FLOPPY DISK CARE AND HANDLING

connector. Configuration procedures are different when a twisted cable is used rather than a straight cable. INSTALLING A FLOPPY DRIVE WITH A STRAIGHT CABLE.— To install a floppy drive with a straight cable to be used as drive A, set the drive select jumper to DS0. Connect the end of the cable to this drive. To install a second drive (drive B), place the jumper in the second position (DS1) and connect the drive to the middle connector of the cable. INSTALLING A FLOPPY DRIVE WITH A TWISTED CABLE.— The twisted cable was developed by manufacturers to make assembling computers at the factory easier. With a twisted cable, both floppy drive select jumpers are set to DS1, and the twist in the cable provides the actual drive select. Table 10-7 shows how the twist works to select drives A and B. To select a drive, both the motor enable signal and the drive select signal must be present. To select drive B, the controller would enable pins 12 and 16 and the drive would be turned on. To select drive A, the controller enables pins 10 and 14. Because of the twist, pin 10 is routed to pin 16 on drive A and pin 14 is routed to pin 12. Since drive A thinks it is drive 1, it turns on and works. Terminating Resistor Also on the drive electronics board is a terminating resistor. The terminating resistor looks like a standard 14-pin DIP IC. It maybe labeled TR or T-RES. The terminating resistor provides the proper load to the disk controller card, but only the floppy drive at the end of the cable is terminated. The floppy drive connected to the middle of the cable should have the terminating resistor removed. To remove this resistor, simply pull it out of the socket. Some manufacturers solder the terminating resistor in place and use a jumper to take it out of the circuit. Disk Changeline/Ready Jumper The disk changeline/ready jumper is used to indicate the disk has been changed and therefore the directory must be reread. Media Sensor Jumper The media sensor jumper is only found in 3.5-inch, 1.44M floppy drives. It enables the media sensor to inform a high-density drive when a 720K disk has been loaded into the drive. By sensing the type of disk loaded, the drive can control the write current for high- and low-density disks and prevent improper formatting of a disk. Enabling and disabling the media sensor varies with manufacturer, so you will need to refer to the installation instructions to properly configure the drive. FLOPPY DISK CARE AND HANDLING Floppy disks are very durable and reliable with a minimum of care. Inserting a 5.25-inch disk in its storage envelope and storing the disk in a disk file box is the best practice for storing disks. The 3.5-inch disk’s plastic case and shutter eliminate the need for the storage envelope. These are best stored in a disk file box designed for 3.5-inch disks. Other precautions for handling disks are illustrated in figure 10-9. Referring to figure 10-9, precaution 4 states that you should keep disks away from machines with magnetic parts. We all know that the large power transformers aboard ships can generate electromagnetic fields that can damage disks. But these electromagnetic fields can be in places we don’t ever think about. A Table 10-7.—Interface connections between the Floppy Controller and Drives (Twisted Cable) 10-12