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SCSI bus bandwidth. If the controller held onto the bus while waiting for the
device to seek, then the other devices would be locked out. This is also
sometimes referred to as overlapped operations or multi-threaded I/O on the
SCSI bus. This feature is very important in multitasking environments.
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Synchronous versus Asynchronous
An asynchronous device must acknowledge each byte as it comes from the
controller. Synchronous devices may transfer data in bursts and the
acknowledgments happen after the fact. The latter is much faster than the
former and most newer devices support this mode of operation. The
adapters negotiate with devices on the SCSI bus to ensure that the mode
and data transfer rates are acceptable to both the host adapter and the
devices. This process prevents data from being lost and ensures that data
transmission is error free.
1.6.3 SCSI Adapters
The SCSI adapter provides the interface between the host bus (for example
Micro Channel or PCI) and the SCSI bus. The SCSI adapters that IBM has
developed are:
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IBM Personal System/2 Micro Channel SCSI Adapter
This adapter is a 16-bit Micro Channel bus master adapter adhering to the
SCSI-I interface. It is capable of an 8.3 MBps burst data transfer rate on the
Micro Channel. It uses a 16-bit data path and can use a 24- or a 32-bit
address on the Micro Channel. It can be installed in either a 16- or 32-bit
MCA slot, but if the system has more than 16MB of memory, it must be put in
a 32-bit slot due to the limitations of 24-bit addressing in a 16-bit slot.
The bus master capability of this SCSI adapter optimizes data flow from each
SCSI device configured to the system. This capability can provide
performance benefits in applications where multitasking or high-speed data
flow is essential. It allows the processor to be off-loaded from many of the
input/output activities common to DASD transfers. This adapter also
conforms to the Subsystem Control Block (SCB) architecture for Micro
Channel bus masters.
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IBM Personal System/2 Micro Channel SCSI Adapter with Cache
This adapter provides a superset of the features of the PS/2 Micro Channel
SCSI Adapter. It is a 32-bit Micro Channel bus master adapter containing a
512KB cache buffer. The cache is used to buffer data between system
memory and the device, which permits higher efficiency on both the Micro
Channel and the SCSI buses. It has a burst data transfer rate on the Micro
Channel of 16.6 MBps. This adapter is recommended where improved data
transfer rates and multiple SCSI devices are required and system memory is
constrained.
•
IBM SCSI-2 Fast/Wide Adapter/A
This adapter is a high performance SCSI-II adapter and a 32-bit Micro
Channel bus master adapter capable of streaming data at 40 MBps. It has
dual SCSI-II fast and wide channels (one 20 MBps internal and one 20 MBps
external). It supports devices using either asynchronous, synchronous, or
fast synchronous (10 MBps) SCSI data transfer rates. It also supports
Chapter 1. IBM PC Server Technologies 19