[Fixed!] Completely new to BBC hardware - Master with flashing cursor, no boot

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[Fixed!] Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Wed Mar 04, 2020 3:23 pm

Hi All,

I've just obtained a BBC Master via swapping some Atari gear with a friend, and I need advice on why it's not booting.

On first power up (probably for the first time in decades) it did the low then high beeps, then the power supply exploded.

Handily I had the forsight to order a cap kit which I've just fitted:
bbc01.jpg
and now it stays unexploded when powered on.
But I just get this screen:
bbc02.jpg
It either does the low then high beep, or what sounds like both beeps at once. The CAPS and SCROLL leds toggle on and off with the keyboard, but nothing appears on screen if I use any other keys.

reading the troublshooting PDF seems to indicated that it might be the teletext chip? But before I start snipping components I thought I'd ask, unless it's something simple.
All I have the the computer itself, no tape drive / floppy / expansion cards etc. I'm assuming the battery pack is long dead, it's three duracells shrinkwrapped.
Also, everything is soldered to the board (bar one chip) so nothing appears to be loose:
bbc03.jpg
Advice on what to check next gratefully received :)

slight edit: voltages all read +5.12v and the negative -5.10v, all stable.

edit edit: If I toggle CAPS off, then press break, there is a slight flicker of the cursor and the CAPS LED comes back on
Last edited by spiny on Thu Mar 12, 2020 6:44 pm, edited 1 time in total.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by SteveBagley » Wed Mar 04, 2020 3:43 pm

Turn it on while holding down the 'R' key to reset the CMOS configuration -- that should (hopefully) bring it up in a working state.

Steve

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by marcusjambler » Wed Mar 04, 2020 3:44 pm

That behaviour can be indicative of a keyboard issue... Do you have access to a known working one?

Marcus

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Wed Mar 04, 2020 3:53 pm

marcusjambler wrote:
Wed Mar 04, 2020 3:44 pm
That behaviour can be indicative of a keyboard issue... Do you have access to a known working one?

Marcus
Alas, not to hand, though I may be able to borrow one, however, the keyboard does seem to respond, pics in next message
Last edited by spiny on Wed Mar 04, 2020 3:59 pm, edited 1 time in total.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Wed Mar 04, 2020 3:54 pm

SteveBagley wrote:
Wed Mar 04, 2020 3:43 pm
Turn it on while holding down the 'R' key to reset the CMOS configuration -- that should (hopefully) bring it up in a working state.

Steve

Holding down R when powering up doesn't appear to do anything, but if I do a CTRL+SPACE+Break, and type HELP followed by return I get this:
bbc04.jpg

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Wed Mar 04, 2020 5:11 pm

I've more experience in poking a Model B into life than a Master, but the way those characters repeating in a regular manner across the screen makes me suspect an issue around the RAM addressing.
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 9:14 am

Some more info from this mornings testing.

Reading other threads shows that holding various keys down should boot into different setups.

If I hold F when powering up I get the same random test on screen, but I can use the keyboard, and have characters display, sort of.

If I press a key, nothing shows, but when I press another key, or the same key, the previous character I typed is show, eg in this screenshot I typed AABBCCDD etc. Characters stop displaying after about 20 keypresses.

No idea if this helps, but I have zero knowledge of BBC stuff, I'm an Atari guy :)
bbc06.jpg

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 9:15 am

Kazzie wrote:
Wed Mar 04, 2020 5:11 pm
I've more experience in poking a Model B into life than a Master, but the way those characters repeating in a regular manner across the screen makes me suspect an issue around the RAM addressing.
Any tips on how to troubleshoot this ?

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 9:22 am

This is me typing A to Z then 1 to 0
bbc07.jpg


Also, if I leave this powered on for five minutes or more, the CAPS and SCROLL LEDs light up dimply, but still respond with a keyboard toggle, but I get no respoinse from hitting return ot any other keyboard input. BREAK does seem to reset though.

And, the left hand chip in this pair , IC42, is -much- hotter than the other one
bbc08.jpg

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Thu Mar 05, 2020 10:21 am

That pattern from pressing A-Z and 1-0 is interesting, alternate characters repeated twice, and then starting again at A 32 locations forward. Those are all powers of two, so that may help indicate what's at fault.

When you pressed 'A', did all the 'A's on the screen show up immediately, or after subsequent keypresses?

A loot at http://www.8bs.com/in128.htm tells me that IC42 (on the right in your photo) is the Video ULA. The Video ULA in the Model B runs hot under normal operation, earlier versions were even shipped with a heatsink. I don't think that IC42 being warm is indicative of a fault. But do let us know if you notice anything else warm.

I'll put my thinking cap on, and get back to you later today once I've had the chance to look at the Master's circuit diagrams a bit.
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 10:46 am

Kazzie wrote:
Thu Mar 05, 2020 10:21 am
That pattern from pressing A-Z and 1-0 is interesting, alternate characters repeated twice, and then starting again at A 32 locations forward. Those are all powers of two, so that may help indicate what's at fault.

When you pressed 'A', did all the 'A's on the screen show up immediately, or after subsequent keypresses?

A loot at http://www.8bs.com/in128.htm tells me that IC42 (on the right in your photo) is the Video ULA. The Video ULA in the Model B runs hot under normal operation, earlier versions were even shipped with a heatsink. I don't think that IC42 being warm is indicative of a fault. But do let us know if you notice anything else warm.

I'll put my thinking cap on, and get back to you later today once I've had the chance to look at the Master's circuit diagrams a bit.
doh, confused my right and left, yes, the -right hand side- chip is hotter, if thats expected behaviour, then thats fine :)

Character display is odd, it's doing this:

pressing A shows two A on screen, pressing B moves the cursor but shows nothing, pressing C shows two C, pressing D moves but shows nothing, pressing F shows two F and so on.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Thu Mar 05, 2020 11:46 am

spiny wrote:
Thu Mar 05, 2020 10:46 am
Character display is odd, it's doing this:

pressing A shows two A on screen, pressing B moves the cursor but shows nothing, pressing C shows two C, pressing D moves but shows nothing, pressing F shows two F and so on.
:arrow: So pressing 'A' makes two 'A's display side-by-side immediately? Upthread you suggested that nothing appeared until you did a second keypress.

I'm also interested as to when the extra pairs of 'A's show up, those that are 32 characters along the row.

-------

Having had a dive through the circuit diagrams, it seem that a lot of your Master must be working correctly. The Shift and Caps Lock keys are detected by the same keyboard controller chip (IC16) as every other key (bar Break), and news of the keypress is sent to the CPU through a 6522 VIA (IC8). The same VIA controls the switching on-and-off of the LEDs, but it can't do so on its own, it only executes commands given to it by the CPU. So there's every chance that this fault is confined to the video display circuitry somehow. It's not enough to crash the CPU immediately on boot, anyway.

Drifting back to the earlier screenshot you posted, after running *HELP, the "SS3322" would correspond to alternate characters of the string "OS 3.2" which should be the first line of HELP's output. Likewise "iiwwhhee 1100" may correspond to "Viewsheet 1.0" or similar. It may be notable that it's the even characters that are repeated here, rather than the odd ones as with your A-Z test. Or it may be that there's an initial "carriage return" in HELP's output which gets repeated first, so 'O' is actually the third character in the sequence.*

From the number of characters across your screenshots, I suspect your Master is booting to Mode 0. (It appears to be 80 characters across.)

:arrow: Could you try switching to other MODEs, and see what happens? I'd be particularly interested in Mode 7, as it uses the teletext chip instead. If you can do so, please try typing some characters, and grab a picture of how it's displayed onscreen.

*Edit: the lowercase characters showing up in the A-Z,1-0 screenshot you posted may be the "ssaaee" from "Mistake", when you typed something in and then pressed Return.
Last edited by Kazzie on Thu Mar 05, 2020 11:51 am, edited 1 time in total.
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 11:50 am

Kazzie wrote:
Thu Mar 05, 2020 11:46 am

:arrow: Could you try switching to other MODEs, and see what happens? I'd be particularly interested in Mode 7, as it uses the teletext chip instead. If you can do so, please try typing some characters, and grab a picture of how it's displayed onscreen.
just to check, I need to type

*CONFIGURE MODE 7

to do this yes? I last used a BBC in school, and that was about 30 years ago :)

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Thu Mar 05, 2020 11:53 am

spiny wrote:
Thu Mar 05, 2020 11:50 am
Kazzie wrote:
Thu Mar 05, 2020 11:46 am

:arrow: Could you try switching to other MODEs, and see what happens? I'd be particularly interested in Mode 7, as it uses the teletext chip instead. If you can do so, please try typing some characters, and grab a picture of how it's displayed onscreen.
just to check, I need to type

*CONFIGURE MODE 7

to do this yes? I last used a BBC in school, and that was about 30 years ago :)
Just MODE 7 will be enough to change the display immediately. As I've said, I'm no expert on Masters*, but I think the command you've typed would store settings in the CMOS so the Master would boot to Mode 7 on every boot. (In a Model B, there are physical switches on the keyboard PCB that you'd use instead.)

(*I saw one in a Church Hall once as a teenager, and that's as close as I've come to one!)

(Also, note a minor edit to my earlier post while you were typing.)
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 12:02 pm

Kazzie wrote:
Thu Mar 05, 2020 11:46 am

:arrow: So pressing 'A' makes two 'A's display side-by-side immediately? Upthread you suggested that nothing appeared until you did a second keypress.

I'm also interested as to when the extra pairs of 'A's show up, those that are 32 characters along the row.
I did a vid, as it's just easier :)

I booted holding D
Then slowly type A to Z then 1 to 0
I then hit return
Then I typed @ROMS and hit return

https://youtu.be/nAF4ZRh4Luw

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Thu Mar 05, 2020 1:28 pm

spiny wrote:
Thu Mar 05, 2020 12:02 pm
Kazzie wrote:
Thu Mar 05, 2020 11:46 am

:arrow: So pressing 'A' makes two 'A's display side-by-side immediately? Upthread you suggested that nothing appeared until you did a second keypress.

I'm also interested as to when the extra pairs of 'A's show up, those that are 32 characters along the row.
I did a vid, as it's just easier :)

I booted holding D
Then slowly type A to Z then 1 to 0
I then hit return
Then I typed @ROMS and hit return

https://youtu.be/nAF4ZRh4Luw
Great, thanks. And the ability to slow down video on Youtube is handy too :)

To type out my own narrative of what's happening:
  • The cursor starts off in the second column, the '>' prompt is not displayed
  • On pressing 'A', an 'A' appears in columns 1 and 2, cursor advances to column 3
  • On pressing 'B', the cursor advances to column 4, but column 3 remains blank
  • On pressing 'C', a 'C' appears in columns 3 and 4, and the cursor advances to column 5
  • ...
While these characters appear on row 7 (where the cursor is), they also appear in the same columns of row 5, columns 17 and onward of row 2, columns 33 onward of rows 3 and 5, columns 49 onward of row 6, and columns 65 onward of rows 1 and 3.

As the display mode is currently 80 characters wide, counting character by character, row by row, from the top left (location zero), the pairs of 'A's show up at locations 64, 96, 192, 224, 320, 352, 432, 480. That's not showing much of a pattern in decimal, but if we convert those numbers to hexadecimal, we have:
  • 0x040, 0x060, 0x0C0, 0x0E0, 0x140, 0x160, 0x1B0, 0x1E0
which makes the pattern a little clearer. The video circuitry is displaying duplicates at eight locations, where the second hexadecimal digit is 4, 6, C, or E. And that duplication goes on for 32 characters (from 0x40 to 0x5F). Shoving those four hex digits into binary gives us:
  • 4: 0b0100
  • 6: 0b0110
  • C: 0b1100
  • E: 0b1110
(Note that those bits are for the second hexadecimal digit, so would correspond to bits 7-4 of a memory address.)

So it seems that when the video circuit requests screen data from RAM at an address where bit 6 is set, the data obtained is from the memory at an address with bits 7, 6, and 5 set, whatever the requested values of bits 7 and 5. (Bit 4 is unaffected, as you need five bits (0-4) to describe the 32 characters that are shown.)

Additionally, when requesting the contents of an even address (where bit 0 is zero), the data returned is the contents of the adjacent odd address (where bit 0 is one).

------

So that feels like a pretty comprehensive explanation of the symptoms. The next question is: what could the cause be?

Beebs use a 6845 CRT controller chip to automate the task of fetching sequences of memory locations for the video circuit. The chip is configured directly by the CPU, and also generates the horizontal and vertical sync signals to generate a stable picture on your screen. That bit is working fine, so the CPU is clearly able to talk to the 6845.

The Master's 6845 (IC22) generates 18 address signals (MA0-13 and RA0-3) which are fed to IC31, a custom chip (CF30048) that buffers and rearranges these signals according to whatever screen Mode is currently being used. It then requests the contents of the relevant address from the RAM chips (at the appropriate time), which presents that information on the data bus. The data bus is read by the video chip (IC42, or IC21 and IC32 if in Teletext Mode 7) and used to generate the video signal.

:arrow: My suspicion is that either IC22 or IC31 is faulty. I don't believe the RAM chips are faulty, for reasons I'll explain in a footnote below.*

There are fourteen testing points on the board (numbered TP7-20) near the 6845 (IC22), which correspond to several of the MA and RA lines between the 6845 and the CF30048.

:arrow: If you have a voltmeter, logic probe, or oscilliscope, could you take a reading at each of these points, and report back? I'm not too sure what I'd expect to see, but there may be a smoking gun in the results.

:arrow: Additionally, I'd still be interested in whether you can change Modes with the command MODE 7, and how the display responds to keypresses afterward.



*The Master 128 uses a pair of 4464-type DRAM chips for main memory (including display memory), and another pari for sideways RAM, which we'll ignore. Each chip provides four bits of data for all 64K different addresses in system and (shadow) display memory, so two chips together are needed to provide 8 bits of data for the CPU etc. The 6502 family of processors use the very bottom of RAM for storing system variables, and the OS typically can't boot unless that RAM is working correctly. In a Model B, separate banks RAM chips are used for the CPU's bottom RAM and the upper RAM used for display memory, so it's possible for a faulty chip to corrupt the display without affecting the OS. But the Master uses a single bank for all addresses. RAM faults can be fairly odd, and sometimes affect only some addresses. But if your fault were to be caused by faulty RAM, both chips would have to have the same error on upper addresses, while working fine on the lower ones. The odds on that are fairly long.
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 2:37 pm

Nice info :)

I have a cheap 'scope but I'm not 100% on how to use it, but I''l do a vid in a minute

These are results from probing with my meter:

pins are in right to left, top to down order, as located on the board

19: starts at 2.00v but drops when measured to around 0.6v, takes about 20 seconds to drop
15: 1.95v stable
11: 1.99v stable
8: 2.00v stable
7: 2.02v stable
9: 2.02v stable
12: 2.03v stable
16: 2.03v stable
20: 0.035v stable
10: starts at 0.5v and drops when measured to zero, takes about 20 seconds to drop
13: 2.02v stable
14: 0.1v stable
17: 0.09v stable
18: 2.06v stable

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Thu Mar 05, 2020 3:57 pm

spiny wrote:
Thu Mar 05, 2020 2:37 pm
Nice info :)

I have a cheap 'scope but I'm not 100% on how to use it, but I''l do a vid in a minute

These are results from probing with my meter:

pins are in right to left, top to down order, as located on the board

19: starts at 2.00v but drops when measured to around 0.6v, takes about 20 seconds to drop
15: 1.95v stable
11: 1.99v stable
8: 2.00v stable
7: 2.02v stable
9: 2.02v stable
12: 2.03v stable
16: 2.03v stable
20: 0.035v stable
10: starts at 0.5v and drops when measured to zero, takes about 20 seconds to drop
13: 2.02v stable
14: 0.1v stable
17: 0.09v stable
18: 2.06v stable
Great work. The first five testing points correspond to MA0-MA4, the next four are RA0-3, then MA5-9.

All the voltages that are stable around 2V sound fine to me. The DC voltage will be an average of the voltage over time, and a changing signal that spends roughly half it's time around 0V, and half around 4~5V, will average out at 2~2.5V.

The voltages that are stable near 0V could be fine, depending on how the Master configures its 6845 chip. I'm afraid I don't know on that.

The two points where you find a drooping voltage are interesting. They correspond to MA0 and MA5. We've discovered that the video circuit can't read display memory addresses where bit 0 is meant to be zero, it gets the contents where the bit is one instead. Bit 5 is one of the bits that was implicated in the whole "repeating all over the screen" nonsense, too.

Most (all?) 6845 chips are built using NMOS technology. Their outputs have a resistor that pulls every output voltage high, unless the transistors inside shorts the output to ground by "pulling down". They normally transition quickly from high to low output, but are slower going low-to-high. The slow droop you've measured makes me think that something's broken inside, and it's no longer able to hold the output voltage high as it should.

The MA/RA bus is only connected to the 6845 (IC22) and the CF30048 chip (IC31). When everything's working, the 6845 drives the bus (outputs the voltages), and the CF30048 reads the bus as its inputs. It may be that when you ask the 6845 to drive your voltmeter as well, it's no longer able to shift enough current to meet the demand, and so you see the voltage tail off. (Even when your voltmeter isn't attached, it may not be able to meet the demands of the CF30048 anyway, leading to our video corruption.)

I reckon it's more likely that the 6845 is faulty than the CF30048, as if the CF30048 was pulling certain wires' voltage down (or internally disconnected from them) then its influence would already have taken effect before you touched your probe to the testing point, so no droop (or there'd be no change at all, because the chip wasn't electrically connected to the testing point any more).

:arrow: It would be a good idea to check that the 6845 is getting a good supply voltage (pin 20, 5V), just in case it's being starved of electrons somehow.

Other than that, I'd be leaning toward trying a replacement for the 6845. A few notes of caution on that course of action, though:
  • Desoldering a 40-pin chip is a big job for a novice, as you have to get every pin desoldered before it'll move an inch. Some forum members have recently shared some advice on how to remove faulty chips in a recent thread, here
  • There are several varieties of 6845 chip, with each manufacturer making them to slightly different specifications. You may want to ask members here for advice on which variants will be fully compatible, or if they have any for sale. (Retroclinic (Mark) is one of several members who stock components for Beebs. If they don't stumble across this thread, you could make a post in the 'Wanted' subforum.)
  • It may be a good idea to hold off a bit before digging out a soldering iron and snips, in case someone has a differing opinion. My diagnosis is based on experience with a Model B, and a copy of the Master's circuit diagram - not hands-on experience of fixing Masters! :D
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 5:25 pm

I'm not sure which way to 'read' the pins, but if it's this one, (reading 1,2,3 etc along the bottom row) then yes, it's getting 5.02v
bbc09.jpg
but if it's this one (all lower pins odd, upper pins even) , it's only getting 2.02v
bbc10.jpg
I'm reasonably OK with a soldering iron, I've replaced soldered 68000 chips in various Atari STs with socketed versions, my method was to snip everything off carefully, then pull the legs in turn using fresh solder.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Thu Mar 05, 2020 5:32 pm

DIP chips are numbered counter-clockwise from the "Pin 1" notch/mark, so your first photo, and the reading, are correct.

If you've wrangled 68000s, you should be okay with a piddly 40-pin chip! 8)
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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by 1024MAK » Thu Mar 05, 2020 8:28 pm

Some additional information:

The Master stores various configuration settings in the CMOS RTC/SRAM chip. If there is no back-up battery supply (missing battery or flat battery), this chip will forget the stored settings a short time after the machine is switched off.

When you now power on, the OS will assume that the now corrupt settings are valid and hence various strange affects can occur depending on the state of the relevant RAM memory locations. This includes the option to boot an autoloading file from disk, the default screen MODE on start-up, the keyboard repeat rate, plus many others. The full list is in a link below.

When you ask the OS to reset the contents of the CMOS RTC/SRAM chip, MOS 3.2 unhelpfully puts less than useful values in the chip. Hence you need to manually update the configuration values to more useful values. MOS 3.5 does however put more useful values into the chip. You can shorten the command “CONFIGURATION” to “CO.” :wink:

So to save messing about, don’t switch off, but see if CONTROL - BREAK works (press and hold CONTROL while pressing BREAK, then release BREAK, then release CONTROL). This should reset the machine without losing the configuration settings.

If there is a problem with the display, it is also worthwhile trying the shadow screen MODEs. These are the same as MODEs 0 to 7, but use a different area of RAM. Add 128 to the MODE number to use a shadow screen MODE. For example, shadow screen MODE 128 is MODE 0 but using shadow RAM.

Links:
http://beebwiki.mdfs.net/Master_128

http://beebwiki.mdfs.net/Address_translation

http://beebwiki.mdfs.net/MODE

http://beebwiki.mdfs.net/CMOS_configura ... allocation

How to reset and configure a Master

Mark

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Thu Mar 05, 2020 10:31 pm

1024MAK wrote:
Thu Mar 05, 2020 8:28 pm
Some additional information:
Cool, I'll try this in the morning - the machine is in my office at work :)

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Fri Mar 06, 2020 12:01 pm

OK, I've remade the battery pack, just because it's easy to do.
bbs12.jpg
bbc13.jpg
And now some info on how I'm booting the machine.

If I just power on, I just get the white cursor screen, no keyboard input displays on screen.

If I power on holding R, the same, just a white cursor, no keyboard input displays on screen

If I power on with either F or D held dowm, I can type stuff.

But MODE commands do not appear to do anything, this is me typing MODE 128 then hitting RETURN:
bbc11.jpg
Same if I type CONFIGURATION, nothing new appears on screen.

Though the machine appears to respoding to other commands, I think anyway, if I type *ROMS and hit RETURN, I get the following, sometimes with diferent characters, but always with the text layed out in the same place. Could two of the lines be trying to say 'unplugged' perhaps ?
Note: I can't reliably repeat this, the text is usually garbage, but one time out of twent it shows this:
bbc14.jpg
cheers, Phil.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by 1024MAK » Fri Mar 06, 2020 2:07 pm

If the machine has not been instructed to use a language ROM, it will default to a simple command line interface only. I can’t remember if this CLI will accept a MODE command.

But BASIC will accept a MODE command, and it is recommended that the machine is set to use the BASIC ROM, otherwise when you type a BASIC command, you will just get an error message.

Either type:
CO. LANG 12
(if that does not work, use *CO. LANG 12 )
Then press CONTROL BREAK
which should set the machine to automatically start up in BASIC if the standby power from the battery pack is working (assuming no other faults).

You can then type MODE etc.

OR if you can’t get that to work, you can type
*BASIC
to tell the machine to switch to BASIC.

You can then type MODE etc.

If neither of those work then there is another problem.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Fri Mar 06, 2020 3:19 pm

OK, this is more promising.

typing *BASIC seems to do something, in that it does a carriage return, and then typing MODE n does seem to change the screen res, I did a video:

https://youtu.be/zGnp1V0BBSs

apologies for vertical filming and shaky camera, my phone is resting on a diskbox so I could type with both hands.

Whats happening in the vid:

I type *BASIC and hit return
I then hit return a few times to get the cursor visible
I then type MODE 1 and hit return a few times
then Mode 2
Mode 3 and so on
Each time the cursor seems to change size, so it looks like the screen res is changing, but I can't see anything I type.

Extra info: I'm using composite out, piped to a cheap composite to VGA converter, connected to a monitor that can do oldschool 15hz refresh rates (It's a monitor that works with Atari ST Low resolution for example)

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Fri Mar 06, 2020 3:34 pm

OK, more investigation, and another shaky video:

https://youtu.be/GpmPLTft07o

I'm typing:

*BASIC <return>
MODE 5 <return>

then typing the age old test prog:

10 PRINT "HELLO" <return>
20 GOTO 10 <return>
RUN <return>

and thats when the screen scrolls, but with garbage text.

it stops when I hit <escape>

I then typed some other stuff, LIST then RUN again to restart the prog.

So, it looks like the BBC is working, but I just can't see what I'm typing. Any pointers from here ?

Does it look like a problem with the chip in IC22 ?

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by Kazzie » Fri Mar 06, 2020 3:57 pm

It's good to see that there really is a working Beeb behind that veil of darkness. :)

Could you put together a similar video in Mode 7? I'm more familiar with how that display mode operates than I am with Mode 5 (plus it uses a slightly different set of chips).

P.S. The control signal for when (thus where) to draw the cursor is sent directly from the 6845 (IC22) to the video chip (IC42), so it bypasses the RAM and buffers.
BBC Model B 32k issue 7, Sidewise ROM board with 16K RAM
Archimedes 420/1 upgraded to 4MB RAM, ZIDEFS with 512MB CF card
Acorn System 1 home-made replica

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Fri Mar 06, 2020 4:12 pm

Kazzie wrote:
Fri Mar 06, 2020 3:57 pm
It's good to see that there really is a working Beeb behind that veil of darkness. :)

Could you put together a similar video in Mode 7? I'm more familiar with how that display mode operates than I am with Mode 5 (plus it uses a slightly different set of chips).

P.S. The control signal for when (thus where) to draw the cursor is sent directly from the 6845 (IC22) to the video chip (IC42), so it bypasses the RAM and buffers.
sure:

https://youtu.be/iFjbK1jOCE4

same deal, into BASIC (the text on the screen almost looks like a BASIC prompt too)
then MODE 7
<return> a few times
10 PRINT "HELLO"
20 GOTO 10
RUN

I think I typo'd though as I had to type RUN and <return> twice before the screen scrolls

<escape> stops, and I add a few more lines, try a LIST etc then RUN again.

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by 1024MAK » Fri Mar 06, 2020 5:53 pm

Try MODE 135

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Re: Completely new to BBC hardware - Master with flashing cursor, no boot

Post by spiny » Fri Mar 06, 2020 6:07 pm

1024MAK wrote:
Fri Mar 06, 2020 5:53 pm
Try MODE 135

Mark
I tried with all the mode +128 modes, but they all look the same as the normal mode, i.e. MODE 135 shows the same as MODE 7

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