I have two C64's, a C128, and as for drives: SD2IEC, 1541-U2, 1541-II and a 1581 - that makes for 4 devices. Looking at the drives, those only allow 4 device ID settings (8,9,10,11), but according to IEC bus documentation, it seems that practical limit is about 8 devices (given 40mA current drain per device).

Is there any way to connect more than those 4 devices at once ?

I'm not trying to hook up whole 8 devices, but possibly a 1571 or/and CMD FD-2000.

Given that devices seem to only have 4 ID settings - would my approach require hardware modifications to enable additional ID's ?

I vaguely recall that some drives (aside from 1541-u2 and sd2iec) allow for software ID change, but even if that let me go beyond ID 11, I think that doesn't last through reboots.

I am aware of devices such as IEC switcher, which I do not have, I'm looking for a "permanently connected" solution with assigned addresses.

2 Answers 2


Normally Commodore drives are device numbers 8, 9, 10, and 11, and drives 0 and 1, giving 8 drives, but some newer products support device numbers up to 31 (although 31 is reserved for UNTALK/UNLISTEN command), giving 23 devices with two drives each for a theoretical total of 46 drives.

Unfortunately, the 7406 chip on the IEC bus can only handle 40 mA of load current (5 mA per device), limiting the bus to 8 devices. It may be possible to increase this using both drive numbers 0 and 1.

Edit: An example of a device which could achieve this is the 64HDD.

  • Ok, so 4 device numbers of 2 drives each, but does the bus allow for 2 separate devices to use the same device number but different drive number ? And would any of the mentioned - 1541-II, 1541-U2, SD2IEC, 1581 allow for changing drive number ? - there are only dip switches for the device number, I would assume it's always drive 0... Jun 6, 2016 at 22:37
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    2 separate devices can't use the same device number, the device number identifies which device you want to talk to. once you have identified the device you can then use drive numbers to tell the device which drive to use (e.g. for dual drive devices).
    – Ken Gober
    Jun 7, 2016 at 13:30
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    @KenGober: Were any dual-drive units ever made for IEC (as opposed to IEEE-488)?
    – supercat
    Jun 17, 2016 at 22:40
  • Short answer: No. Longer answer: There are mentions of a 1541D and a 1572 but the 1541D was never produced and the 1572 only made it to the prototype stage. Each drive in the 1572 had it's own controller so it was device 8 and 9, not drive 0 and 1. Sources: floodgap.com/retrobits/ckb/secret/periph.html ist.uwaterloo.ca/~schepers/MJK/1572.html
    – Tim Locke
    Jun 18, 2016 at 15:44
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    @supercat - the MSD SD-2 is a dual-drive dual-interface drive (both IEEE-488 and IEC). Commodore had planned to release a 1572 IEC dual-drive but cancelled it.
    – Ken Gober
    Jun 19, 2016 at 12:00

The CMD HD series hard drives and their modern day Thunderdrive clone made by MyTech Electronics can be configured for any default device number from 8 to 29. Also, the MyTech FD2K and FD4K Firedrive clones of the CMD FD-2000 and FD-4000 floppy disk drives are dip switch configurable for device numbers 8 through 15. A total of 8 devices can adequately share the IEC bus through daisy-chaining. Adding a 9th device will cause read errors on all the devices connected.

  • 1
    Welcome to Retrocomputing Stack Exchange. Please read the tour. What's the difference between the configurations you mentioned in the first and the second sentences?
    – wizzwizz4
    Jul 15, 2017 at 15:11

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