1541 Disk Drive with VC 20

[zspzs]

Hi All,

I am trying to connect my VC 20 to a 1541 disk drive. I learned from the Internet that because of the speed difference between the VC 20 and the 1541 drive, the VC 20 compatibility mode must be activated for 1541 by this command: OPEN 15,8,15,"UI-":CLOSE 15. After doing this I issued a simple LOAD"$",8 command. The drive started searching but failed loading the disc directory. The READY. prompt does not come back until I press the RUN/STOP key and the red LED on the drive remains on even after the disc stops spinning.

My first assumption was that the disc should be formatted by a VC 20 with the drive in VC 20 compatibility mode but this did not help. The format command seems to be processed OK but the LOAD"$",8 command fails again in the same way. The floppy disc itself is a known good DS DD disc.

Does anyone had similar experience or have any idea how to solve this problem?
Thank you in advance!

[Mike]

I learned from the Internet that because of the speed difference between the VC 20 and the 1541 drive, the VC 20 compatibility mode must be activated for 1541 by this command: OPEN 15,8,15,"UI-":CLOSE 15.

This is clearly wrong, and not necessary.

The bus protocol was slightly slowed down from the 1540 to the 1541 because the C64 couldn't keep up with the allowed time out delays of the 1540. The UI- command just restores the slightly faster 1540 protocol, but the VIC-20 can equally handle the 1541 bus speed.

...

It doesn't matter whether a VIC-20 or a C64 issues the format command. That is performed entirely by the DOS in the drive. And there, 1540 and 1541 use the same disk layout.

Can you confirm the 1541 works with a C64? If that is the case, there's a hardware problem with your VIC-20, with one of the VIAs or with the bus buffer.


Oh, and welcome to Denial!

Michael

[zspzs]

Thank you for the quick reply.

Yes, the 1541 drive works perfectly with my C64, so I think the drive is OK.

Regarding your comment

This is clearly wrong, and not necessary.

My experience is the following: If I issue the LOAD"$",8 command without adjusting the speed of the 1541 to the VC 20 I always get FILE NOT FOUND error. After activating the VC 20 compatibility mode the VC 20 starts loading but fails as I wrote in my original post. So the OPEN 15,8,15,"UI-":CLOSE 15 commands have some effect on the system.
Nevertheless you are most probably right and there is a hardware problem with my VC 20. What do you suggest which chip should be changed first? Which is the most likely defective?

Thank you
Peter

[Mike]

The drive seems to react to sent data, but the computer can't seem to receive data.

Just for a test, please run this short program after powering up the drive. It returns the drive status, which gives the DOS type after power on:

Code: Select all

1 OPEN15,8,15:FORP=-1TO0:GET#15,A$:P=ST=0:PRINTA$;:NEXT:PRINT:CLOSE15

(Trying it in direct mode doesn't work due to the GET# command)

If there's no response by the drive either, you should first locate UB4 (a 7404 hex-inverter) and replace it (use a socket). That chip easily gets shot, when the serial cable is plugged while computer and/or drive are powered on.

If that still doesn't work, VIA #1 (MOS 6522, UAB3) is the most likely culprit. If that chip is socketed, try lifting it by 1/2 mm and then reseating it. Sometimes, the pins oxidate, and then that procedure is sufficient to make it work again. Otherwise you'd have to replace the chip.

A very similar scenario has been covered in this thread: Resurrecting an old VIC.

[zspzs]

The result of the test program is the following:
On the VC 20

On the properly working C64

The red circles indicate the result of the first run of the program after powering the 1541 drive. The result of the second attempt marked with a yellow circle. The VC 20 freezes during the second try.

So it seems there is some kind of response from the drive. Does this mean that the 7404 is OK and I have to start with the 6522?

[mrr19121970]

I think Mike means the 7406 this is right next to the uab3. I had "searching for..." forever yesterday. Replaced uab3, still bad it. Replaced hex inverter and it worked again (with original uab3).

[Mike]

Another difference between schematics and real h/w (@#$%&*!). Yes, I meant the 7406, of course.

You should start with the TTL. It's much easier to source and replace than the VIA. Only if replacing the 7406 doesn't help, then you should work on the VIA. As noted in the other thread, there exist CMOS versions of the VIA which are not 100% compatible, please take a thorough look over there.

[eslapion]

Another difference between schematics and real h/w (@#$%&*!). Yes, I meant the 7406, of course.

I have all the schematics for the 1541 drives and different boards. None of them ever mention a 7404...

[zspzs]

Another difference between schematics and real h/w (@#$%&*!). Yes, I meant the 7406, of course.

You should start with the TTL. It's much easier to source and replace than the VIA. Only if replacing the 7406 doesn't help, then you should work on the VIA. As noted in the other thread, there exist CMOS versions of the VIA which are not 100% compatible, please take a thorough look over there.

I forgot to mention that the 7406 was replaced in the VC 20 earlier because the very first error message was the DRIVE NOT PRESENT error. After inserting a new 7406 the situation changed to the previously described FILE NOT FOUND error.

So if I understand correctly there is nothing to do with the 7404 in the VC 20. The next step is to replace the 6522. (Unfortunately it is not socketed...)
Anyway I red the another topic you suggested. The solution there was the 6522 replacement.

I have a non working (mechanically damaged) 1541 drive. Can I use it's 6522 in the VC 20?

Thank you.

[mrr19121970]

I have a non working (mechanically damaged) 1541 drive. Can I use it's 6522 in the VC 20?

Yes, you can use the one from the 1541 (or swap UAB1 & UAB2)

[zspzs]

Yes, you can use the one from the 1541 (or swap UAB1 & UAB2)

I am afraid, swapping UAB1 and UAB3 could cause another malfunction in the VC 20.
Or is it a usual solution?

[Mike]

Swapping both VIAs is only useful to check whether the fault goes with the suspect VIA.

In your case, you'd get some problems with the keyboard instead, but of course that doesn't solve the problem.

Just take the VIA from the damaged 1541 and replace VIA #1 in your VIC-20.

[zspzs]

OK, I will do it in the coming days and will come back with the result.
Thank you all!

[zspzs]

Today I replaced the UAB3 6522 in the VC 20. The problem solved. It works perfectly now. Thank you all for the useful help.