Another black screen topic

Champion (377)

07-12-2016, 14:12

Ok, it turns out the point I was using as 5v for the logic probe was not really 5v.
So I found a proper 5v point and did some more testing.
z80:
address lines are blinking
data lines are blinking
refresh is blinking
m1 is blikining
reset is high, goes low when the reset button is pressed
busrq is high
wait blinks
busak is high
wr an rd blink
clk blinks
mreq and iorq blink
nmi high
int high
halt high
That all looks normal to me.

So I checked the bios rom:
After reset oe blinks for some seconds then goes high.
ce blinks
The address and data lines blink.

subrom:
oe is high for some seconds and then starts and keeps blinking.
ce blinks
The address and data lines blink

So it seems to start with the bios and the switches to the subrom which also makes sense i guess..

Then the vdp:
64 xtal 2 blinks
63 xtal 1 is low (or the frequency is to high for the logic scope as it's limit is 21mhz)
8 cpuclk blinks

So any clues ?

By PingPong

Prophet (3624)

07-12-2016, 22:21

YABST ;-)
I also suppose vdp is generating interrupts to cpu...
maybe you need to check if image is outputted using the different video outs of your msx.
(RF, SCART, CVBS)

By Philip

Champion (377)

07-12-2016, 22:56

The int pin on the z80 stays high, so that is actually not normal as I wrote before.
It seems to me either the vdp does not generate the interrupts (and is thus broken) or the communication between the cpu and vdp (to enable the interrupts) does not work somehow.
Tomorrow some more tests on the vdp (I've been reading the v9938 data book, so hopefully I understand somewhat how it talks to the cpu )

I just tested CVBS which also does not work. RGB I already tested and I could not find a antenna cable to test RF...

By RetroTechie

Paragon (1563)

08-12-2016, 11:48

Given the readings above, I'd suspect VDP or VRAM chips at this point. Maybe Z80 hangs in a subROM routine that initializes VDP and (tries to) display boot logo? You might want to take that logic probe to some VDP pins: (and report results)

9 /RESET (and see how it responds to pressing reset button)
31 /CSR
30 /CSW
62 /RAS
61 /CAS0
60 /CAS1
57 VRAM R/W select

By sd_snatcher

Prophet (3425)

08-12-2016, 14:03

The mainROM in this machine is socketed from factory, isn't it?

So, replace the mainROM with one EEPROM containing the "Hotbit v1.3" BIOS. This is available at the MSXpro website.

If then the machine boots properly, the problem is probably in the RTC circuit.

By Philip

Champion (377)

08-12-2016, 21:45

Today I almost bought this eeprom programmer (warning cheap chinese stuff :P ) but thought I would not need it for now :) The bios/basic rom in my hb-75p is also in a socket, I wonder if an msx1 bios would work in an msx2 ?

Anyway here are the results from the measurements (almost more exciting as hamlet's quiz...)
9 reset high, becomes low when reset is pressed
30 csw high, blinks once shortly after reset.
31 csr high
62 ras blinks
61 cas0 blinks
60 cas1 blinks
57 vram r/w select high

28 mode 1 blinks
29 mode 0 blinks

32,34-40 CD0-7 all blink

25 int high

By Alexey

Guardian (2989)

08-12-2016, 22:37

Btw, this is quite good EEPROM prorgammer despite being from China. Works like a charm for me...

By Philip

Champion (377)

08-12-2016, 23:44

Alexey wrote:

Btw, this is quite good EEPROM prorgammer despite being from China. Works like a charm for me...

Interesting... Which eeproms do you use with them ? I saw quite some winbond 27c512's on ebay, are those any good ?
Linux support would be nice, but one can't have it all I guess...

By Alexey

Guardian (2989)

09-12-2016, 12:30

I programmed many 2716, 2732, 2764, 27c128, 27c256, 27c512 (both UV erasable and electrically erasable, DIP and PLCC casing, both OTP and FLASH), Atmega 16 and 32 microcontrollers. Hell, it even managed to program some of the old Russian EPROM chips that require 25v while this programmer supports only 21v. Too bad I can't program 42 pin DIPs, but you can't have it all - there you are right.

Those Winbond chips work like a charm too.

By sd_snatcher

Prophet (3425)

09-12-2016, 22:31

Philip wrote:

The bios/basic rom in my hb-75p is also in a socket, I wonder if an msx1 bios would work in an msx2 ?

Since the NMS-8250 has a built-in memory mapper, it has to be initialized. And the HB-75P ROM won't do that. The Hotbit 1.3 BIOS is a community enhancement over the v1.2 that adds memory-mapper initialization, amongst many other enhancements. The keyboard layout will be incorrect for your machine, but that doesn't matter for this test.

Quote:

Anyway here are the results from the measurements (almost more exciting as hamlet's quiz...)

Try to check also if the /CS pin of the RP5C0A is blinking.