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The Serial Port

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The Serial Port History

• Accessing remote locations before LAN and Internet: Telephone line and modem

• The information has to be transferred in a serial way

• From this scenario we have the terms DTE and DCE

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DTE / DCE

• DTE : Data Terminal Equipment

• DCE : Data Communication Equipment

DTE

Modem

DCERS232 / V24

Telephone line

Modem

DCERS232 / V24

DTE

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DTE / DCE continued…

• IBM defined the PC as a DTE

• An input pin on a DCE is an output on a DTE and vice versa

• The signal definitions were made in the Recommended Standard RS232

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RS-232• Standardized by EIA/TIA EIA-232 or later

TIA-232 with a following letter for the version (in the 60’s…)

• Initially for teletypewriters, but the use was soon extended to printers and other peripherals

• Great confusion in the usage over the years (and still…)

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RS-232C

• Defines a 25 pole D-sub connector

• About 20 signals for communication and synchronization

• Actual communication uses 3 pins – TxD (transmitt data) DTE out

– RxD (receive data) DCE out

– SG (signal ground) -

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RS-232C continued…

• Handshaking pins e.g.– RTS (request to send) DTE out

– CTS (clear to send) DCE out

– DSR (data set ready) DCE out

– DCD (data carrier detect) DCE out

– DTR (data terminal ready) DTE out

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RS-232C signals

• Voltage

• Bipolar signals1 is -3..-25 V

0 is +3..+25 V

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Connecting DTE/DCE

• A DTE is connected to a DCE pin to pin

• DTEs are most common today

• A DTE can connect to another DTE using a “null modem cable”

• In this cable the wires are crossed so that input meets the corresponding output.

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The serial port on the PC

• In the beginning the full 25-pole RS232C was used

• Later the 9-pole version was defined EIA/TIA-574

• Low voltage definitions EIA/TIA-562

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Why Use this Dinosaur?

• USB is intended to replace but -• Implementing USB is a huge task• Serial port relies on simple definitions and

is easy to use both on the software and hardware sides

• Encapsulated chip-solutions for using USB exist but use the serial port definitions on both sides

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The asynchronous serial format

1 0 1 0 0 0 1 1

1

idlestart

parity stop

LSB MSB

Logical levels shown

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About the asynchronous format

• Character oriented (maximum 8 bits)

• Agreement on the speed (bits/s)

• Standardized Baud rates

• Synchronization for each character/byte

• A UART (Universal Asynchronous Receiver Transmitter)handles the conversion to/from serial format

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Handshaking

• Receiver may be choked by too much data

• Hardware pins can be used e.g. RTS-CTS

• XON/XOFF characters can be used if only characters are transmitted

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American Standard code for Information Interchange