An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

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An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by banman »

Not sure if anyone has come across this before.....

Recently, I was observing the behaviour of several different MOS 6522 VIA chips in a Mitsumi style 1541 disk drive.

I had received recently 4 X cheap MOS 6522 VIA chips from an Asian supplier.
The condition of these was very interesting. They came wrapped in a thick Glad wrap (Saran wrap) like material stacked into each other. They looked clean, the legs had all been dipped in solder. One of the chip's had 3 of the legs fall off on 2 insertions into a socket. I was able to save it with some bodge wire.
They had MOS branding and all the same date code. I didn't go further with trying to see if the labels were counterfeit. I was too worried that I may further compromise the chip's pins.

I am fairly certain that these are pulls from something else.
They did look convincingly original though.

I had these bought chips plus a few that I had and knew were original and in various states of operation. Some were outright dead others would partially work, others were ok.
I had tried them in a Vic 20 in the 2 different VIA sockets the marginal ones would say have no joystick control where as in the other socket the keyboard would be fine.

Just randomly, I noticed that if one placed a 6522 VIA chip under test in socket UC2 of a 1541 disk drive mainboard and had a JiffyDOS 5.0 kernal active and a full format was initiated. A fully functional VIA chip would complete the activity.
A failed or marginal 6522 VIA chip would not complete this operation. I checked as many different configurations as I could. This was the one that seemed to give a pass no-pass case.
Using the original Commodore factory kernal will pass marginal MOS 6522 VIA chips. Be aware.
Now, I did use a X1541 cable hocked up to a PC laptop running CBM Transfer just for easy usage.


Here are the steps I used ......

I initialized the disk by powering up the drive.

Read the error channel (check the drive status button in CBM Transfer).

If it reports anything other than.... 73,jiffydos 5.0 1541,00,00
It's failed the test.

Then start a complete disk format (warning this will erase everything on the disk). It's fast.

Again read the error channel (check the drive status button in CBM Transfer).

Check to see if OK appears.

Load the disk's directory and list it.

If it comes back OK then the MOS 6522 VIA chip has successfully passed testing.

I would say without a doubt that this operation can be achieved on the real C64 breadbins and C64C hardware without any JiffyDOS running inside the C64 computers themselves. JiffyDOS needs to in the 1541 disk drive, though.

There must be some 1541 JiffyDOS 5.0 kermal routines that fully exercise the MOS 6522 when it's in socket UC2.

Maybe someone out there can confirm or deny this observation.


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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by banman »

Here's some photos.
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by banman »

Here's some info on execution on C64 computers....

READ THE ERROR CHANEL:
type
10 OPEN 15,8,15
20 INPUTt15,A,AS,B,C
30 PRINT A,A$,B,C
type run


(Format)


OPEN 15,8,15
PRINT#I5,"N0:yourdiskname,2 digit id"
CLOSE 15

Or

OPEN1,8,15,”N0:”DISKNAME,ID”:CLOSE 1
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by eslapion »

banman wrote: Fri May 28, 2021 8:49 pm Just randomly, I noticed that if one placed a 6522 VIA chip under test in socket UC2 of a 1541 disk drive mainboard and had a JiffyDOS 5.0 kernal active and a full format was initiated. A fully functional VIA chip would complete the activity.
A failed or marginal 6522 VIA chip would not complete this operation. I checked as many different configurations as I could. This was the one that seemed to give a pass no-pass case.

...

Here are the steps I used ......

I initialized the disk by powering up the drive.

Read the error channel (check the drive status button in CBM Transfer).

If it reports anything other than.... 73,jiffydos 5.0 1541,00,00
It's failed the test.

Then start a complete disk format (warning this will erase everything on the disk). It's fast.

Again read the error channel (check the drive status button in CBM Transfer).

Check to see if OK appears.
I would say there is no need to go that far. Your test is valid but is somewhat overkill.

In the VIC-1541 or 1541, UC3 is used for communications with the IEC (aka serial) bus but PA0-PA-7 are not connected so putting a 6522 in socket UC3 will never be able to test the chip completely no matter what you do.

However, UC2 is used to control the encoder/decoder clock (track density), the stepper motor of the R/W head as well as communication with the encoder/decoder chip (the 325572-01 which can be replaced by GandALF). Just reading the status channel (aka error channel) and reading the directory of a good floppy is plenty enough.

Every single pin of UC2 except CB1 (pin 18) is tied to a critical function so any problem there and it's impossible to read or write anything on a disk.
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by banman »

Hi eslapion,


Great, this is very useful information as always thank you.

This is exactly the explanation I was observing.

Yes, again I have made an error in my typing. :oops:
Many apologies for the confusion and thanks eslapion for spotting this.

I should have said :

"Just randomly, I noticed that if one placed a 6522 VIA chip under test in socket UC3 (not UC2) of a 1541 disk drive mainboard and had a JiffyDOS 5.0 kernal active and a full format was initiated. A fully functional VIA chip would complete the activity.
A failed or marginal 6522 VIA chip would not complete this operation. I checked as many different configurations as I could. This was the one that seemed to give a pass no-pass case.…"

Thank you for providing me with the the correct IC socket location. The photos show the correct location for the MOS 6522 VIA .


Now I know what the GandALF is for. I have seen it advertised for sale however never really knew what it replaced. This is UC1 IC socket.
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by banman »

Actually while trying to determine the correct IC socket locations from the photos to correct my terrible typos I noticed something else that is equally fascinating...

Look carefully at the first photo in this post, the one that contains text and shows the UC1, UC2, UC3 and UC4 in a row. It's in hi Res. Tap on the photo for more detail.

The printing (if that's what it is) on the chip that is in UC3 from AliExpress is remarkably different from the other original Commodore factory chips.

The text alignment is in a totally different position to the other chips as is the way it's marked too. Looking at it in my hand in the light it definitely appears to be etched (embossed) not printed in ink as the other original chips are. I can run my fingernails into the etched surface.

Notice too, that the date is 3286, is younger than the older original chips next to it. All 4 chips I purchased from AliExpress had the same date stamp!
Why would Commodore go from this etching to plain white printed surface markings on the later dated chips?
There seems to be some circular abrasions on the surface of this AliExpress chip as well.

Hands up who think this chip's been doctored?
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by eslapion »

banman wrote: Tue Jun 01, 2021 7:42 am The printing (if that's what it is) on the chip that is in UC3 from AliExpress is remarkably different from the other original Commodore factory chips.

...

Hands up who think this chip's been doctored?
Yeah, I posted something about this on Amibay.

If you want compatible replacements for the MOS 6522 that are currently in production, it's better to buy the W65C22N from Western Design Center (not to be confused with Western Digital).

These ICs are still available from Mouser at a decent price.

From Amibay:
I can only say for sure the W65C22N from Western Design Center (not to be mistaken with Western Digital) is still in production and is safe to use. The W65C22S is not unless you add a diode and the same is true for some R65C22 but I can't tell which ones. To be safe, I suggest you avoid them since you can only get old chips and lately China has been a champion at selling fakes of just about anything.

See: http://www.westerndesigncenter.com/wdc/w65c22-chip.cfm
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by banman »

Hi eslapion,


Thank you for that information. Nice to know some new parts can still be obtained.

I see they sell brand new 6502 CPU chips as well. Am I correct in thinking that these are drop in replacements for Vic 20 computers and 1541 disk drives?
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Re: An interesting way to test MOS 6522 VIA chips as found in the 1541 disk drive and Vic 20

Post by eslapion »

banman wrote: Wed Jun 16, 2021 7:18 amI see they sell brand new 6502 CPU chips as well. Am I correct in thinking that these are drop in replacements for Vic 20 computers and 1541 disk drives?
The W65C22N are direct drop-in replacements to the MOS 6522. The W65C02 however has differences in the pins and a few instructions.

See: https://www.westerndesigncenter.com/wdc ... ements.php
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