I/O BUSES

What is BUS ? A bus connects all the internal computer components to the CPU and Main memory. Every bus has a clock speed measured in MHz. A fast bus allows data to be transferred faster, which makes applications run faster.

I/O BUS System Bus: Connecting to CPU, memory and Cache. Address Bus Data Bus Control Bus I/O Bus: I/O bus defines the pathway for signals to flow between functional elements of computer. The I/O buses differ from the system bus in speed. Their speed will always be lower than the system bus speed.

GENERAL TYPES OF BUSES FRONT SIDE BUS (FSB):- FSB is also known as the Processor Bus, Memory Bus, or System Bus and connects the CPU with the main memory. BACK SIDE BUS (BSB):- Back-side bus is a computer bus that connects the CPU to the Cache and was first introduced with the Intel Pentium Pro. The back-side bus is a part of the CPU and has a speed that is dependent on the speed of the processor.

Types of I/O Buses ISA VESA Local Bus MCA PCI EISA PCI Express

ISA BUS Industry standard architecture(ISA) is the bus architecture that was introduced as an 8-bit bus with the original IBM PC in 1981 ; later was expanded to 16- bit with IBM PC/AT in 1984. Features :- Available in 8 bits and 16 bits. Number of devices to be attached :- upto 6 Has 20 address lines and 16 data lines

LIMITATIONS OF 8-BIT ISA BUS 1. Low data transfer rate :- The 8-bit ISA bus is driven at speed 4.77MHz. Data transfer rate = data bus width(bits) * bus speed = 8 * 4.77MHz = 38.16 Mbit/sec =38.16/8 MBps = 4.77MBps 2. Complex expansion board installation :- There is no PnP technique in it.So if any new card is to be used ,then a number of jumpers are to be used.

Features of 16-bit ISA 3.Very limited IRQ 4.Very limited DMA channel 1.Maximum data transfer rate :- The 16-bit ISA bus is driven at speed 8.33MHz. Data transfer rate = data bus width(bits) * bus speed = 16 * 8.33MHz = 133.28 Mbit/sec = 133.28 /8 MBps = 16.66MBps 2. Additional data and address lines :- Eight more data lines and 4 more address lines are introduced. 3. DMA :- 4 more DMA channels are added. 4. IRQ :- 5 more IRQ levels are added for more I/O connectivity.

ISA BUS 8-bit 16-bit

ISA BUS 8-bit ISA BUS 16-bit ISA BUS Pins Pins Bus width 8 - bit Compatible with 8 bit ISA card Pins 62 Power +5 V, -5 V, +12 V, -12 V Clock 4.7727266 MHz Bus width 16 - bit Compatible with 16 bit ISA card Pins 98 Power +5 V, -5 V, +12 V, -12 V Clock 8.333333 MHz

ISA BUS is used with sound cards, disk drives or most network and video cards. 8-bit ISA(XT) CARD 16-bit ISA CARD

ISA BUS 16 bit Original 8 bit ISA DMA lines More DMA lines IRQ lines 8 - 15 ± 5, ±12V Power IRQ lines 2-7 clock Extra 8 bits of I/O bus Extra 4 bits of Address bus 20 bits of Address bus 8 bits of I/O bus 16 bit Original 8 bit ISA

Elimination of ISA Bus ISA Bus is slow, hard to use and bulky. ISA plug in cards to be replaced by either PCI plug-in cards or USB add-on peripherals Limited number of interrupts.

MCA BUS Micro channel architecture bus was IBM’s effort to replace ISA with some more features, introduced in 1987. It was introduced with IBM PC/AT and was extended to 32 bits. MCA bus is incompatible with ISA bus. MCA bus has speed of 10MHz.

FEATURES OF MCA BUS 1). 32-bit Bus width :- MCA bus provides a full 32-bit bus width. Hence, had superior throughput to the ISA bus. 2). Plug and Play :- PnP technology was introduced in MCA bus ,thus there is no need to use jumpers and switches. 3). Interrupt sharing :- A limitation of ISA was that it was not possible to assign the same IRQ level to more than one devices at a time. But in MCA it’s possible to design a board that shares its interrupts with other boards in same PC.

4). Bus mastering :- Bus mastering is a technique that is basically used to decide the device next wants the access of the bus. If more than one devices are competing to have the access of the bus , then the master will choose the appropriate device to use it first according to its priority. Three types of devices compete for control of the bus :- a). System Master :- A system master controls and manages the system configuration .It is the default master i.e. it will have the control of channel/bus when no other master requires it.

b).Bus master :- A bus master supports data transfers with an I/O slave or a memory slave. c). DMA controller :- A DMA controller supports data transfers with DMA slaves and memory slaves. 5). MCA slave devices :- A MCA slave is a participant that sends and receives data under the control of an MCA master. a). I/O slave :- I/O slaves are selected by the address within the I/O address space. b). Memory slave :- Memory slaves are selected by their address within the memory address space.

c). DMA slave :- DMA slaves are selected by the address within the I/O address space. 6). Priority levels :- In MCA , all devices are given priority codes to ensure that order is preserved within the system. Memory refreshing is given highest priority. Next in the priority level is DMA channels followed by Bus masters in I/O slots. If any NMI(Non maskable interrupt) occurs, then control is returned to CPU immediately.

EISA BUS Developed by the "Gang of Nine" (nine non-IBM manufacturers of IBM-compatible PCs, led by Compaq) in 1988. 32 Bit Bus. Runs at 8.33 MHz. The theoretical maximum transfer speed is 33 Mbytes/s. A main advantage is that ISA boards can be plugged into the EISA bus.

FEATURES OF EISA BUS 1). ISA compatibility 2). 32 Bit bus width 3). Bus mastering 4). PnP 5). Data transfer rate 6). Interrupt sharing

3 SLOTS OF EISA BUS

ELIMINATION OF EISA BUS EISA could not deliver high speed video performance.

LOCAL BUS A local bus is a computer bus that connects directly, from the CPU to one or more slots on the expansion bus. The significance of direct connection to the CPU is avoiding the bottleneck created by the expansion bus, thus providing fast throughput. There are several local buses built into various types of computers to increase the speed of data transfer.

Bus layout in traditional PC

Why Local bus was introduced? The slow speed buses could be used with earlier computers because of their CLI (command line interface) nature i.e. they were using DOS based environment . With evolution of GUI(graphic user interface), in Windows we needed to have better graphics and videos transfer of better speed. Thus, we uses local buses.

VESA LOCAL BUS (VLB) Video Electronics standard associations. It was introduced in 1992 mainly for increasing the video performance in PCs. FEATURES :- VLB is a 32-bit bus . A VLB slot is a 16 bit ISA slot with third and fourth slot connectors added on the end. Its speed is 33 MHz As it is an extension of ISA bus,it is compatible With ISA card

LIMITATIONS OF VESA BUS Its design was based on 486 processor i.e. difficult to be adapted by Pentium. The number of cards that can be used was low (1 or 2). It doesn’t support bus mastering.

VESA 32-BIT SLOT

PCI LOCAL BUS Peripheral Component Interconnect was introduced in 1993 by Intel. FEATURES :- 32 bit bus. Speed of 33MHz can be extended to 66MHz. Adopted by 5th and 6th generation PCs. Parallel transfer.

What was new? Burst Mode: allows multiple sets of data to be sent. Full Bus Mastering: the ability of devices on the PCI bus to perform transfers directly. High Bandwidth Options: allows for increased speed of the PCI.

Typical PCI cards used in PCs include: network cards, sound cards, modems, extra ports such as USB , TV tuner cards and video cards .

32 bit PCI Card

PCI EXPRESS LOCAL BUS (PCIe) It was introduced in 2004 by Intel · Dell · HP · IBM Features :- It is a 64-bit bus. Its speed is 133MHz. Compatible with PCI. Serial Transfer yet Faster. Parallel transmission cant go longer.

PCIe Bus slots of different sizes Various PCI slots. From top to bottom: PCI Express ×4 PCI Express ×16 PCI Express ×1 Conventional PCI (32-bit)

PCIe Bus Card