©F.M. Rietti Communication Lines Fundamentals

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 2 Communication Lines Generally used to connect external devices –P2P (Point to Point) Point to Point cabling –EIA RS232 Multipoint cabling –Ethernet

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 3 Communication Lines (cont) EIA RS232 –RS-232 was first introduced in 1962 by the Radio Sector of the EIA. [2][3] The original DTEs were electromechanical teletypewriters, and the original DCEs were (usually) modems. When electronic terminals (smart and dumb) began to be used, they were often designed to be interchangeable with teletypewriters, and so supported RS-232.EIA [2][3]teletypewriterselectronic terminals

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 4 Communication Lines (cont) RS232 electrical –Independent channels are established for two- way (full-duplex) communications. The RS232 signals are represented by voltage levels with respect to a system common (power / logic ground). The "idle" state (MARK) has the signal level negative with respect to common, and the "active" state (SPACE) has the signal level positive with respect to common. RS232 has numerous handshaking lines (primarily used with modems), and also specifies a communications protocol. –The RS-232 interface presupposes a common ground between the DTE and DCE. This is a reasonable assumption when a short cable connects the DTE to the DCE, but with longer lines and connections between devices that may be on different electrical busses with different grounds, this may not be true.

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 5 Communication Lines (cont) RS232 electrical (cont) –RS232 data is bi-polar TO +12 volts indicates an "ON or 0-state (SPACE) condition" while A -3 to -12 volts indicates an "OFF" 1-state (MARK) condition.... Modern computer equipment ignores the negative level and accepts a zero voltage level as the "OFF" state. In fact, the "ON" state may be achieved with lesser positive potential. This means circuits powered by 5 VDC are capable of driving RS232 circuits directly, however, the overall range that the RS232 signal may be transmitted/received may be dramatically reduced. –The output signal level usually swings between +12V and -12V. The "dead area" between +3v and -3v is designed to absorb line noise. In the various RS-232-like definitions this dead area may vary. For instance, the definition for V.10 has a dead area from +0.3v to -0.3v. Many receivers designed for RS-232 are sensitive to differentials of 1v or less.

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 6 Communication Lines (cont) RS232 connections –Data is transmitted and received on pins 2 and 3 respectively. Data Set Ready (DSR) is an indication from the Data Set (i.e., the modem or DSU/CSU) that it is on. Similarly, DTR indicates to the Data Set that the DTE is on. Data Carrier Detect (DCD) indicates that a good carrier is being received from the remote modem. –Pins 4 RTS (Request To Send - from the transmitting computer) and 5 CTS (Clear To Send - from the Data set) are used to control. In most Asynchronous situations, RTS and CTS are constantly on throughout the communication session. The DTE transmits when it sees CTS up. –Full on RS232 on this LinkLink

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 7 Communication Lines (cont) RS232 minimum connections –Possible communicating only with 3 wires Common GND DCE Tx DTE Rx DCE Rx DTE Tx –For small speeds in microcontrollers environment without serial RS232 chips, transmit & receive can be implemented with a loop controlling Digital Inputs & Outputs (remember electrical tension standards), but not recommended, best using a dedicated chip, for tension adjustment and insulating circuits.

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 8 Coomunication Lines (cont) RS232 limits –Single msg exchanged is 7 or 8 bits, then a maximum set of 255 symbols –On a single cable, distance from DTE-DCE may be: RS232 cable length according to Texas Instruments Baud rate Maximum cable length (ft-m) : – – – –

©F.M. Rietti LM-18 Computer Science SSI Embedded Systems I 9 Communication Lines (cont) Point to Point (P2P) –One physical connection for device Multiplexed Devices (MUX) –One physical connection for devices