RE: CBM 1541 (fwd)

From: COPLIN, Nicholas. (
Date: 2000-11-16 05:52:18

The way I understand it the UI- command (if required) must be issued once
per diskdrive reset, including the sending of UI and UJ commands. It's a bit
like the software "re-numbering" of drives which is forgotten once a UI is
issues (a real pain!!). Hence, its possible that the Forth program issues a
UI and clears the setting...

The UI- command sets Tv =20us (1540 default) instead of Tv=60-70us (which is
the 1541 default).  The Tv (time data is valid) signal was lengthened with
the introduction of the C64 which has bad-lines lasting up to 43us and hence
may hang if it missed reading the signal in time....

Interestingly you can use the faster speed with UI- if you blank the screen
on the C64/128 before doing the disk I/O.

Whether you need to use UI- with a VIC20 or not I would question. My PAL set
up works with the 1541/1571 without changing the setting.  In theory it is
no problem as the VIC20 does its job and waits for the the clock line to
toggle back.

- Nick

> -----Original Message-----
> From:	Marko Mäkelä []
> Sent:	Wednesday, 15 November 2000 18:20
> To:
> Subject:	CBM 1541 (fwd)
> Dear John,
> Since I'm buried under work at the moment, I'm forwarding your question to
> the cbm-hackers mailing list.  (Note: when replying to the message, be
> sure to send a CC to John.)
> I have never required the UI- or UI+ commands, but your situation could be
> different, since the PAL VIC-20 runs at 1.108 MHz while the NTSC VIC-20
> runs at 1.023 MHz.  The PAL C64 is much more slower than the PAL VIC-20:
> 0.985 MHz minus the cycles stolen by the VIC-II.  The NTSC C64 runs at
> 1.023 MHz.  The drive runs at 1 MHz in any case.
> I'd rather suspect some kind of a hardware problem.  The UI-/UI+ setting
> is definitely not written on the disk; it just alters some delays in the
> drive's serial bus protocol.
> 	Marko
> ---------- Forwarded message ----------
> Date: Tue, 14 Nov 2000 13:40:59 -0800
> From: "Kotrba, John" <>
> To: "'Marko.Makela@HUT.FI'" <Marko.Makela@HUT.FI>
> Cc: 'home' <>
> Subject: CBM 1541
> Marko;
> I see you have considerable interest in details of old CBM equipment.
> Maybe
> you can advise me since I see you provided a disassembly of 1541 ROM.  I
> am
> using a 1541 drive with my VIC-20, operating BASIC and FORTH (Datatronic
> v1.1).  The owner's guide for 1541 refers to "matching drive to computer"
> by
> sending "UI-" to control channel if VIC and "UI+" if C64 computer.  I have
> found I need to do this reading BASIC program files, else drive spins
> without finding program.  When I use FORTH (ramdom file format), it seems
> that it I forget to send command "UI-" command, my disc reads fine
> (written
> previously on same drive).  Now I may have used that command possibly when
> I
> first wrote to that disc, but not now.  However, I'm making a new system
> disc and having trouble getting it to write to a newly formatted disc.
> Now
> I'm wondering it this is because I got used to not sending the UI- and
> this
> is how its showing up.  I guess my question is: should that command be
> manditory anytime disc is used, or is it possible its needed when first
> creating disc and then accomodates even if not sent?  Also, might the
> nature
> of file system eg sequential program vs ramdom blocks matter.  I
> eventually
> find answers to these type of questions experimentally, but its a big
> waste
> of time.  I'm currently trying to create several FORTH system discs for
> backup purposes so I don't have an accident and loose much effort.  I can
> only devote so much time to a hobby!
> Your opinion?
> John Kotrba
> -
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