Which ROM for Minstrel 3?

[8008guy]

Hi Ien,

did you try out "grey border" jumper. Even if Processor is in reset state this should give a grey output, so shift register and things around that can be tested.
If reset goes to high(check if it does) there should be some data seen in D0-D7.

Next you may pull ram and eprom out of the board and pull each data line to ground using a separate resistor per line(something between 1k and 10k).
So CPU may execute NOP operations. Signals on M1, MREQ and RFSH are expected to be visible.

Best regards,

Max

When I select the "grey border" I can see a change on the screen, though not in the form of a border. It appears pixelated.

I am getting MREQ and RFSH signals. Also I can see data on D0/D1/D2 lines.. (I ran out of inputs)

[8008guy]

Hi Len

When you say the screen lights up, do you mean the display goes to a white colour (assuming a colour display)? If not, could you either take a photo or give a more detailed description.

What signals do you get on the following pins of the Z80 CPU?
Be very careful, if you short the probe between adjacent pins you may damage one or more of the chips.

Pin Function
16 /INT (should be pulsing)
17 /NMI (should be pulsing)
18 /HALT (should be pulsing)
19 /MREQ (should be pulsing rapidly)
21 /RD (should be pulsing rapidly)
22 /WR (should be pulsing)
24 /WAIT (should be pulsing)
25 /BUSRQ (should be high)
26 /RESET (should be high, except for a very short time after power is applied)
27 /M1 (should be pulsing)
28 /RFSH (should be pulsing)

Then test address lines A14 and A15. These give an idea of which memory the CPU is accessing and if it is trying to generate a TV display.
You can also test the remaining address lines A0 to A13 and the data lines D0 to D7. A lack of activity is suspicious.

Next, test the /CE (also known as /CS ) pin on the RAM chip (pin 20). Then test the /CE (also known as /CS ) pin on the EPROM chip (pin 20). Both should show activity.

Z80 pin-out here.

Mark

CPU signals as you describe, except /HALT always stays high

[1024MAK]

It doesn’t look like it is following the normal run loop, as /IORQ never changes state.

Definitely need to see A14 and A15 and A0 to A7...

As the TV picture is entirely generated by the CPU executing the display file, with a little help from the hardware logic chips, if it is not running the normal run loop, you will never get a display.

Mark

[8008guy]

It doesn’t look like it is following the normal run loop, as /IORQ never changes state.

Definitely need to see A14 and A15 and A0 to A7...

As the TV picture is entirely generated by the CPU executing the display file, with a little help from the hardware logic chips, if it is not running the normal run loop, you will never get a display.

Mark

I see action on A14, not A15

[8008guy]

Looking at the IORQ it appears to always stay low

I put a meter on the line to check for a ground, with the power off it reads in the megs. The connection between the cpu pin 20 and pin 5 of IC 19 is 0 ohms. So it is not open or shorted to ground.

What would cause the Z80 to keep that low?

[1024MAK]

/IORQ (pin 20 on the Z80 CPU) should normally be high, as it is an active low signal. The Z80 takes it low to signal that the current operation is an input or output to a I/O port, and hence the address on the address lines is the required I/O port address.

Contrast this to /MREQ, which is normally high, but goes low when the Z80 CPU either reads from memory (ROM or RAM) or writes to memory (RAM).

The /IORQ line should go low whenever the the machine is reading the I/O port that scans the keyboard for key presses and to see if 50Hz or 60Hz screen mode should be used. In addition it is used for certain display signals.

Mark

[1024MAK]

If A15 is always low, then the system is not even trying to display a video picture.

Can you please recheck that your EPROM is correctly programmed (get the programmer to read back the ROM, then use a suitable program on your PC to compare to the file you downloaded).

Mark

[1024MAK]

Can you also please check what you get on the Z80 clock input, pin 6.
And check that the /RESET line (Z80 pin 26) changes to low (briefly) before going back to high when the reset button is pushed and released.

Mark

[8008guy]

Hi, and thanks everyone!!!

The Minstrel is working. The answer was EPROM. So for some reason after a windoz update my programmer appeared to be working, but was running in a demo mode. After a day of soldering and stuffing I failed to notice that. It even looked like it was verifying the EPROM. I uninstalled and reinstalled my programmer SW, programmed the chip, and when I plugged in the EPROM and it worked perfectly!

I really appreciate everyone's help. For me it was a really good learning experience into understanding the "guts" of the machine.

Cheers,

len

[mrtinb]

And again Mark’s invaluable help, knowledge and kindness to help.