Semi-Virtual Diskette (SVD)
Apple //c Notes

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For Info:

SVD Support for Apple //c  -  Notes on db19 Connector   -  Booting from the SVD   -  Apple 5.25" Disk Secret

SVD Support for the Apple //c

You will probably not be surprise to hear that the SVD works well with the Apple //c. In most ways, the Apple //c is compatible with the other Apple 2's. However, there are a couple of important issues that you should know when using the //c with the SVD. Each of these issues is described in more detail below.
  • Connecting to the SVD from the 19-pin connector
  • Booting from the SVD

The Apple //c included many new features with its introduction. An important one, from the SVD's point of view, is that it didn't plug directly into the old Apple Disk II's. Note that it is quite compatible with the old Disk II's, it's just that the connections to the //c are different.

Note that the above image was lifted from
It's a nice site!
There was a new line of disk drives that were released starting with the //c. The first was a Disk IIc, followed then by the Unidisk 5.25", and then the Apple 5.25" drive. Each of these new drives were compatible with the old Apples (as well as the old Disk II), just the connectors were different. An Apple 5.25" drive is shown here.

For more information about the old Apple disk drives, please see Phil Beesley's site at

Connecting to the Apple's db19 Connector

As described above, the Apple //c didn't plug into the old Disk II's. It used a new 19-pin (db19) connector instead.

Unlike the good ole' Disk II's that connected to the older Apple II's using a 20-pin flat ribbon cable and connector the disk drives compatible with the //c uses a 19-pin (db19) connector. This connector serves the same purpose as the 20 pin connector on the disk interface card that you would normally have inside an older Apple. One nice thing about this connector, however, is that it won't allow you to connect the cable incorrectly... the possibility of which caused many sad events with the older Apple II line.

To the left you see a picture of the back side of a 2c, click on the image for a more detailed view. The floppy connector is the one to the right of the RCA connector in the middle.

To connect the SVD to the Apple //c you need an adapter cable that converts from the db19 to the old 20-pin IDC connector that the SVD uses. This cable will have a female db19 on one end, and a female 20-pin IDC connector on the other.

You can still find people who sell this cable: IEC (in Colorado, USA) And it's not too terribly expensive. I haven't contacted them, however, to determine if they actually do still carry this cable, or if the link above is just a cob-web.

Assuming, however, that you can find a db19 connector yourself and want to build the cable yourself, the pin-out is given below. Note that finding a db19 connector is no small task. I did consider putting a db19 on the Apple Dongle but couldn't find one that was cost effective.

Note that much of this information can be found on the Internet. I found much of it by using my voltmeter and reading schematics. A nice treatment of the subject can be found here too:

You don't have to buy or build one of these cables if you have a spare Apple 5.25" or Unidisk drive laying around. In these disks, there is already a db19 to idc20 converter cable! And you don't have to destroy the disk to get it. See below for more information.

DB-19 Pin IDC-20 Pin Pin Description
1 1 GND
2 3 GND
3 5 GND
4 7 GND
5 9 -12v
6 11 +5
7 13,15 +12
8 17,19 +12
9 NC Select 2
10 20 Write Protect
11 2 Step 0
12 4 Step 1
13 6 Step 2
14 8 Step 3
15 10 Write Request
16 12 or NC +5
17 14 Select 1
18 16 Read Data
19 18 Write Data


  • This table is valid for the Apple 5.25" disk, the Unidisk, and the Apple //c disk.
  • The db19 pin 9 is used as a second disk select line and gets wired through to pin 17 on the daisy-chained db-19 on the Unidisk and Apple 5.25" drive. On the Apple //c disk, this is an external interrupt line.
  • On the Apple //c disk, db19 pin 16 is not connected.

Booting the Apple //c from the SVD

Another new feature introduced in the Apple //c was a built-in floppy drive. This drive is permanently wired as the "boot drive" or Apple people would say, as slot #6 drive #1. This poses a bit of a problem for booting off of the SVD...or any external drive for that matter. The Apple //c simply makes it difficult to boot off of an external drive.

Some of you are lucky, however, some versions of the //c had ROMs that allowed booting from the external drive...but very few of you. If you want to see if you can boot from the external drive, try the following:
1. Reset the //c to get to the "]" prompt.
2. Try the external boot sequence "PR#7".

This may or may not work for you. For more information on the differences in the ROMS, see

Even If You Boot Externally...

Even if you are able to boot externally, you may have problems. I found it quite difficult to boot and run bootable games on the SVD connected to the external drive connector. Half-way through the process, the game tries to get data from the //c internal drive.

Maybe someone out there can help out?

A Secret Inside the Apple 5.25'' Disk Drive

Hidden inside the Apple 5.25" disk drive, and probably the Unidisk and Apple IIc Disk is a cable that can be used to connect the //c directly to the SVD, or an Apple Disk II for that matter! The external cable that connects the Apple 5.25" drive to the computer has a female idc20 on the other end, inside the drive itself. This cable, therefore, has a female db-19 on one end, and a female idc20 on the other. The pin-out is ALMOST perfect...just one thing needs to be changed as described below.

Removing the cable is a fairly simple matter. You simple dissassemble the case (left as an exercise for the reader). The cable can be removed with a screw, disconnecting the grounding wire, and VIOLA! A converter cable. HOWEVER, there is one very important change you need to make:

1. Remove the cable from the disk drive

2. Pull pin 17 from the idc20 connector

Click on this picture to see a close-up.

This idc20 pin 17 is internally connected in the disk drive to be the second drive select.

The actual pin-out for this cable is given in the table below, along with some notes about the connections relative to the SVD.

DB-19 Pin IDC-20 Pin Pin Description SVD Notes
1 1 GND
2 3 GND
3 5 GND
4 7 GND
5 9 -12v
6 11 +5 SVD connects 11 & 12 together
7 13 +12 all four +12 wired together in the SVD
8 15 +12 all four +12 wired together in the SVD
9 17*** Select 2 MUST BE PULLED OUT OF THE IDC20
(otherwise it will be connected to +12v)
10 20 Write Protect
11 2 Step 0
12 4 Step 1
13 6 Step 2
14 8 Step 3
15 10 Write Request
16 NC +5 SVD connects 11 & 12 together
17 14 Select 1
18 16 Read Data
19 18 Write Data