Ask Mr. Tech # 3
II Alive Volume 1 Number 1
March / April 1993
QUESTION: I’m a member of a local user group. Most of our members own Apple IIGS or lie computers, but I own a Laser 128EX. I’m very happy with this computer with the exception of one minor problem. I am having problems using 5.25″ disks written by the other Apple II computers in our club. They can use the disks that I produce on my Laser computer, and I can read the disks that they produce. However, as soon as I try to write to one of their disks it gets corrupted. What is going on here? My drive doesn’t seem to be defective in any other ways.
ANSWER: There are many factors that can affect the way that diskettes (5.25″ format especially) work in different drives. We might start with a little theory about how computers store information on diskettes.
A disk drive stores information on the diskette by rotating the disk media at a specific speed and pulsing a signal through the drive’s read/write head . The media on the diskette “remembers” this signal because it is coated with a magnetic layer. Sounds simple, doesn’t it? Well, there is a little more to it than that. The pulses that the computer encodes onto the diskette is limited to ON and OFF signals. Since all of the information inside of your computer is stored in a similar format, diskettes seem perfectly suited for data storage. But suppose the byte you want to write to the diskette looks like this (in binary form) : “11110000” (four ON bits and four OFF bits). All of the ON bits and OFF bits can run together, making the number look like “10” to the diskette drive. Because of this, the computer takes all of the information to go on the diskette and encodes it before writing it to diskette. It does this by adding extra zero bits between all of the data bits to separate them.
In addition, the computer needs a way to find the information that it writes out. The diskette does this by dividing each of the circular tracks into sectors or blocks. The drive writes additional identifying information at the beginning and end of each sector. Some of this information is just a set of repeating patterns to let the computer know it is coming up on a sector and to allow the controller hardware to synchronize with the diskette’s rotation. (A simple copy protection technique is to simply change some or all of this information in such a way that standard software can’t figure out where it is on the disk.)
The two variables that can affect the use of diskettes between different drives are speed and tracking. When you format a new diskette, the computer writes brand new address information for each of the circular tracks. If the speed or tracking of another drive is significantly different from the drive that created the disk, the tracks may not line up, or the bits may come in at a speed too different for the drive controller to sync up with.
Apple 5 .25″ drives spin at 300 RPM. If the drive is spinning too slowly, the information gets written to the disk in a much smaller space. Therefore, the space between bits written to the diskette is much smaller, and “on” bits may begin to blend together, which will cause the computer to have difficulty reading the data. If the drive is spinning much faster the data will be written on a much larger space. This may cause the data to be written over the top of the identifying information at the end of the current sector or at the beginning of the following sector. If the computer cannot find the end of the sector it is reading, it may think the diskette itself is faulty. Because there is some space allotted between sectors, a drive may vary in speed by a small amount without the danger of misreading or or mis-writing a diskette.
I suspect this is what’s happening with your drive. Fortunately, speed is easily adjusted if you have the right software. Copy II Plus is a good program for this . Just insert a blank diskette in the drive to be tested, select VERIFY from the menu, and select DRIVE SPEED from the sub-menu. On the bottom of the Laser, you’ll find a small hole under the diskette drive. Insert a jewelers flat-head screwdriver while running the drive speed test to adjust the timing. I have found that the internal Laser drives tend to change speed easily. Temperature, movement and just normal use can cause any drive to speed up or slow down a little. Tracking is not so easy to fix. When you format a diskette you may have noticed a disconcerting rattling sound. The computer moves the drive’s read/write head all the way to the outside track, where it runs it into a rubber bumper several times (causing the grinding noise). This action, called re-calibration, makes certain that the head is directly over track 0. All other tracks are written by moving the read/write head a certain distance from that position.
If the rubber bumper is damaged or not in the correct location, the re-calibration action doesn’t place the head in the correct track 0 position, and all diskettes formatted in the drive will be off-track. They will function properly in the original diskette drive, but may not work in any other drives. There is no real way to calibrate tracking with software. The procedure requires a special test disk and, usually, an oscilloscope.
If adjusting your disk drive’s speed doesn’t help, you might try cleaning the drive with a 5.25″ drive cleaning kit (sometimes oxide builds up on the heads and causes intermittent problems with reading and writing disks). If all else fails, and you don’t want to send the computer to an authorized Laser service center, you could always buy an external 5.25″ drive. You can use the external drive for writing diskettes that other computers can use, and still use the internal drive for your own purposes.
Note: All Tech Questions are answered either by Editor Jerry Kindall or by Bill Carver, the Quality Computers Technician