Semi-Virtual Diskette (SVD)
TRS-80 Model 3/4

 SVD Information 

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The SVD 

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For Info:

Connecting  -  Using   -  General Trouble Shooting   -  Model 3/4 Trouble Shooting   -  Supported Features   -  Finding Diskette Images

Connecting the SVD to the TRS-80 Model 3/4

To connect the Model 3 or 4 to the SVD, or any external floppy drive for that matter, you need to have a floppy cable connected to the external floppy connector located on the bottom of the main unit to the right (when facing the front of the unit). Of the two connectors on bottom (on the right side), the floppy connector is the one closest to keyboard.

This edge-card connector is part of the floppy controller inside of the Modle 3/4. Although is is normally well anchored, be careful when connecting the floppy cable just in case.

Below is a picture of the bottom side of one of my Model 3's. I was surprised to see that it was so scuffed!

It is best to use the cable that came with the machine, but any 34-conductor ribbon cable with 34-pin edge-card connectors will due. If the cable you use has drive-selecting "pulled-pins" then you should refer to the pulled-pin discussion on the TRS-80 Model I page regarding this type of cable.

The card-edge connector on the controller has pin #1 on the right side, when looking at the bottom of the unit with the keyboard up. Most controllers have a notch in the card-edge connector closest to pin-one. If your cable connector doesn't have the plastic key that fits the notch, be extra careful to orient the stripe on pin 1.

When connecting external floppy drives (or the SVD) to the external floppy connector on the Model 3/4, one of the external floppy drives on the cable needs to be "terminated". The terminated drive should be the last one on the cable.

NOTE: You must use one of the supplied connectors with the SVD. For information on these connectors, click here .

This termination can be done with a real "terminated drive" or the SVD can be the terminated drive. To use the SVD as the terminated drive, use the terminated connector that comes with the SVD. If you are using the SVD along with a real disk drive that is terminated, use the un-terminated connector with the SVD.

Different Controllers & Floppy Number Trickery

There is a top-side and a bottom-side to the Model 3/4 floppy controllers. The top-side is connected to the internal floppy drives, and the bottom-side (which protrudes from the bottom of the machine) is where you connect external floppy drives.

Each one of these connectors allows two floppy drives to be connected to it. The two floppies are numbered #0 and #1 for each of the two connectors. That is, the floppy control signals for each connector can control floppies acting as drives #0 and #1.
The weird part about this arrangement is that the bottom-side connector is used to connect drives #2 and #3...even though the floppy control signals on this connector are used to control drives #0 and #1. In other words, the floppy drive acting as drive #0 on the bottom-side connector is used as drive #2 to the TRS-80. And the floppy drive #1 on the bottom-side connector is used as drive #3 to the TRS-80.
Drive Number
Drive Number
# 0top# 0
# 1top# 1
# 2bottom# 0
# 3bottom# 1

But What Does This Mean to Me?

The question you may be thinking is "why do I care?" The point is that you need to care. When the SVD is connected to the bottom-side of the controller it must respond to the TRS-80 just like a real floppy drive. This means that it needs to act like floppy #0 or floppy #1, even though the TRS-80 sees it as floppy #2 or #3.

The point is, when you connect the SVD to the bottom-side of the controller, you need to load the SVD virtual drive #0 when you want it to respond as drive #2. You need to load the SVD virtual drive #1 to have it respond as drive #3.

But Wait...

Some of you have it easier. There are after market floppy controllers for the TRS-80 that don't have the the same "behavior" with the different controller connectors.

For example, the Micro Mainframe controller has two connectors just like the stock controller. However, the drive signals on both connectors are the same. This means that for the SVD to act as drive #2 to the TRS-80 you load the virtual drive #2 in the SVD software.

Using the Model 3/4 with the SVD

Booting from the SVD

04/25/1985 17:24:45 NEWDOS/80 READY
With the SVD, you can boot a tremendous number of different operating systems on your TRS-80 Model 3/4. Here's one of my favorites shown booting here.

However, booting the Model 3/4 from the SVD is a bit of a challenge. Unless you connect the SVD to the inside connector of the standard controller, disconnecting the physical drive #0, the SVD cannot be drive #0. And, you can only boot the Model 3/4 from drive #0.

There are basically three options to boot the Model 3/4 from the SVD:


Connect the SVD Internally

The standard floppy controller on the Model 3/4 has an internal floppy connector and an external floppy connector. The external connector controls floppies #2 and #3 - with the internal connector controlling floppies #0 and #1.

If you open the Model 3/4 case, you can fairly easily connect the SVD to internal connector by disconnecting the cable leading to the internal drives, then connecting the SVD to the internal connector. If you use a 34-pin cable with non-keyed (pulled-pin) connectors the SVD will be able to act as drives #0 and #1. Personally, I have connected the SVD as drive #0 and one of the internal drives as #1.

This picture shows the connector and its location within a Model 4. Click on the image to blow it up.


Make a Boot Disk

The easiest method to "boot from the SVD" is really not to boot from the SVD. Instead, make a backup copy of a boot disk from the SVD, and boot that. For this, you need a boot disk...but if you don't have one, you'll need to do step 1 above.

Assuming that you're booting TRSDOS, you can then use the BACKUP program to make a backup. Use the SVD drive, assumed to be #2 in this example, as the source and either #0 or #1 as the destination.

As described above, it is important to understand that using the standard TRS-80 Model 3/4 controller, that although the external connector controls drives #2 and #3, the signals on that connector use the lines for drives #0 and #1. That is, when the SVD is connected to the external connector, you should load drives #0 and #1 in the SVD Control Program.


Source disk number? 2
Destination disk number? 1
There's more information on make disk copies below.


Use a Different Controller

This option is included for completeness...though I doubt anyone will run out and buy a new controller just to work with the SVD easily!

Some after-market floppy controllers for the Model 3/4 don't have the same limitation that the external floppy connector cannot act as drive #0. However, the SVD hasn't been tested with those controllers and can't be guaranteed to work. In fact, the one after-market controller that the SVD has been tested with didn't work well: Micro Mainframe. Although you can easily boot from the SVD with this controller, you can't write to the SVD using it. The problem is that the write pulses generated by the controller are only 250ns wide...too small for the SVD to reliably read.

If you have an after market controller, though, you probably also have internal drives. So although the SVD can be connected to the external connector and can, therefore, act as drive #0, one of the internal drives is probably ALSO connected as drive #0. So for you to be able to boot from the SVD even using an external controller, you will need to disconnect, or re-jumper, the internal drive #0.

Sample Directory Listing

Assuming that the SVD is connected as drive #2, which as noted means that the SVD Control Program loaded drive #0 (confusing I know), then the following will generate a directory listing in NEWDOS/80.
DIR :2
(screen clears)
DRIVE 2   NEWDOS80  00/00/00   40 TRKS   30 FDES    64 GRANS



Running a Program

Once you are able to "list" a virtual floppy from the SVD, running programs is easy. Here are a couple of examples of running programs from some disks.


Basic Program

Running basic programs on the Model 3/4 and even Model 1 for that matter, requires that you first start BASIC. In almost all OS's you simply type "BASIC" to get it running.

After you are in BASIC, you can either "LOAD" or directly "RUN" a BASIC program. You can either specify the file extension "/BAS" or if you don't, it will be assumed.

The different OS's have different BASIC's on them. This means that some BASIC programs will NOT run on one OS while they may on others. Annoying, but true.

If your BASIC program won't run, try another OS.

BASIC (enter)

(screen clears)

> RUN "PROGRAM/BAS" (enter)



Non-basic Programs (binary)

Running a binary program (or "/CMD") is easy, just type in its name. If the file is on a different diskette, other than # 0, depending upon the operating system you are running, you may need to append the drive number, such as ":2" to the back-end of the command. You may even need to specify the whole filename such as "DIRCHECK/CMD:2".

This command, by-the-way, is a great way to see the details of what is on the diskette, virtual or otherwise.

DIRCHECK (enter)

Creating a REAL floppy from an SVD image

One of the best things about the SVD is that it allows you to both:

  • create real floppies from a floppy image
  • backup your real floppies to an image that you can upload to your PC

Here is an example of making a real copy of a virtual floppy image on the SVD. It assumes that the target real floppy drive is drive #1 and the SVD is drive #2. It assumes also that you have downloaded some example image.

It is important to note that there are different commands for the different OS's for formating and copying diskettes. In the following examples, NEWDOS/80 commands are used.


Format the real floppy

FORMAT,1 (enter)
It isn't completely necessary to format the floppy first. But it isn't a bad idea. Note, however, that you should never try to format the image in the SVD! More on this in the next section.

Note, too, that in NEWDOS/80 you can specify that the COPY command is to format the floppy.


Copy the SVD image

COPY,2,1,6/39/05 (enter)

Creating an SVD image of a REAL floppy

To make a virtual copy of one of your real floppies, essentially you simply reverse the floppy drive numbers in the command above...but there is another important step as described below.


Download a blank floppy to the SVD

Included with the SVD software are a couple of blank floppy images. They do, however, have a DOS filesystem on them. This makes it easy to copy files over to the "blanks." Or, if you don't care about the filesystem, you can just copy over them. The SVD doesn't care.


DO NOT try to format the SVD image

Unlike in the above example, when making a real floppy, you should NOT try to format the image in the SVD.

The SVD emulates generation of the sectors as well as the format of the sectors. "Formatting" of the image is, therefore, not necessary. It isn't fatal to the SVD, but it will mess up the image causing you to have to download the blank image again.


Copy the Real image

COPY,1,2,6/30/05 (enter)
After downloading the blank, run the command that will copy the real floppy to the blank in the SVD. This command copies the image sector by sector to the SVD.


Upload the new image to your PC

Now you need to return to the PC, and upload the changed image from the SVD back to the PC. Please see the PC Software reference pages Section 5 - Uploading a floppy image for information on how to do this.


Save the new image

Finally, you can save the uploaded image to a new PC file. Again see the PC Software reference pages Section 6 - Saving a floppy image for more information.

Chicken and the Egg

There's an obvious "chicken and the egg" problem when getting a Model 3/4 running, even with the SVD. Since the stock floppy controller doesn't allow booting from the external floppy connector, it is impossible to boot the Model 3/4 without a real floppy diskette...that is, unless you open up the machine. If you have a boot floppy of some type, it is a simple matter to create new boot disks from images loaded on the SVD.

The point here is that if you don't have a boot disk for the Model 3/4, you can still use the SVD to create one, but you'll need to open up the machine. Sad, but true.

Copy Programs

An annoying "feature" of some TRS-80 DOS's is that the included backup or copy programs will only work with disks or disk images that are written with the same OS. This can be a problem if you are trying to create a bootable DOS disk for a different DOS than the one doing the copy.

A nice tool that I have used to alleviate this problem is the CopyCat program. It does a sector by sector copy of a disk no matter what DOS created it. You can find this program on Ira's TRS-80 site.

Note that there are different versions of the CopyCat program. Some of them have a bug where they try to copy tracks 0 through 40 of a 40-track disk. Those of you who know how to count will realize that this adds up to 41 tracks. This may confuse the SVD to the point that you will have to reload the image on it.

General Trouble Shooting (non Apple 2)

Can't boot from the SVD

When your vintage computer won't boot from the SVD there is often a simple problem that you can correct. The LEDs on the SVD can help you figure out the problem. Follow these steps to do so.

  1. Plug the SVD into the floppy cable.
  2. Make sure that the computer is off. Or in in the case of an external floppy controller (as with the TRS-80 Model I Expansion Interface) make sure it is off as well.
  3. Looking at the LEDs on the SVD, you should see only the Power LED lit. Here are a few different possibilities for the SVD LEDs:
    Disk 0
    Disk 1
    Track 0
    This is what you should see. The SVD is getting power and the cable appears to be connected least so far so good.
    Disk 0
    Disk 1
    Track 0
    (all LEDs off) The SVD isn't receiving power. Check the power connector and the power supply.
    Disk 0
    Disk 1
    Track 0
    If the Disk 0 and 1 LEDs are on as well as the Write LED, then you most probably have the floppy cable connected "backwards". Be careful to note where the cable stripe is as well as the plastic key on the connector. Worse come to worse, just try reversing the connector.
  4. Download a bootable image to floppy #0 on the SVD from the PC Software.
  5. Turn on the external floppy controller if there is one, then turn on the main computer. This should cause it to try to boot from the SVD. When you turn the main unit on, watch the LEDs on the SVD, and compare to the following:
    Disk 0
    Disk 1
    Track 0
    When trying to boot from the SVD, the SVD Disk 0 LED along with the Track 0 LED should come on. This indicates that the computer is trying to boot from Disk 0 and that the SVD is responding with data from the first track of the downloaded floppy image (Track 0). Note, however, that the Track 0 light will only come on for a short time. As soon as the boot process gets going, the computer will "seek" the virtual drive to a different track, causing the Track 0 LED to go out.
    Disk 0
    Disk 1
    Track 0
    BE CAREFUL - the Track 0 LED may have come on for a short time right after you try to boot. Watch the LED while turning on the unit. However, if the Track 0 LED doesn't come on but the Disk 0 LED comes on, that indicates that the floppy controller is working and that it is trying to boot from Disk 0. The fact that that Track 0 light doesn't come on indicates that the diskette image wasn't downloaded to Disk #0 in the PC Software or that it wasn't downloaded to the SVD.
    Disk 0
    Disk 1
    Track 0
    If the Write LED is on, there isn't termination on the cable. You should use the terminated connector on the SVD or use a terminated real floppy on the same floppy cable. (see here for more info on termination).
    Disk 0
    Disk 1
    Track 0
    If the Disk 0 LED doesn't come on, the computer isn't trying to boot from the SVD. This indicates that the cable to the SVD (and other floppy drives) isn't connected correctly.
  6. If the Drive 0 and Track 0 lights both come on, then after a very short time, the Track 0 light should go out.
  7. If this all worked correctly, but the computer didn't boot, it probably means that you downloaded an incorrect floppy image. Try a different one.

TRS-80 Model 3/4 - Trouble Shooting

Can't boot from the SVD

The Model 3/4 with the normal "stock" floppy controller only knows how to boot from drive #0. Unfortunately, one of the internal drives is always drive #0. Further, the external floppy connector is always drives #2 or #3 (although it looks like #0 and #1 to the SVD). So this means, without some work, you can't natively boot from the SVD.

I have personally connected the SVD to the internal floppy connector - disconnecting the interal drive #0 and it booted just fine. Most of the time, however, I don't open up the machine. Instead, I just create a real floppy of the boot image from the SVD. You can do this with a simple sector-by-sector copy of the image on the SVD. See above for how to do this copy. Note that operating system boot images are normally not copy protected. This means that a sector-by-sector copy works just great. Trying to copy bootable games, however, sometimes creates problems because copy-protection causes sector-by-sector copies not to work right. In this case you should use something like CopyCat to do a track-by-track copy from the SVD.

Can't read from the SVD

This problem is normally caused by a bad cable. And when I say "bad" I mean that a cable that is either mis-connected or has features that may work for real drives and not for the SVD. Take a look at the following:

  • Pin 1 (the stripe) - make sure that pin #1 (the one with the stripe) is aligned correctly. See above for a lot of detail on this.
  • Pulled Pins - make sure that the cable you are using either DOES NOT have pulled pins, or you are using the connector that matches the drive you the SVD is emulating.

Try using a real floppy drive

If you are having problems, the best thing to do is to get the TRS-80 to boot from a real diskette before trying the SVD. If you don't have a real diskette, first try the general trouble shooting tips above.

Another way to check this problem is to connect the SVD as the ONLY drive (remember to use the terminated connector) but use a connector WITHOUT pulled pins - like the one included with the SVD. If the cable isn't working properly when booting you may see:
Disk 0
Disk 1
Track 0
This indicates that the TRS-80 is signalling both floppy lines at the same time. This could be either a cable issue or a controller issue.
Disk 0
Disk 1
Track 0
This indicates that you either there is NO signal going to the floppy (i.e. the cable or controller isn't working at all) or that the cable isn't terminated.

  • Regarding termination - don't forget that you must have either a terminated floppy drive or a terminated SVD connected to the end of the floppy cable. There are two types of early Model I drives: 26-1161 which is the un-terminated drive, and the 26-1160 (and the later 1164) is the terminated drive. So you must have a 1160 or a terminated SVD at the end of the floppy chain. You can also have 1161's on the chain along, too, if you like.

  • Sometimes you get ahold of a bad floppy image. You really can't do anything about this. But to make sure that it isn't another problem, try loading an image that you know works. Or download one of the included OS images...these are known to work.

  • You may have downloaded the wrong virtual floppy drive from the PC software. For example, if you downloaded an image to virtual floppy 0, but you have the SVD connected to the second floppy connector, the TRS-80 won't see the SVD. Check out the section above for more information about getting the SVD hooked up right. My advice is to use the non-keyed connector that comes with the SVD so you can safely ignore most of these problems.

  • Don't forget to download your floppy images! After you load them to the virtual drives please hit the Download All button.

    Two Floppy Images Won't Work

    • This problem almost always indicates an inappropriate SVD connection to the TRS-80. See the section above for more information.

    Can't Format the Floppy

    • Unfortunately, you can't format the SVD floppy image. You should, instead, download a blank floppy image to the SVD that is already formated.

  • TRS-80 Model 3/4 - Supported Features

    DMK, JV1, and JV3 image file support. (most copy-protected ones too!)
    double density (SS DD) - some third party controllers shown NOT to work
    single-sided single density (SS SD)
    double-sided (the current design doesn't have enough memory for a DS DD image so it really doesn't make sense to support it)
    boots many different OS's - or allows boot disk creation if you only use the SVD externally
    reading/booting the diskette image
    connects to external floppy connector
    connects internally if you open up the Model 3/4
    writing the diskette image
    formatting the diskette image
    downloading individual files - you can download individual BASIC programs or command files

    TRS-80 Model 3/4 - Links to Images The consumate site for TRS-80. Includes images, manuals, magazines, everything!
    File ExtensionCommon MachineDescription
    .dmk TRS-80 1/3/4 The .dmk format is quite common for TRS-80 disk images. Originally named for its creator, David Keil, this format is a raw image of the bytes found on a TRS-80 diskette. This includes sector headers and such, although special clocks and bit transitions for FM or MFM are not represented. Most copy-protected diskettes can be represented in this format.
    .jv1 TRS-80 1/3/4 Name for its creator, Jeff Vavasour, the .jv1 format is a simple contiguous set of floppy sectors. .jv1 is only used for single-sided single-density diskettes, and normally only for Model I files.
    .jv1 TRS-80 1/3/4 Also named for its creator, Jeff Vavasour, the .jv3 format includes custom header information in addition to sector data. This allows some copy-protected images to be represented. Double-density diskettes are often represented in this format.
    .dsk TRS-80 1/3/4 All of the above formats (.dmk, .jv1, .jv3) can have the .dsk extension. Fortunately, due to the very different formats, the SVD can determine which format is in a .dsk file.