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A "bus" is just a number of wires in parallel used to transfer
information from one device to another device. The wires may
be built into a printed wiring board, PWB, or may be in
a flexible cable.
The most important specification for a bus is its protocol.
The protocol defines the method for accessing the bus, read
requests, write requests, address and data sequencing, etc.
There may be many devices on a bus. In order for all the
devices to work together, all must follow the protocol.
A possible bus may have the following sets of lines.
The Control lines are used to implement the protocol.
There may be a bus master, hardware, that arbitrates when
two devices want to get on the bus at the same time.
When a bus has a clock, the bus is called synchronous.
All signals change on rising edge, falling edge or both.
An asynchronous bus is driven at the speed of the device
currently driving the bus.
Diagram showing how busses might be connected in a computer:
The bandwidth, speed, of a bus may be measured in
bits per second, bps Mbps is 10^6 bps, not 2^20 bps
bytes per second, Bps communication is typically powers of 10
megahertz, MHz
words per second
transactions per second
A transaction is a complete protocol sequence.
An example with time progressing down:
Device 1 Device 2
wait for bus available
put address on bus
set request to 1
wait for Ack = 1, acknowledge
wake up because request = 1
save address from bus
set Ack to 1
wait for request = 0
wake up because Ack = 1
release address lines
set request to 0
wait for ready = 1
wake up because request = 0
set Ack to 0
put data on the bus
set ready to 1
wait for Ack = 1
wake up because ready = 1
save data from bus
set Ack to 1
wait for ready = 0
wake up because Ack = 1
release data lines
set ready to 0
finished this transaction
wake up because ready = 0
set Ack to 0
finished this transaction
bus is available
Often, the bus protocol is implemented as a Deterministic Finite
Automata, DFA. The state diagram for the above protocol could be
shown as:
Examples of Busses circa 2012 including older (changes with time)
Bus name Max Max Max width comment
Mbits MBytes MHz
per sec per sec
front side 17,024 2,128 133 128 many possible
34,048 4,256 133 256
19,200 2,400 150 128
85,248 10,656 333 256
136,448 17,056 533 256
204,800 25,600 800 256
225,280 26,160 880 256
256,000 32,000 1,000 256
320,000 40,000 1,250 256 (PPC Mac G5)
307,200 38,400 1,600 192 (I7 extreme, 1 channel)
AGP 2,112 264 66 32
AGP8X 17,056 2,132 533 32
PCI 1,056 132 33 32
PCI 2,112 264 33 64
PCI 2,112 264 66 32
PCI 4,224 528 66 64
PCI 4,224 528 133 32
PCI 8,448 1,056 133 64
PCIX 17,056 2,132 533 32 extended, compatible
PCIe 64,000 8,000 2000 32 express, one way, full duplex
1,2,4,8,12,16 or 32 lanes
ATA 100 800 100 25 32
ATA 133 1064 133 33 32
ATA 160 1280 160 40 32
SATA 150 1200 150 600 2 one way, full duplex
SATA std 1500 187 1500 1 one way, full duplex
limited by motherboard
SATA II 300 2400 300 1200 2
SATA II std 3000 375 3000 1 no forcing to build standard
SATA 3.0 6000 750 6000 1
SCSI 1 40 5 5 8
SCSI 2 160 20 10 16
SCSI 3 1280 160 80 16
SCSI UW3 2560 320 160 16
SCSI 320 5120 640 320 16 has cable terminators
Firewire1394 400 50 400 1
Firewire1394b 800 100 800 1 many video cameras
Firewire S16 1600 200 1600 1
Firewire S32 3200 400 3200 1
Firewire S80 6400 800 6400 1
USB 1.1 12 1.5 12 1 slow
USB 2 480 60 480 1 new cable
USB 3 3200 400 1600 2 new cable, dual differential
5000 625 2500 2 new connectors, optional speed
6400 800 3200 2 micro, mini, connectors etc.
Fiberchannel 1000 125 1000 1 1062.5
Fiberchannel 2000 250 2000 1 >mile
Fibre 16GFC 3200 14000 full duplex 10Km
Fibre 20GFC 5100 21000 full duplex
Ethernet 10 10 1.25 10 1
Ethernet 100 100 12.5 100 1
Ethernet 1Gig 1000 125 1000 1
Ethernet 10G 10000 1,250 10000 1
ISA 400 50 25 16 really old
IEEE 1284 ECP 2.5 0.31 0.31 8 half duplex
printer port
V.90 56 0.056 0.005 0.056 1 modem, one way, full duplex
OC-48 2,500 optical cross country
OC-192 STM64 10,000 Optical Carrier
OC-768 STM256 40,000 5,000 light
Mbps MBps MHz
The speed of light limits the amount of information that can be
sent over a given distance. Many busses have length restrictions.
Light can travel about
300,000,000 meters per second
300,000 meters per millisecond
300 meters per microsecond
0.3 meters per nanosecond (about 1 foot)
Unchanged in last few decades. (slower inside integrated circuit)
Pentium 4 busses and PCI-X vs PCIe
Note one example of AGP being replaced by PCI-e and the mention
of many "busses" in the advertisement:
SCSI and printer port, wave forms
For HW12, read the directions carefully. Every bus is different.
Example of HW12 solution method
Now you can do HW 12
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