. I'm fairly sure I've seen cases of the 6502 "chattering" when reset is asserted, depending on what state the 6502 when this happens. If it's important, I could try to confirm...
by JannievanZyl
Sun Feb 04, 2018 6:09 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

1024MAK wrote:Pin 40 (/RST) low resets the CPU. For normal CPU operation, it should be high.

Mark

Yup, that's what I get.
by JannievanZyl
Sun Feb 04, 2018 5:52 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

1024MAK wrote:In this thread, there are details of a fault on the reset circuit.

Mark

Just measured again. Pin-40 of the CPU is high unless I push reset which makes it go low.

This is definitely correct, right?
by JannievanZyl
Sun Feb 04, 2018 5:47 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Unplugged the CRTC and still get the address bus activity when the CPU is in reset. At this point there's not much left on the board in the state where I get this bus activity in CPU reset. Just to confirm, from what I've read the system will still generate a banner in this state, correct? Yes, wit...
by JannievanZyl
Sun Feb 04, 2018 5:44 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Confirmed, with no CPU the add bus lines jump to about 1.5V and stay static. So, it really looks like activating RST on the CPU generates activity on the address lines. I've tested this CPU in a working beeb. Am I going nuts? :? No! Either the CPU is faulty, or more likely, there is a fault with th...
by JannievanZyl
Sun Feb 04, 2018 5:42 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Also useful reading (from 8bs.com/inbbc/html): (I can't guarantee all the following is correct, but it made sense to me as I wrote it.)......... Hopefully this will help you compare your Beeb's behaviour to what should be happening. Thanks Kazzie Will carefully go through this and trace. But to jus...
by JannievanZyl
Sun Feb 04, 2018 5:30 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Confirmed, with no CPU the add bus lines jump to about 1.5V and stay static.

So, it really looks like activating RST on the CPU generates activity on the address lines. I've tested this CPU in a working beeb.

Am I going nuts? :?
by JannievanZyl
Sun Feb 04, 2018 5:24 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Erm, what you got is a very strange fault :shock: What happens on the CPU address pins (well the socket pins) if you power up without a CPU? Mark Thank God nothing happens without a CPU! [-o< Otherwise I would've sprinkled holy water on the thing. :) After I saw removing the CRTC still gave add bus...
by JannievanZyl
Sun Feb 04, 2018 4:56 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Unplugged the CRTC and still get the address bus activity when the CPU is in reset. At this point there's not much left on the board in the state where I get this bus activity in CPU reset. Just to confirm, from what I've read the system will still generate a banner in this state, correct? beeb-5.jpg
by JannievanZyl
Sun Feb 04, 2018 4:32 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

Depends on which bit of the address bus you are monitoring. I see this on all 16 bits. I first noticed this on the input of IC24 (A5-A9) so checked the CPU and see it on the whole bus. The 6845 CRTC generates addresses for the DRAM, even when the there is no TV picture information to display. It is...
by JannievanZyl
Sun Feb 04, 2018 4:02 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

A question: I don't think you have said, but which issue PCB does this Beeb have? Mark Thanks Mark, this is an Issue 7. Will definitely try and get it all on 1MHz. But here's another question, please.....(think I'm losing my marbles here :) ) On a working beeb, if I look at the address bus it shows...
by JannievanZyl
Sun Feb 04, 2018 2:21 pm
Forum: hardware
Topic: 2 Faulty Beebs
Replies: 89
Views: 2286

Re: 2 Faulty Beebs

The BBC should have to switch between the 2MHz and 1MHzE clocks after reset to use the VIAs and CRT controller. As a result, I'm suspecting the 74LS74 flip-flops that glue the Video clock output to the processor. I'd check the relevant D and Q (/Q) of ICs 30, 31 and 34 to check if the clock signals...