VIC 1540

Pedro Lambrini · Post by **Pedro Lambrini** » Sun Feb 15, 2009 1:15 pm

Ah, so a 1541 on the Vic just automatically runs slower on the Vic than a 1540. I thought it was a bug on the C64 side of things and that if a 1541 was used on the Vic it would run at a faster rate a la the 1540.

As an aside, is there a difference between the VIC-1541 and the newer ones?

shem · Post by **shem** » Sun Feb 15, 2009 1:23 pm

Ok here is my understanding

when the vic20 was released the drive was a 1540
later when the c64 was released the drive ( 1540 ) was too quick for the c64 so they modded the 1540 did some tweaks and a rom change so that it would be compatible with the 64 and re branded it so it became the 1541
as far as I am aware the 1541 will work with the vic and the 64 the problem only shows up if you try to use the original 1540 ( the faster drive ) with the c64 that's when you need to slow the 1540 down so that it works correctly with the c64 ...

There are several board versions of the original 1541 the older 1541 with the full size board is basically the same as the original 1540 but with a different rom later on commodore have altered the board layout in the 1541 but kept it in the 1541 box and later still they released the 1541 MKii

Roelie · Post by **Roelie** » Sun Feb 15, 2009 1:26 pm

As far as I know the unaltered 1540 is too fast for the c64. Because of an error in the VIA 6522 (in the 1540 and 1540) the shift register could not be used anymore and they had the go back to bit-banging (bit by bit reading). This is very power consuming for the 6500 microprosessor in the computer (vic 20 and c64). Since the VIC II chip in the c64 needs more often access to the memory less is available for the 6502. Too less for bit-banging with the 1540. Commodore made a new rom to even slow the 1540 more down so that it did work with the c64 and then they called the drive a 1541.
Now with a special command you can again speed up the 1541 drive. But then it only works with a vic 20.

P.S. The 1570 and 1571 are the first drives that can use the serial shift register as meant to be, since they also have the CIA 6526 chip inside.

Correct me if I am wrong.

shem · Post by **shem** » Sun Feb 15, 2009 1:30 pm

Ahh now we are getting somewhere
This is an excellent history lesson

Pedro Lambrini · Post by **Pedro Lambrini** » Sun Feb 15, 2009 1:33 pm

So, what is the special command to speed up the 1541? Does anyone know, at all?

Roelie · Post by **Roelie** » Sun Feb 15, 2009 1:42 pm

From the 1541 user manual:

Using With a VIC 20 or Commodore 64
The 1541 Disk Drive can work with either the VIC 20 or Commodore 64 computers. However, each computer has different requirements for speed of incoming data. Therefore, there is a software seitch for selecting which computer's speed to use. The drive starts out ready for a Commodore 64. To switch to VIC 20 speed, the foIlowing command must be sent after the drive is started (power-on or through software):
OPEN 15,8,15, "UI-": CLOSE 15
To return the disk drive to Commodore 64 speed, use this command:
OPEN 15,8, 15, "UI+": CLOSE 15
More about using this type of command is in chapter 4, with a detailed
explanation of the U (user) commands in chapter 7.

carlsson · Post by **carlsson** » Sun Feb 15, 2009 2:30 pm

Anyway, wouldn't POKE 53265,0 on the C64 let you use it with a 1540 ?

Yes, the display will shut down but that is a price you'd have to pay. Just type in commands in blind, the computer still works although you have no display.

brain · Post by **brain** » Sun Feb 15, 2009 8:17 pm

Turning off the display does indeed work.

For those with the SD2IEC/uIEC, UI+- is implemented. Speed up your access today!

For reference, here's the sd2iec code that implements the change in speed:

Code: Select all

    set_clock(1);
    if (globalflags & VC20MODE)
      _delay_us(34);  // Calculated delay
    else
      _delay_us(69);  // Calculated delay

This happens during the send of data from 1541 to 64. in UI- mode, we set the clock HI and wait 69uS before setting clock LO again. In UI+ mode, we only wait 34uS. So, effectively, UI- is half as fast.

Jim

carlsson · Post by **carlsson** » Mon Feb 16, 2009 1:20 am

Would those delays also apply to PAL computers? I guess as soon as mine arrive, I can experiment a bit and see for myself.

ral-clan · Post by **ral-clan** » Mon Feb 16, 2009 8:48 am

So what will happen if you connect a 1541 to a VIC-20 and DON'T execute the command to make it VIC-20 compatible (speed it back up)?

In my experience, no problems....but perhaps others have experienced otherwise.

shem · Post by **shem** » Mon Feb 16, 2009 12:21 pm

I guess it will run just a little slower

Schema · Post by **Schema** » Mon Feb 16, 2009 12:29 pm

brain wrote:For those with the SD2IEC/uIEC, UI+- is implemented. Speed up your access today!

Is this one of the settings that's saved with XW?

carlsson · Post by **carlsson** » Mon Feb 16, 2009 5:04 pm

Yes, a 1541 runs fine with a VIC-20 although not quite as fast as it could... eeh... not as fast as a 1540 would. A poor man's C64 disk turbo is to issue the UI- command then close the display. On the other hand almost any fastloader will outperform that ~15% speed increase.

brain · Post by **brain** » Mon Feb 16, 2009 5:55 pm

ral-clan wrote:So what will happen if you connect a 1541 to a VIC-20 and DON'T execute the command to make it VIC-20 compatible (speed it back up)?

In my experience, no problems....but perhaps others have experienced otherwise.

There won't be any problems, but you will get slower performance.

Jim

brain · Post by **brain** » Mon Feb 16, 2009 5:56 pm

Schema wrote:
brain wrote:For those with the SD2IEC/uIEC, UI+- is implemented. Speed up your access today!
Is this one of the settings that's saved with XW?

It is not, because there was a concern that someone would save UI+ on a VIC setup, and then take it to a C64 and it would not work, and then call/email with issues.

You might be able to lobby for it to be saved in the config.

Jim