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RS-232 signals, handshaking, DTE-DCE

Short answer

RS-232C connects DTE (computer) to DCE (modem) with signals: TxD, RxD, RTS, CTS, DTR, DSR, DCD. Handshaking: DTE asserts DTR, DCE replies DSR; DTE raises RTS, DCE asserts CTS, then data flows on TxD/RxD. DCD indicates carrier. Null modem cross-connects for direct DTE-DTE.

Explain the signals, connections and handshaking signals used in the RS-232 (RS-232C) serial interface standard. Describe how data is transferred from DTE to DCE and vice versa.

RS-232C connects a DTE (computer/terminal) to a DCE (modem) with a 25-pin (or 9-pin) connector. Logic 1 is −3 to −15 V and logic 0 is +3 to +15 V.

Signals (DB-25 pin number)

SignalPinDirection (from DTE)Function
TxD2OutSerial data from DTE
RxD3InSerial data to DTE
RTS4OutDTE requests permission to send
CTS5InDCE is ready to receive
DSR6InDCE is powered and ready
GND7-Signal ground
DCD8InCarrier detected from remote modem
DTR20OutDTE is ready
RI22InTelephone ring detected

Handshaking signals: DTR/DSR (device readiness) and RTS/CTS (flow control for each transmission), with DCD showing that the line is connected.

Data transfer

 DTE (computer)                        DCE (modem)
   DTR  ------------------------------>   (DTE ready)
   DSR  <------------------------------   (DCE ready)
   RTS  ------------------------------>   (request to send)
   CTS  <------------------------------   (clear to send)
   TxD  ------------------------------>   serial data out
   RxD  <------------------------------   serial data in
   DCD  <------------------------------   (carrier present)

DTE → DCE: the DTE turns on DTR; the DCE responds with DSR. The DTE then asserts RTS; when the DCE is ready (carrier established) it asserts CTS. The DTE now sends the bits on TxD (start bit, data, parity, stop). At the end RTS is dropped and CTS goes off.

DCE → DTE: when a remote carrier is detected the DCE asserts DCD, and sends the received data on RxD; the DTE reads it, using DTR to indicate that it can accept data (hardware flow control). Where no modem is used, a null modem cross-connects TxD↔RxD, RTS↔CTS and DTR↔DSR.

✓ Answer checked by Claude Opus 5.5 (high effort)

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