--- title: "RS-232 signals, handshaking, DTE-DCE" subject: "Microprocessor" chapter: 4 key: "semester_4/microprocessor:rs232-signals-dte-dce" source: https://www.bctengineer.com/q/semester_4/microprocessor/rs232-signals-dte-dce site: BCT Engineering --- # RS-232 signals, handshaking, DTE-DCE Microprocessor · Chapter 4: Microprocessor System - Asked in: 2070 Magh, 4 marks; 2069 Poush, 4 marks; 2068 Magh, 5 marks (asked 3 times) - Video: [RS-232 Serial Communication Protocol Explained: DB9 and DB25 Connectors, and DTE/DCE](https://www.youtube.com/watch?v=Qx2d20elziw) — Engineering Funda - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/microprocessor/rs232-signals-dte-dce - Whole chapter: https://www.bctengineer.com/md/semester_4/microprocessor/chapter-4 ## Question 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. ## 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. ## Answer 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) | Signal | Pin | Direction (from DTE) | Function | |---|---|---|---| | TxD | 2 | Out | Serial data from DTE | | RxD | 3 | In | Serial data to DTE | | RTS | 4 | Out | DTE requests permission to send | | CTS | 5 | In | DCE is ready to receive | | DSR | 6 | In | DCE is powered and ready | | GND | 7 | - | Signal ground | | DCD | 8 | In | Carrier detected from remote modem | | DTR | 20 | Out | DTE is ready | | RI | 22 | In | Telephone 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 ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are RTS and CTS used for? A. Sending and receiving the data bits B. Showing that the telephone line is ringing C. Per-transmission handshake: DTE requests to send and DCE signals it is clear to send D. Showing that the modem is powered Answer: C. RTS (from DTE) requests permission to send and CTS (from DCE) replies that it is ready to receive; they form the flow-control handshake. (Key point: RTS and CTS) 2. In DTE to DCE transfer, what must happen before the DTE sends bits on TxD? A. DCD goes off B. DTR/DSR are set, then DTE asserts RTS and DCE answers with CTS C. RI is asserted by the DCE D. The DTE sends a null-modem signal Answer: B. DTR is answered by DSR, then RTS is asserted and when the DCE asserts CTS the DTE sends data on TxD. (Key point: DTE to DCE sequence) 3. What does DCD indicate? A. A carrier has been detected from the remote modem B. The DTE is ready C. The DCE has finished sending D. A telephone ring is detected Answer: A. DCD (data carrier detect) shows the DCE detects a carrier from the remote end, meaning the line is connected; RI is the ring indicator. (Key point: DCD signal) 4. What does a null modem connection do when no modem is used? A. It adds a MAX232 level converter B. It converts the signals to TTL C. It connects DTR to ground D. It cross-connects TxD with RxD, RTS with CTS and DTR with DSR Answer: D. A null modem crosses the lines so that two DTEs can talk directly, each seeing the other as a DCE. (Key point: Null modem) ## Related in Microprocessor - [Short note: RS-232 standard](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ashort-note-rs232) — Microprocessor, chapter 4, asked 3× - [Short note: serial and parallel interface](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Aserial-parallel-interface) — Microprocessor, chapter 4 - [Synchronous vs asynchronous serial transmission](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Async-async-serial) — Microprocessor, chapter 4, asked 3× - [Single and double handshaking](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ahandshaking-single-double) — Microprocessor, chapter 4 ## Related in other subjects - [RS-232C handshaking signals](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-handshake-signals) — Instrumentation II, chapter 3, asked 2× - [Connecting two DTE RS-232 devices](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Adte-dte-problem) — Instrumentation II, chapter 3, asked 2× - [Long line by short model and vice versa](https://www.bctengineer.com/question.md?key=semester_4%2Fpower_system_analysis_i%3Along-line-short-model-vice-versa) — Power System Analysis I, chapter 5, asked 2× - [Importance and working of RS-232C](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-importance-signals) — Instrumentation II, chapter 3