External 3.5" floppy only works one sided

[ajcasado]

Hi,

I have a problem with the external floppy, this happens with four different drives, so I guess that is a problem with the floppy management hardware of my 664.

The issue is that the side select signal doesn't change. If I measure the voltage on /S1 (pin 3 of the external floppy connector) is always ~0V. In Parados I have to select single sided head 1 for the drive to work properly. If I disconnect the /S1 signal from the connector then I have to select head 0 in Parados. With /S1 disconnected the floppy /S1 pin is at 5V and the CPC /S1 pin is at 0V.

Looking at the schematic I guess that the problem could come from the chip IC206, that seems to works as a NOT gate for this signal. I don't have a 74ls38 for test this guess, but I have several 74ls04. It's a good idea to try to bypass the '38 with the '04 to drive directly the external floppy connector pin 3?

[Nworc]

It's a good idea to try to bypass the '38 with the '04 to drive directly the external floppy connector pin 3?

Maybe for testing you can try that for a short period, but I wouldn't do that as it's not a good idea, for two reasons:
1) The NAND gate works as a driver, to protect the 765 FDC and to deliver the power to drive multiple connected floppies.
2) The 765 delivers SIDE, and the NAND gate makes the /SIDE out of it, which is the floppy standard.

When you select side 0 and side 1 in Parados, does it have any effect on PIN 27 of the FDC?

[ajcasado]

Thanks for the response @Nworc

Maybe for testing you can try that for a short period, but I wouldn't do that as it's not a good idea, for two reasons:

1) The NAND gate works as a driver, to protect the 765 FDC and to deliver the power to drive multiple connected floppies.

That was my concern, thanks for the confirmation. Using the 6 NOT gates of the 74ls04 in parallel will get me closer to the safer side?

When you select side 0 and side 1 in Parados, does it have any effect on PIN 27 of the FDC?

Yes, it switches between 0V and 3.5V, but have no effect in pin 11 of the IC206. In pin 40(Vcc) of the FDC the voltage is 4.3V.

[Nworc]

Okay, putting all that info together, it appears that the NAND of the 74LS38 is broken that deals with the Side.

The reason for that you have to set the correct side in Parados to 0 ot 1 depending on whether you open or close the Pin 3 of the external floppy connector is that the Sector ID which is read from the floppy must match the requested identification when accessing the disk, and the floppy indeed switched the side when you close or open Pin 3.

There is the faint chance that it's the voltage which is not sufficient at the 74LS38, since if anyone says he measures the VCC, and it is not somewhere near 5V, people become suspicious, and they are right. If Pin 40 on the FDC is at 4.3V, so is the voltage at Pin 14 of the 74LS38. As a first action I would make sure that 5V is indeed 5V, and then change the 74LS38 if that didn't help.

All of that is just my opinion.

[ajcasado]

All of that is just my opinion.

And is a wise one.

Looking the datasheets is shown that the minimum rated Vcc is 4.75V for the 74ls38 and 4.5V for the D765, so the measured 4.3V is a probable source of problems.
The supply I'm using is a fused atx power supply that delivers 5V at no load, but switching on the cpc it drops to 4.5, too low I guess to make everything behave properly.

Anyway, even though the 0V at any input at the pin 11 of the 38 makes me suspect that it is not in a very good shape, I'll start getting a proper 5V source taking care that it won't deliver enough current to fry something (more?).

Thanks for the advice.