--- title: "Connecting two DTE RS-232 devices" subject: "Instrumentation II" chapter: 3 key: "semester_5/instrumentation_ii:dte-dte-problem" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/dte-dte-problem site: BCT Engineering --- # Connecting two DTE RS-232 devices Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System - Asked in: 2071 Shrawan, 5 marks; 2078 Kartik, 5 marks (asked 2 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 Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/dte-dte-problem - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3 ## Question Describe the problem that occurs when you attempt to connect together two Rs.232 devices that are both configured as DTE. Draw a diagram which shows how this problem can be resolved. ## Short answer Connecting two DTEs directly causes TXD-to-TXD and RXD-to-RXD contention, preventing communication. A null modem cable crosses TXD with RXD and handshake lines (RTS to CTS, DTR to DSR) to resolve it. ## Answer #### The problem RS-232 is defined for a **DTE to DCE** connection with a straight-through cable: the DTE transmits on TXD (pin 2) and receives on RXD (pin 3); the DCE does the opposite. If two DTEs (for example two PCs) are connected with a straight cable: 1. TXD of one is connected to TXD of the other, so **two output drivers are connected together** (contention, possible damage) and neither receives data. 2. RXD is connected to RXD, so both receivers are listening to nothing. 3. The handshake lines clash in the same way: RTS to RTS and DTR to DTR (outputs together), and the inputs CTS, DSR and DCD get no signal, so a device waiting for CTS or DSR waits forever. ```text DTE A (PC) DTE B (PC) TXD 2 ------------------------ TXD 2 (output to output) RXD 3 ------------------------ RXD 3 (input to input) RTS 4 ------------------------ RTS 4 CTS 5 ------------------------ CTS 5 GND 7 ------------------------ GND 7 ``` #### Solution: null modem (crossover) cable The cable makes each DTE think it is talking to a DCE by crossing the transmit/receive lines and the handshake lines. In the sketch, the number after each arrow is the pin on DTE B. ```text DTE A (DB-25) DTE B (DB-25) TXD 2 ------------------------------> 3 RXD RXD 3 <------------------------------ 2 TXD RTS 4 ------------------------------> 5 CTS CTS 5 <------------------------------ 4 RTS DTR 20 ----------------------------+-> 6 DSR +-> 8 DCD DSR 6 <-+--------------------------- 20 DTR DCD 8 <-+ GND 7 -------------------------------- 7 GND ``` Connections: TXD(A) -> RXD(B) and TXD(B) -> RXD(A); RTS(A) -> CTS(B) and RTS(B) -> CTS(A); DTR(A) -> DSR and DCD (B) and the reverse; signal ground straight through. The simplest null modem needs only three wires (TXD, RXD, GND), and the handshake inputs are looped back at each end (RTS to CTS, DTR to DSR and DCD) so the software never waits. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What goes wrong when two DTEs are joined by a straight cable? A. Voltage too high B. TXD is connected to TXD, so two outputs clash and neither receives C. The baud rate doubles D. Ground is missing Answer: B. RS-232 straight cable expects DTE-DCE; DTE to DTE ties output to output and input to input. (Key point: Straight cable problem) 2. What happens to handshake inputs like CTS and DSR in that case? A. They get no signal so a device may wait forever B. They are always high C. They are ignored by hardware D. They produce data Answer: A. Inputs get no signal from the other DTE, so a device waiting for CTS/DSR waits forever. (Key point: Handshake clash) 3. What is the solution? A. A longer cable B. A null modem (crossover) cable C. A higher baud rate D. Removing ground Answer: B. A null modem crosses TXD/RXD and handshake lines so each DTE sees a DCE. (Key point: Solution) 4. Which crossing is correct in a null modem? A. TXD(A) to TXD(B) B. TXD(A) to RXD(B) and RTS(A) to CTS(B) C. RXD(A) to RXD(B) D. CTS(A) to CTS(B) Answer: B. Transmit goes to receive and RTS to CTS; ground goes straight. (Key point: Crossings) ## Related in Instrumentation II - [USB to RS-232 adapter design](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ausb-rs232-adapter) — Instrumentation II, chapter 3 - [Simplex, half and full duplex using RS-232](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232-simplex-half-full) — Instrumentation II, chapter 3, asked 2× - [TTL to RS-232 interfacing circuit](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Attl-rs232-interface) — Instrumentation II, chapter 3 - [Importance and working of RS-232C](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Ars232c-importance-signals) — Instrumentation II, chapter 3 - [Sports car dashboard coupling problem](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Asports-car-coupling) — Instrumentation II, chapter 6 - [8255 mode 2 operation with timing diagram](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Appi-mode2) — Instrumentation II, chapter 2 ## Related in other subjects - [RS-232 signals, handshaking, DTE-DCE](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ars232-signals-dte-dce) — Microprocessor, chapter 4, asked 3× - [Short note: RS-232 standard](https://www.bctengineer.com/question.md?key=semester_4%2Fmicroprocessor%3Ashort-note-rs232) — Microprocessor, chapter 4, asked 3× - [Resection, two point problem, intersection](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Aresection-two-point-problem-ab-p) — Surveying II, chapter 5 - [Two point problem; advantages, disadvantages](https://www.bctengineer.com/question.md?key=semester_3%2Fsurveying_i%3Atwo-point-problem-adv-disadv) — Surveying I, chapter 6