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A Timex TS1000 is definitely not the first thing that I would use as a video source. Nothing against the cheapest ever Z80 based computer (I have a TS1000 and various ZX81s...), but... It uses a ceramic resonator (normally used as part of a filter) instead of a quartz crystal, so it’s master clock f...

I think it is highly unlikely that these are preproduction models. Firstly, because of the PCB revision and the date codes on the chips used. Secondly, there are too many about. Preproduction models are often produced in very low numbers, and often don’t survive for long before being scrapped. As th...

Could it be some sort of light pen interface? No. The standard Beeb already has a light pen input on the analogue port. Some of the text on the bottom of the PCB (maybe home-made using a photocopy from a magazine?) is legible: VID IN Frequency set U -> ... pin 10 of the 74LS00 but not connected V -...

What is the condition of the 6502 socket? What is the condition of the socket for IC51 (the OS ROM)? Do you know if this machine has ever had any ROM, RAM/ROM or any other internal expansions fitted? If you think that the socket contacts are okay, or especially if you think there may be problems, th...

Yep, makes sense. I can see how it works. Not really sensible though is it? So, If I had 4x ROMS that I put onto one of the 512 IC's one after the other, does it see them without mods, or do I need to make changes to the board first? On a standard BBC B, the 6502 won’t be able to access (see) anyth...

I'm sure Mark will correct me if I'm wrong here, but I think when programming Tricky's Test ROM into a 27C512 it needs to be placed at offset 0xC000 within the EPROM, not 0x4000. This is because a 27C512 is a 64KB EPROM, and both A14 (pin 27) and A15 (pin 1) are connected on IC51 on the Model B. Da...

The OS ROM socket (IC51) is different. In that pin 27 is not connected to +5V/VCC on all machines (unless it has been modified). But instead pin 27 is connected to the main address bus, in fact to the 6502 A14 line. Despite this, as the OS ROM socket is only selected when the 6502 A14 and A15 are hi...

Unless the 6845 stops generating addresses (which refreshes the DRAM), the 6502 should continue running even if the display system stops producing a video output. When the fault occurs, can you probe all the pins on the 6502, as that may give a clue as to what’s happening (or not). Then test the DRA...

The important point here is the way that a Beeb accesses the EPROM or EEPROM. The Beeb can only ever see a 16k byte area (the sideways ROM area) no matter how large or small the actual EPROM or EEPROM is. Modern programmers can however see the whole memory area of the EPROM or EEPROM. The size of th...

With respect to just reading the normal ROMs and the normal EPROMs as used in a BBC B, selecting any manufacturers 2764 will read the 8k byte (64k bit) chips. Selecting any manufacturers 27128 will read the 16k byte (128k bit) chips. You only need to match the middle part of the chips part number. I...

No, the divide by 10 (10X) position is better than the 1X because it reduces both circuit loading (increases the ‘scope impedance) and reduces the extra combined capacitance from the probe, lead and ‘scope input capacitance. Whereas the X1 switch position is just a straight through connection that d...

What they said above, but the ballpark values of most pull-up resistors are between 2.2kΩ and 4.7kΩ where the stray capacitance is high, assuming the chips on the bus can drive the lines down to at least 0.5V. So for a 2.2kΩ pull-up resistor, the outputs need to sink 2mA to 0V/GND. For a 4.7kΩ pull-...

The HN4827128G needs 5V at 100mA and 21V at 30mA during programming. That’s a total of 1.13W of power. The M5L27128K needs 6V at 100mA and 21V at 30mA during programming. That’s a total of 1.23W of power. The uPD2764 needs 5V at 100mA and 21V at 30mA during programming. That’s a total of 1.13W of po...

The issue is not just the rated programming voltage of the programmer, but also it’s ability to supply enough current (power) at this voltage without the voltage dropping. To step up a USB 5V supply to 21V at the current (power) required needs a very good DC-DC converter. Unfortunately if the manufa...

Inside is actually worse. How many infractions of the UK electrical code can we find? That’s normal for items made in China for export... Mark A little surprised Amazon are supplying them from a UK shipping address in this fashion though Not had that before. Can feel a call/email to trading standar...

Also note that on the Acorn BBC B and Master 128, some of the domino 5 pin DIN plug/sockets do not have any physical polarisation, so the plug can go in the wrong way. No damage will be caused, but it won’t work.

Mark

And me please!

Mark

Well, I'm utterly stuck with these. The UV seems to blank them perfectly as the programmer now sees them as blank (and reports as such). But can I get them to write? Nope. Just getting errors. This sucks. I presume you are talking about your 2764 EPROMs. Are they 12V, 21V or 25V devices? Some USB p...

Colday wrote: ↑

Sun Aug 09, 2020 8:15 pm

Inside is actually worse.

How many infractions of the UK electrical code can we find?

That’s normal for items made in China for export...

Mark

Are the probes you are using switched to the divide by 10 position? Have you calibrated then? What are you using for the 0V/GND connection? These things can affect the displayed waveforms. Aside from the dodgy waveforms, IC45 pins 9 to 12 do look to be attempting to work. Do you have an EPROM progra...

Have you tried a minimal configuration?

Keep in mind that with a minimal configuration it’s normal to get a continuous sound. And the keyboard won’t work.

Mark

Good that you nailed it in the end

Mark

Hi

Can you please add me for the afternoon session only. I can only join for a couple of hours between around 3pm and around 5.30pm.

Mark

Encoding the nTUBE, nPGFC and nPGFD to two lines can be done by a 74LS148, 74HCT148 or maybe a 74HCT147. I’ve not looked at propagation times though. The DATA nOE can be derived from a single three input AND logic gate (e.g. 74LS11, 74HCT11 or similar), as the Beeb’s address decoding will not allow ...

We have a topic for that! See here

Mark

The legend next to D15 says "2xIN414148" in the user guide, but "2xIN4148" on the Tribbeck and original circuit diagrams. Do please note that the part number is '1'N4148 Dave H. For those that are not aware, excluding the markings used on surface mounting devices/components, the vast majority of pa...

I’m glad you got it going

Have fun

Mark

Here are some past topics where this subject has come up:

Power connectors on a model B mobo

BBC Micro Issue 7 board

Model B female spade power connectors

There are more, but the above should be sufficient.

Mark

VCC +5V GND ground power spade terminal terminals connector connectors

This subject has come up before, somewhere in these topics, there is either a Farrell or a RS part number.

For small numbers, Mark of RetroClinic may have some.

Mark

How long before we get USB10 to go with Windows10

Mark