RS232 & RS449 standards

RS232 standard(considering Motorola ACIA-Asynchronous communication interface adapter IC )

  • RS stands for Recommended Standards

  • Serial data is sent through TxD line.

  • It requires output voltage level of driving amplifier for mark (logic 1, between
    -5V & -15 V) and space ( logic 0, between 5V &15V).

  • The terminating circuit receive data on receiver side through RxD line.

  • For control signal , voltage value is same but logic is positive i.e., for logic 1 it is between 5V & 15V and for logic 0 it is between -5V & -15V.

Main control signals are:
1.Request to send(RTS):

  • Send by UART to MODEM

  • MODEM responds by asserting clear to send (CTS).

  • The purpose is to provide handshaking between modem & UART.

2.Carrier detect (CD), data carrier detect (DCD) or receive line signal detect(RLSD)

  • When MODEM detects RTS from UART one of its response is to send mark logic 1 to telephone line, it responds by asserting CD.

  • UART detects CD & interprets that remote station is online & data link is complete.

3.Data terminal ready(DTR) from DTE and data set ready(DSR) from MODEM

  • It informs data link are electrically ready to establish data link.

  • MODEM asserts DSR saying that it is ready.

4.Common signal return

  • This establish common signal reference point between MODEM & DTE.

5.Ring indicator(RI)

  • It is asserted by modem capable of detecting incoming telephone ring.

  • DTE connected to auto answer mode responds by asserting RTS line beginning process to establish link.

  • Remaining lines are used for signalling & timing control for modems requiring data synchronization.

  • Secondary data rates are smaller used for acknowledgement in opposite direction of data channel.

Electrical and physical specifications:

  • Maximum ±25 V can be applied to line.

  • Cable impedance ( 330 maximum line drivers & 7000 load circuits)

  • Maximum line capacitance 2500pf.

  • Capacitance per foot must not exceed 50pf/foot.

  • Maximum cable length 50 feet due to capacitance specifications.

RS449 interface

  • It has two connectors: 37 pin main connector & 9 pin secondary channel connector specified by RS449 channel.

  • Increased pins share same functions as RS232, with reduced line capacitance.

  • It allows data rates of 2 Mbps & cable length of 200 feet.

New control functions include:

  • Local loop(LL), data placed in TxD line is looped back through MODEM onto receiver
    side to DTE.

  • For local equipment remote loop back (RL),data sent by station is looped back to sending station.

  • In remote loopback DTE is not involved.

  • A similar control signal data mode(DM) indicates data is in normal transfer mode.

  • If DTE desires to prepare DCE to receive new signal line(NS), cause receive data line(RD) to mark state & disables carrier detect(CD) line.

  • Once MODEM receives new signal, it places receive data line & asserts CD line.

  • The send common(SC) & receive common(RC) indicates direction in which data flows.

Reference: Analog & digital communication by Michel Miller

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