5. Interface Cabling Requirements
This standard provides the capability of operating on the AT Bus in a daisy
chained configuration with a second drive that operates in accordance with
these standards. One drive (selected as Drive 0) has been referred to as the
master in industry terms and the second (selected as Drive 1) has been
referred to as the slave (see Figure 5-3).
The designation as Drive 0 or Drive 1 is made by a jumper plug or switch on
Data is transferred in parallel (8 or 16 bits) either to or from host memory
to the drive's buffer under the direction of commands previously transferred
from the host. The drive performs all of the operations necessary to properly
write data to, or read data from, the disk media. Data read from the media is
stored in the drive's buffer pending transfer to the host memory and data is
transferred from the host memory to the drive's buffer to be written to the
+-----------------------------------------------------+
+---------------^-------------------------------------+
FIGURE 5-1: ATA INTERFACE TO EMBEDDED BUS PERIPHERALS
+-----------------------------------------------------+
+------+====== AT Bus ======+-------------------------+
FIGURE 5-2: HOST BUS ADAPTER AND PERIPHERAL DEVICES
+-----------------------------------------------------+
+---------------^-------------------------------------+
| | |__________________ Device Interface
| ___|_________________ | e.g. ESDI, SCSI
FIGURE 5-3: ATA INTERFACE TO CONTROLLER AND PERIPHERAL DEVICES
5.2 Addressing Considerations
In traditional controller operation, only the selected controller receives
commands from the host following selection. In this standard, the register
contents go to both drives (and their embedded controllers). The host
discriminates between the two by using the DRV bit in the Drive/Head Register.
5.3 DC Cable and Connector
The drive receives DC power through a 4-pin or a low-power application 3-pin
The pin assignments are shown in Table 5-1. Recommended part numbers for the
mating connector to 18AWG cable are shown below, but equivalent parts may be
Connector (4 Pin) AMP 1-480424-0 or equivalent.
Contacts (Loose Piece) AMP 60619-4 or equivalent.
Contacts (Strip) AMP 61117-4 or equivalent.
+------------------------+------------+
| POWER LINE DESIGNATION | PIN NUMBER |
+------------------------+------------+
+------------------------+------------+
The pin assignments are shown in Table 5-2. Recommended part numbers for the
mating connector to 18AWG cable are shown below, but equivalent parts may be
Connector (3 Pin) Molex 5484 39-27-0032 or equivalent.
+------------------------+------------+
| POWER LINE DESIGNATION | PIN NUMBER |
+------------------------+------------+
+------------------------+------------+
System ground may be connected to a "quick-connect" terminal equivalent to:
Drive Connector Terminal AMP 61664-1 or equivalent.
Cable Connector Terminal AMP 62137-2 or equivalent.
Provision for tying the DC Logic ground and the chassis ground together or for
separating these two ground planes is vendor specific.
The I/O connector is a 40-pin connector as shown in Figure 5-4, with pin
assignments as shown in Table 6-1.
The connector should be keyed to prevent the possibility of installing it
upside down. A key is provided by the removal of Pin 20. The corresponding pin
on the cable connector should be plugged.
The pin locations are governed by the cable plug, not the receptacle. The way
in which the receptacle is mounted on the Printed Circuit Board affects the
pin positions, and pin 1 should remain in the same relative position. This
means the pin numbers of the receptacle may not reflect the conductor number
of the plug. The header receptacle is not polarized, and all the signals are
relative to Pin 20, which is keyed.
By using the plug positions as primary, a straight cable can connect drives.
As shown in Figure 5-4, conductor 1 on pin 1 of the plug has to be in the
same relative position no matter what the receptacle numbering looks like.
If receptacle numbering was followed, the cable would have to twist 180
degrees between a drive with top-mounted receptacles, and a drive with
bottom-mounted receptacles.
+-----------------------+
==+==== Circuit Board ====+== ==+==== Circuit Board ====+==
+-----------------------+
.. * Location of pin number stamped as 1 when receptacle mounted upside down
FIGURE 5-4: 40-PIN CONNECTOR MOUNTING
Recommended part numbers for the mating connector are shown below, but
equivalent parts may be used.
Connector (40 Pin) 3M 3417-7000 or equivalent.
Strain relief 3M 3448-2040 or equivalent.
Flat Cable (Stranded 28 AWG) 3M 3365-40 or equivalent.
Flat Cable (Stranded 28 AWG) 3M 3517-40 (Shielded) or equivalent.
The cable specifications affect system integrity and the maximum length that
can be supported in any application.
TABLE 5-3: CABLE PARAMETERS
+-------------------------------------+------+--------+
| Cable length of 0.46m (18 inches) * | Min | Max |
+-------------------------------------+------+--------+
| Driver IoL Sink Current | 12mA | |
| Driver IoH Source Current | | -400uA |
| Cable Capacitive Loading | | 200pF |
+-------------------------------------+------+--------+
* This distance may be exceeded in circumstances where the
characteristics of both ends of the cable can be controlled.
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