Serial Communication Interfaces

Communication Hardware &
Protocols
Prof. Anish Goel
Embedded Systems

Communications
Embedded Systems Prof.Anish Goel2

Serial Communications

Asynchronous Serial (RS‐232)

RS232 Voltage levels

RS232 – Handshaking

Serial Peripheral Interface

SPI and the clock

I2C

I2C Write a Single Byte

Other Features

RS-485
RS232 is an interface to connect one DTE, data terminal equipment to one DCE, data
communication equipment at a maximum speed of 20 kbps with a maximum cable
length of 50 feet.
This was sufficient in the old days where almost all computer equipment were
connected using modems, but soon after people started to look for interfaces capable
of one or more of the following:
•Connect DTE's directly without the need of modems
•Connect several DTE's in a network structure
•Ability to communicate over longer distances
•Ability to communicate at faster communication rates
RS485 is the most versatile communication standard in the standard series defined by the
EIA, as it performs well on all four points.That is why RS485 is currently a widely used
communication interface in data acquisition and control applications where multiple
nodes communicate with each other.

Differential signals with RS485:
Longer distances and higher bit rates
•Main problems with RS232 is the lack of immunity for noise on the signal lines.
•The transmitter and receiver compare the voltages of the data- and handshake lines
with one common zero line.
•Shifts in the ground level can have disastrous effects.
•Therefore the trigger level of the RS232 interface is set relatively high at ±3Volt.
•Noise is easily picked up and limits both the maximum distance and communication
speed.
•With RS485 on the contrary there is no such thing as a common zero as a signal
reference.
•Several volts difference in the ground level of the RS485 transmitter and receiver
does not cause any problems.
•The RS485 signals are floating and each signal is transmitted over a Sig+ line and a Sig-
line.
•The RS485 receiver compares the voltage difference between both lines, instead of
the absolute voltage level on a signal line.
•This works well and prevents the existence of ground loops, a common source of
communication problems.
•The best results are achieved if the Sig+ and Sig- lines are twisted.

Noise in straight and twisted pair cables
•Noise is generated by magnetic fields from the
environment.
•The picture shows the magnetic field lines and
the noise current in the RS485 data lines that is
the result of that magnetic field.
•In the straight cable, all noise current is flowing in
the same direction, practically generating a looping
current just like in an ordinary transformer.
•When the cable is twisted, we see that in some
parts of the signal lines the direction of the noise
current is the opposite from the current in other
parts of the cable.
•Because of this, the resulting noise current is
many factors lower than with an ordinary straight
cable. Shielding—which is a common method to
prevent noise in RS232 lines—tries to keep hostile
magnetic fields away from the signal lines.

RS 485 Advantages
•Twisted pairs in RS485 communication however adds immunity which is a much
better way to fight noise.
•The magnetic fields are allowed to pass, but do no harm.
•If high noise immunity is needed, often a combination of twisting and shielding is
used as for example in STP, shielded twisted pair and FTP, foiled twisted pair
networking cables.
•Differential signals and twisting allows RS485 to communicate over much longer
communication distances than achievable with RS232. With RS485 communication
distances of 1200 m are possible.
•Differential signal lines also allow higher bit rates than possible with non-differential
connections. Therefore RS485 can overcome the practical communication speed limit
of RS232. Currently RS485 drivers are produced that can achieve a bit rate of
35 mbps.

Characteristics of RS232, RS422, RS423
and RS485

CAN
A Controller Area Network (CAN) bus is a high-integrity serial bus system for
networking intelligent devices. CAN busses and devices are common components
in automotive and industrial systems.
Automotive Applications

Benefits of CAN
• Lower cost from reduced wiring compared to two wire, point-to-point
wiring
• Highly robust protocol
– Built-in determinism
– Fault tolerance
– Reliable—More than a decade of use in the automotive industry
CAN Specifications
• CAN data (up to 8 bytes in a frame)
• Maximum 1 Mbaud/s
• 40 Meters at 1 Mbaud/s
• 6 km at 10 kbaud/s
•Theoretical maximum of 2,032 nodes per bus
– Practical limit is approximately 100 nodes due to transceiver
– Most buses use 3–10 nodes
• Data fields—Arbitration ID (11 bit or 29 bit)
– Indicates message priority

CAN Frame
•CAN devices send data across the CAN Network on packets called frames.
•A typical CAN frame contains an arbitration ID, a data field, a remote frame, an
error frame, and an overload frame.

CAN Arbitration
The arbitration ID determines the priority of the messages on the bus. If
multiple nodes try to transmit a message onto the CAN bus at the same time,
the node with the highest priority (lowest arbitration ID) automatically gets
bus access. Nodes with a lower priority must wait until the bus becomes
available before trying to transmit again.The waiting devices wait until the
end of the frame section is detected.

Data Field
•The data field contains the actual data being transmitted.
•The CAN protocol supports two data field formats as defined in the Bosch Version
2.0 specifications, the essential difference being in the length of the arbitration
•ID.
•In the standard frame format (also known as 2.0A), the length of the ID is 11 bits.
•In the extended frame format (also known as 2.0B), the length of the ID is 29 bits.

Introduction to USB
General Introduction
The Universal Serial Bus (USB) is a specification
developed by Compaq, Intel, Microsoft and NEC,
joined later by Hewlett-Packard, Lucent and Philips.
These companies formed the USB Implementers
Forum, Inc as a non-profit corporation to publish the
specifications and organise further development in
USB.
The aim of the USB-IF was to find a solution to the
mixture of connection methods to the PC, in use
at the time. We had serial ports, parallel ports,
keyboard and mouse connections, joystick ports,
midi ports and so on. And none of these satisfied
the basic requirements of plug-and-play.
Additionally many of these ports made use of a
limited pool of PC resources, such as Hardware
Interrupts,and DMA channels.

Data Speeds
The USB specification defines three data
speeds, shown to the right. These speeds are
the fundamental clocking rates of the system,
and as such do not represent possible
throughput, which will always be lower as the
result of the protocol overheads.
Low Speed
This was intended for cheap, low data rate devices like mice. The low speed
captive cable is thinner and more flexible than that required for full and high
speed.
Full Speed
This was originally specified for all other devices.
High Speed
The high speed additions to the specification were introduced in USB 2.0 as a
response to the higher speed of Firewire.

Architecture
The USB is based on a so-called 'tiered star topology' in which there is a single host
controller and up to 127 'slave' devices.
The host controller is connected to a hub, integrated within the PC, which allows a
number of attachment points (often loosely referred to as ports).
A further hub may be plugged into each of these attachment points, and so on.
However there are limitations on this expansion.
As stated above a maximum of 127 devices (including hubs) may be connected.
This is because the address field in a packet is 7 bits long, and the address 0 cannot be
used as it has special significance. (In most systems the bus would be running out of
bandwidth, or other resources, long before the 127 devices was reached.)
A device can be plugged into a hub, and that hub can be plugged into another hub and
so on. However the maximum number of tiers permitted is six.
The length of any cable is limited to 5 metres.This limitation is expressed in the
specification in terms of cable delays etc, but 5 metres can be taken as the practical
consequence of the specification.

USB
This means that a device cannot be further
than 30 metres from the PC, and even to
achieve that will involve 5 external hubs, of
which at least 2 will need to be self-powered.
So the USB is intended as a bus for devices
near to the PC. For applications requiring
distance from the PC, another form of
connection is needed, such as Ethernet.

Host is Master
•All communications on this bus are initiated by the host.
•This means, for example, that there can be no communication directly between USB
devices.
•A device cannot initiate a transfer, but must wait to be asked to transfer data by the host.
The only exception to this is when a device has been put into 'suspend' (a low power
state) by the host then the device can signal a 'remote wakeup'.

Serial Communication Interfaces

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Serial Communication Interfaces