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
