5.25" DRIVES
Are You READY (or DISK CHANGE)?
In the following figure you can see the BUS signals for the even PINs (2 to 34) for a number of 5.25" drives. For the most part they are identical, but only almost. The differences lie in PINs 2, 4, 6 and 34. All drives use the so-called Shugart interface with 34 signals or lines. On the far right I have listed the interface for the IBM compatible controllers. This supports only two drives, not four! The motor control is also different. Take a close look at the PINs 10 to 16.
(full size)
About Pin's and Lines
- PIN 2 ... HEAD LOAD - DENSITY SELECT
- PIN 4 ... IN USE - HEAD LOAD
- PIN 6 ... DRIVE SELECT 4 - READY
- PIN 8 ... INDEX/SECTOR
- PIN 10 .. DRIVE SELECT 1
- PIN 12 .. DRIVE SELECT 2
- PIN 14 .. DRIVE SELECT 3
- PIN 16 .. MOTOR ON
- PIN 18 .. DIRECTION SELECT
- PIN 20 .. STEP
- PIN 22 .. WRITE DATE
- PIN 24 .. WRITE GATE
- PIN 26 .. TRACK 0
- PIN 28 .. WRITE PROTECT
- PIN 30 .. READ DATE
- PIN 32 .. SIDE SELECT
- PIN 34 .. READY - IN USE - DISK CHANGE - DRIVE SELECT 4
PIN 2 / HEAD LOAD - DENSITY SELECT
This line can have two different functions. With the 8" drives, PIN 2 has a very important meaning (TG43), but more about that later.
1. HEAD LOAD / controller >>> disk
With this signal, the read-write HEAD of the drive is LOADed, i.e. pressed onto the disk surface and premagnetised. You can recognise the head loading by the characteristic crackling sound.
2. DENSITY SELECT / controller >>> disk
This signal is used to switch the operating mode of DUAL SPEED drives between normal/double density (DD) and high density (HD). The controller outputs a LOW when double density disks are detected, or a HIGH when high density disks are detected.
PIN 4 / IN USE - HEAD LOAD
This line can have two different functions.
1. IN USE / controller >>> disk
This signal is used to tell the drive that accesses are coming soon. Often, however, this signal can only be used to switch on the lamp on the front bezel.
2. HEAD LOAD / controller >>> disk
With this signal, the read-write HEAD of the drive is LOADed, i.e. pressed onto the disk surface and premagnetised. You can recognise the head loading by the characteristic crackling sound.
PIN 6 / DRIVE SELECT 4 - READY
This line can have two different functions:
1. DRIVE SELECT 4 / controller >>> disk
I always use the numbers 1 to 4 for the DRIVE SELECT, but in the literature you will sometimes also find the number ordering 0 to 3. So, watch out!
On the floppy drive, four jumpers can be used to choose which DRIVE SELECT line is to be used to select and activate the drive.
Very important: On the North Star Horizon controller the DS4 signal is provided on line 34; not 6!
2. READY / controller <<< disk
This line is used to indicate the readiness of a drive: a floppy disk is inserted, the motor is running but the HEAD is not yet LOADED. READY is usually required on XT systems.
This signal can also be on PIN 34.
PIN 8 / INDEX / controller <<< disk
The selected drive applies a pulse to this line each time the index hole on the disk passes the index sensor. The pulse marks the beginning of a track on soft-sectored disks.
PIN 10 / DRIVE SELECT 1 / controller >>> disk
PIN 12 / DRIVE SELECT 2 / controller >>> disk
PIN 14 / DRIVE SELECT 3 / controller >>> disk
I always use the numbers 1 to 4 for the DRIVE SELECT, but in the literature you will sometimes also find the number ordering 0 to 3. So, watch out!
On the floppy drive, four jumpers can be used to choose which DRIVE SELECT line is to be used to select and activate the drive.
PIN 16 / MOTOR ON / controller >>> disk
This line is used to switch ON the MOTOR of the floppy drive. With many drives this can be done independently of the DRIVE SELECT signal. If your drive offers the possibility, then MOTOR ON and HEAD LOAD should be done at the same time.
Since modern controllers do not know the HEAD LOAD signal, you can alternatively use the MOTOR ON signal if your old floppy disk drive requires a HEAD LOAD! With IBM PCs, you should always connect HEAD LOAD and MOTOR ON.
PIN 18 / DIRECTION SELECT / controller >>> disk
When this line is HIGH, the selected disk drive moves the R/W heads one track away from the center (inside to outside) for each pulse on the STEP line. When LOW, the selected disk drive moves the heads one track toward the center (outside to inside) for each pulse on the STEP line. This signal is only read if a STEP pulse (PIN 20) is present at the same time.
PIN 20 / STEP / controller >>> disk
During a Seek operation, the controller issues pulses on this line. The selected drive moves the R/W heads one track per pulse. The direction of the movement is in accordance with the condition of the DIRECTION SELECT line (PIN 18).
If old floppy drives, such as the Shugart SA400L, generate R/W errors even though the drive is OK, then this may be due to the STEP RATE. With newer drives, the STEP RATE is 3 to 6 msec. However, the SA400L only has a STEP RATE of 20 msec. And this is the problem! The controller generates the pulses much too fast, the „slow“ drive cannot keep up.
PIN 22 / WRITE DATA / controller >>> disk
For each HIGH to LOW logic transition on this line, the disk drive stores a flux change on the disk. This action depends on the WRITE GATE line being activated. The DRIVE must be SELECTed and READY.
PIN 24 / WRITE GATE / controller >>> disk
With this signal, the deletion of the old data, necessary for writing the new data, is switched on and the WRITE DATA is enabled. The DRIVE must be SELECTed and READY.
PIN 26 / TRACK 0 / controller <<< disk
When the R/W heads of the selected drive are positioned over track 0, the drive’s „track 00 sensor“ activates this line.
PIN 28 / WRITE PROTECT / controller <<< disk
The presence of a write protected disk in the selected drive activates this line.
Important note: With 5.25" floppy drives, a floppy disk is write-protected if the write-protection notch is taped over. On 8" drives, however, a diskette is write-protected if the write-protection notch is NOT taped over.
PIN 30 / READ DATA / controller <<< disk
DATA is READ (transferred) from the diskette to the controller when WRITE GATE (PIN 24) is inactive and the DRIVE is SELECTed and READY.
PIN 32 / SIDE SELECT / controller >>> disk
This line is used to determine which head is used for two-sided drives. When the line is HIGH, the upper R/W head (HEAD 0) is activated. When LOW, the lower head (HEAD 1) is activated.
Instead of SIDE SELECT, the term HEAD SELECT may also be used.
PIN 34 / READY - IN USE - DISK CHANGE - DRIVE SELECT 4
This line can have four different functions:
1. READY / controller <<< disk
This line is used to indicate the readiness of a drive: a floppy disk is inserted, the motor is running but the HEAD is not yet LOADED. READY is usually required on XT systems.
This signal can also be on PIN 6, see BASF 6106/6108.
2. IN USE / controller >>> disk
This signal is used to tell the drive that accesses are coming soon. Often, however, this signal can only be used to switch on the lamp on the front bezel.
3. DISK CHANGE / controller <<< disk
This line is used to signal that the floppy disk in the drive has been changed. DISK CHANGE is usually required on AT systems.
The following is a very comprehensive description of the READY and DISK CHANGE signals.
Reference: Diskussionsforum des Vereins zum Erhalt klassischer Computer e.V. Thread: 360KB Half-Height PC Floppy Laufwerk gesucht. (Peter z80.eu)
Disk Change Signal - why it's important for PCs (so you can't isolate pin 34 on floppy drives with Ready Signal except it's a 360KB drive)
The standard PC floppy controller and drive use a special signal on pin 34 called Disk Change to determine whether the disk has been changedor more accurately, to determine whether the same disk loaded during the previous disk access is still in the drive. Disk Change is a pulsed signal that changes a status register in the controller to let the system know that a disk has been either inserted or ejected. This register is set to indicate that a disk has been inserted or removed (changed) by default.
The register is cleared when the controller sends a step pulse to the drive and the drive responds, acknowledging that the heads have moved. At this point, the system knows that a specific disk is in the drive. If the Disk Change signal is not received before the next access, the system can assume that the same disk is still in the drive. Any information read into memory during the previous access can therefore be reused without rereading the disk.
Because of this process, systems can buffer or cache the contents of the FAT or directory structure of a disk in the system's memory. By eliminating unnecessary rereads of these areas of the disk, the apparent speed of the drive is increased. If you move the door lever or eject button on a drive that supports the Disk Change signal, the DC pulse is sent to the controller, thus resetting the register and indicating that the disk has been changed. This procedure causes the system to purge buffered or cached data that had been read from the disk because the system can't be sure that the same disk is still in the drive.
One interesting problem can occur when certain drives are installed in a 16-bit or greater system. As mentioned, some drives use pin 34 for a "Ready" (RDY) signal. The RDY signal is sent whenever a disk is installed and rotating in the drive. If you install a drive that has pin 34 set to send RDY, the system thinks it is continuously receiving a Disk Change signal, which causes problems. Usually, the drive fails with a Drive Not Ready error and is inoperable. The only reason the RDY signal exists on some drives is that it happens to be a part of the standard Shugart SA400 disk interface; however, it has never been used in PC systems.
The biggest problem occurs if the drive should be sending the DC signal on pin 34 but isn't. If a system is told (through CMOS setup) that the drive is any type other than a 360KB (which can't ever send the DC signal), the system expects the drive to send DC whenever a disk has been ejected. If the drive is not configured properly to send the signal, the system never recognizes that a disk has been changed. Therefore, even if you do change the disk, the system still acts as though the first disk is in the drive and holds the first disk's directory and FAT information in RAM. This can be dangerous because the FAT and directory information from the first disk can be partially written to any subsequent disks written to in the drive.
If you have ever seen a system with a floppy disk drive that shows "phantom directories" of the previously installed disk, even after you have changed or removed it, you have experienced this problem firsthand. The negative side effect is that all disks after the first one you place in this system are in extreme danger. You likely will overwrite the directories and FATs of many disks with information from the first disk.
If it's even possible at all, data recovery from such a catastrophe can require quite a bit of work with utility programs, such as Norton Utilities (part of the Norton SystemWorks suite). These problems with Disk Change most often are traced to an incorrectly configured drive. A damaged floppy cable can also cause Disk Change to fail.
Windows Explorer doesn't always display the new contents of a drive. Press the F5 key to refresh the display after you change floppy disks to force the computer to read the new disk.
If the drive you are installing is a 5 1/4" 1.2MB or 3 1/2" 720KB, 1.44MB, or 2.88MB drive, be sure to set pin 34 to send the Disk Change (DC) signal. Most drives come permanently preset this way, but some older drives have used a jumper (usually labeled DC) to set this option.
4. DRIVE SELECT 4 / controller >>> disk
Very important: On the North Star Horizon controller the DRIVE SELECT 4 (DS4) signal is provided on line 34; not 6!
8" DRIVES
... comes later ...
Philips BUS
In addition to the Shugart bus, there was also the Philips bus in the beginning. The assignment of the signals, however, differed greatly from the Shugart system. The Philips bus (5,25", 50 pin) did not become a standard and disappeared over time.
Janich & Klaas, FDC 8/5