hooking up two computers to one 1541c

[vic2000]

Hello, I'm trying to hookup my Vic20 and C64c to the same diskdrive (1541c).
Since the diskdrive has two serial connections on the back I want to use both instead of swapping cables all the time.
I only ever use one computer at a time, the C64 has no problem connecting to the diskdrive but the Vic20 can't connect with the diskdrive if both cables are connected at the same time! Even if the C64 is shutoff, the Vic20 still can't use the diskdrive until I unplug the C64..
Is this a limitation of the Vic20 or am I missing something ?
Can I bypass this somehow?

Thanks for any response Danny

[darkatx]

I would imagine that would lead to channel issues with the drive. The fact you can unplug the C64 and then it kicks over means that the drive is still seeing the c64 connection and dismissing the Vc20. I wonder if something is left 'floating' - certain someone will shed some more detailed light on the issue.

[vic2000]

Yes I have to physically unplug the C64 serial cable from the diskdrive , then the Vic20 can use the diskdrive.
I'm thinking that when the serial cable is plugged in , it "short circuits" two or more pins in the diskdrive thus telling the diskdrive it's there? Even if no computer is turned on or not.

[eslapion]

Both computers try to control the IEC bus but I guess it should be possible to make some sort of solid state switchbox using a few 4066 circuits.

Back in the days a switchbox to use a single drive with multiple computers was actually almost as expensive as another drive.

[vic2000]

But how can the C64 control the IEC when it is shutoff and no signal goes thru the cable..
And it's only the Vic20 that has problems with this setup .. even if I swap the cables on the back of the diskdrive.
The C64 will always work but the VIC20 not.

[eslapion]

But how can the C64 control the IEC when it is shutoff and no signal goes thru the cable..
And it's only the Vic20 that has problems with this setup .. even if I swap the cables on the back of the diskdrive.
The C64 will always work but the VIC20 not.

Any Commodore 8 bit computer left off and connected to an IEC bus will bog it down. Inside the VIC-20, C64 and C128 computers are open-collector output circuits and pull-up resistors connected to the 5V line.

Without power, the pull-up resistors become pull-down and no communication is possible anymore. On top of that, the left off computer will receive a small amount of power from the reset line trigerring unpredictable behavior on all devices turned on.

On the VIC-20, if you have a RAM expansion that uses modern SRAM chips, a 1541/1571/1581 drive left attached to a VIC-20 turned off can prevent the purging of the content of the expander. If you have ram expansion in BLK5, it can have very unpleasant consequences.

Connecting directly 2 different Commodore computers to the same IEC bus is a guaranteed failure. You can't do it.

Even having one computer and 2 drives with one of the drives left off is problematic.

[ken]

I would use a simple A-B switch and save yourself a lot of issues

[vic2000]

Thanks for responding, I'm not sure if you understood me correctly.. as I said the C64 have no problem with this setup. No matter if I plug it in the 1st or 2nd serial connector on the diskdrive and the Vic20 is turned off , the C64 can use the diskdrive without problems.
So i'm still thinking it's a Vic20 limitation issue..

Any idea where to find this A-b Switch ??

[eslapion]

Thanks for responding, I'm not sure if you understood me correctly.. as I said the C64 have no problem with this setup. No matter if I plug it in the 1st or 2nd serial connector on the diskdrive and the Vic20 is turned off , the C64 can use the diskdrive without problems.
So i'm still thinking it's a Vic20 limitation issue..

Any idea where to find this A-b Switch ??

You're lucky and you run the chance of damaging components in the VIC or C64.

Added edit:
Many C64 of later generations have internal protection diodes on the IEC bus which the VIC-20 doesn't have. These diodes will definitely short all lines on the IEC bus when the computer is turned off. The VIC-20 will not and this means components inside the VIC can get damaged if it is off and the drive and C64 are on and communicating.

[groepaz]

the IEC bus has no problems with two connected hosts at all. its not a terribly uncommon setup either

if the C64 is powered on, enter POKE 56576,199 so it releases the CLK line - then the VIC20 should be able to access the drive (this has been a common problem for people who connect a C64 and a PC using X1541 cable to the same drive). (alternatively just access the drive by whatever means)

however, if one of the hosts is not powered on, lines are being pulled up due to the already mentioned pullup resistors and you will get problems - the same way you get problems when having a switched off drive on the IEC chain.

[vic2000]

Thanks again for the information. I'm a newbie when it comes to hardware and programming.. but I kinda understand what you guys mean.
I wasn't planning on having both computers turned on at the same time (it also seemed more risky).
So the C64 will not be on when using the Vic20..
I did find this "POKE 37137,PEEK(37137) OR 3" and tried it on my Vic20 but it did nothing to help.

[darkatx]

This is from the Star Commander docs this should help clarify what you need to do -

If you use the built-in drive of a C128D or an SX64 or you want to use the
same Commodore drive from a Commodore machine and a PC then you must execute a
POKE command on the Commodore machine:

- Commodore 64/128: "POKE 56576, PEEK(56576) AND 239" or simply "POKE 56576,
199".

- Commodore VIC20: "POKE 37137, PEEK(37137) OR 3".

This decouples the Commodore machine from the common serial bus, by
switching the CLK line to high. Every time you also access the drive from the
Commodore machine, you'll have to issue this command again afterwards. Note
that the Commander decouples the PC automatically from the serial bus one
second after having completed a disk operation.

[vic2000]

As mentioned the poke code didn't do anything to help the issue.
I guess I'll just have to swap cables instead.. thanks for all . But feel free to fill in if there are any more things to try out:)
Any switch or product that could help me?

[groepaz]

that poke will only help if both computers are running.... if one is switched off, there is little you can do

[Schema]

There was the VIC-Switch....pretty rare though!

http://www.richardlagendijk.nl/cip/mix/ ... -switch/en


Can the Quad Port IEC be used in this way?

http://store.go4retro.com/quadportiec/