RS232
In telecommunications, RS-232 is a standard for serial
communication transmission of data. It formally defined the
signals connecting between a DTE (data terminal
equipment) such as a computer terminal, and a DCE
(data
circuit-terminating equipment, originally
defined as data communication equipment[1]), such as a modem. The RS-232
standard is commonly used in computer serial ports.
In RS-232, user data is sent as a time-series of bits. Both synchronous
and asynchronous transmissions are supported by the standard. In addition to
the data circuits, the standard defines a number of control circuits used to
manage the connection between the DTE and DCE. Each data or control circuit
only operates in one direction, that is, signaling from a DTE to the attached
DCE or the reverse. Since transmit data and receive data are separate circuits,
the interface can operate in a full duplex manner, supporting
concurrent data flow in both directions. The standard does not define character
framing within the data stream, or character encoding.
Connectors
RS-232 devices may
be classified as Data Terminal Equipment (DTE) or Data Communication Equipment
(DCE); this defines at each device which wires will be sending and receiving
each signal. The standard recommended but did not make mandatory the D-subminiature 25-pin connector.
According to the standard, male connectors have DTE pin functions, and female
connectors have DCE pin functions. Other devices may have any combination of
connector gender and pin definitions.
Comparison of RS-232, RS-422, and RS-485
Serial Communication Interfaces
RS-232 is the most common serial interface and ships as a standard component on most Windows-compatible desktop computers. RS-232 only allows for one transmitter and one receiver on each line. RS-232 also uses a Full-Duplex transmission method. Some RS-232 boards sold by National Instruments support baud rates up to 1 Mbit/s, but most devices are limited to 115.2 kbits/s. Note that RS-422/RS-485 interface is not available on most IBM PCs.
RS-422 (EIA RS-422-A Standard) is the serial connection used on Apple computers. It provides a mechanism for transmitting data up to 10 Mbits/s. RS-422 sends each signal using two wires to increase the maximum baud rate and cable length. RS-422 is also specified for multi-drop applications where only one transmitter is connected to, and transmits on, a bus of up to 10 receivers.
The following table compares mode of operation,
total number of drivers and receivers, maximum cable length, and maximum data
rate.
Comparision
table
Specifications RS-232 RS-422 RS-485
Mode
of Operation Single-Ended Differential Differential
Total Number of Drivers
an 1 Driver 1 Driver 32 Driver
Receivers on One Line. One 1 Receiver 10
Receiver 32 Receiver
active at a
time for RS-485
networks
Maximum Cable Length 50 ft (2500 pF) 4000
ft 4000 ft
Maximum Data Rate 160 kbits/s 10 Mbit/s 10 Mbit/s
(40 ft - 4000 ft for (can be up to 1Mbit/s)
RS-422/RS-485)
Transceiver
A transceiver is a device comprising both a transmitter and a receiver which are combined and share common circuitry or a single housing. When
no circuitry is common between transmit and receive functions, the device is a
transmitter-receiver. The term originated in the early 1920s. Technically,
transceivers must combine a significant amount of the transmitter and receiver
handling circuitry.[citation needed] Similar devices include transponders, transverters, and repeaters.
RS-232
Transceiver(MAX3245E)
MAX3245E
are 3V-powered EIA/TIA-232 and V.28/V.24 communications interfaces with
automatic shutdown/wakeup features, high data-rate capabilities, and enhanced
electrostatic discharge (ESD) protection.
All devices achieve a 1μA supply current using
Maxim’s revolutionary Auto Shutdown Plus™ feature. These devices automatically
enter a low-power shutdown mode when the RS-232 cable is disconnected or the
transmitters of the connected peripherals are inactive, and the UART driving
the transmitter inputs is inactive for more than 30 seconds. They turn on again
when they sense a valid transition at any transmitter or receiver input. Auto Shutdown
Plus saves power without changes to the existing BIOS or operating system.
The MAX3245E also feature Mega Baud™ operation, guaranteeing
1Mbps for high speed applications such as communicating with ISDN modems. The
transceivers have a proprietary low-dropout transmitter output stage enabling true
RS-232 performance from a +3.0V to +5.5V supply with a dual charge pump. The
charge pump requires only four small 0.1μF capacitors for operation from a 3.3V
supply. All devices are available in a space-saving TQFN, SSOP, and TSSOP MAX3245E
packages.
Figure:
Pin Configuration
Table:
Pin Description
Table:
Pin Description
Dual
Charge-Pump Voltage Converter
The
MAX3245E’s internal power supply consists of a regulated dual charge pump that
provides output voltages of +5.5V (doubling charge pump) and -5.5V (inverting
charge pump), over the +3.0V to +5.5V range. The charge pump operates in
discontinuous mode: if the output voltages are less than 5.5V, the charge pump
is enabled; if the output voltages exceed 5.5V, the charge-pump is disabled.
Each charge pump requires a flying capacitor (C1, C2) and a reservoir capacitor
(C3, C4) to generate the V+ and V- supplies.
RS-232
Transmitters
The
transmitters are inverting level translators that convert CMOS-logic levels to
5.0V EIA/TIA-232 levels. The MAX3245E guarantee a 1Mbps data rate with
worst-case loads of 3kΩ in parallel with 1000pF, providing compatibility with
PC-to-PC communication software. Transmitters can be paralleled to drive
multiple receivers.
When FORCEOFF is driven to ground or when the Auto-
Shutdown Plus circuitry senses that all receiver and transmitter inputs are
inactive for more than 30s, the transmitters are disabled and the outputs go
into a high impedance state. When powered off or shut down, the outputs can be
driven to ±12V. The transmitter inputs do not have pull up resistors. Connect
unused inputs to GND or VCC.
RS-232
Receivers
The
receivers convert RS-232 signals to CMOS-logic output levels. The MAX3245E have
inverting three-state outputs that are high impedance when shut down (FORCEOFF
= GND).The MAX3245E feature an extra, always active, non inverting output,
R2OUTB. R2OUTB output monitors receiver activity while the other receivers are
high impedance, allowing ring indicator applications to be monitored without
forward biasing other devices connected to the receiver outputs. This is ideal
for systems where VCC is set to ground in shutdown to accommodate peripherals
such as UARTs.
The MAX3245E feature an INVALID output that is
enabled low when no valid RS-232 voltage levels have been detected on all
receiver inputs. Because INVALID indicates the receiver input’s condition, it
is independent of FORCEON and FORCEOFF states.
Auto
Shutdown Plus Mode
The
MAX3245E achieve a 1μA supply current with Maxim’s Auto Shutdown Plus feature, which
operates when FORCEOFF is high and a FORCEON is low. When these devices do not
sense a valid signal transition on any receiver and transmitter input for 30s,
the on-board charge pumps are shut down, reducing supply current to 1μA. This
occurs if the RS-232 cable is disconnected or if the connected peripheral
transmitters are turned off, and the UART driving the transmitter inputs is
inactive. The system turns on again when a valid transition is applied to any
RS-232 receiver or transmitter input. As a result, the system saves power
without changes to the existing BIOS or operating system.
±15kV
ESD Protection
As
with all Maxim devices, ESD-protection structures are incorporated on all pins
to protect against electrostatic discharges encountered during handling and
assembly. The driver outputs and receiver inputs of the MAX3245E have extra protection
against static electricity. Maxim’s engineers have developed state-of-the-art
structures to protect these pins against ESD of ±15kV without damage. The ESD structures
withstand high ESD in all states: normal operation, shutdown, and powered down.
After an ESD event, Maxim’s E versions keep working without latch up, whereas
competing RS-232 products can latch and must be powered down to remove latch up.
Capacitor
Selection
The
capacitor type used for C1–C4 is not critical for proper operation; polarized
or non polarized capacitors can be used. The charge pump requires 0.1μF
capacitors for 3.3V operation. Increasing the capacitor values (e.g., by a
factor of 2) reduces ripple on the transmitter outputs and slightly reduces
power consumption. C2, C3, and C4 can be increased without changing C1’s value.
However, do not increase C1 without
also increasing the values of C2, C3, C4, and CBYPASS, to maintain the proper
ratios (C1 to the other
capacitors).
Applications
§ Notebook, Subnotebook, and Palmtop Computers
§ Cellular Phones
§ Battery-Powered Equipment
§ Handheld Equipment
§ Peripherals
§ Printers
ADVANTAGES
Ø Designing single
PCB Layout for both PCB’s i.e.,
§ Synchro PCB.
§ Resolver PCB.
Ø Saves
PCB Layout cost.
Ø It is a indigenous technology.
Ø Design control.
DISADVATAGES
Ø Take
care when designing the PCB routing.
Ø If any two wires are shorted, the overall PCB
layout is waste for our requirement.
FUTURE SCOPE
Ø Planning to place Daughter card on the PCB by
using PMC connector.
Ø If in future require processor for high speed
calculations, daughter card can be placed on the PCB.
Ø In compact WCM-30 planned to place two SRIF
cards.
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