--- title: "Null modem connection" subject: "Instrumentation II" chapter: 3 key: "semester_5/instrumentation_ii:null-modem" source: https://www.bctengineer.com/q/semester_5/instrumentation_ii/null-modem site: BCT Engineering --- # Null modem connection Instrumentation II · Chapter 3: Serial Interfacing With Microprocessor Based System - Asked in: 2070 Chaitra, 4 marks; 2076 Chaitra, 5 marks (asked 2 times) - Video: [16. What is RS232C? What is Null Modem?](https://www.youtube.com/watch?v=SM2g4uzTs3w) — Shriram Vasudevan - Answer checked by Claude Sonnet 5.5 on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_5/instrumentation_ii/null-modem - Whole chapter: https://www.bctengineer.com/md/semester_5/instrumentation_ii/chapter-3 ## Question Explain how communication takes place between two data terminal equipment (DTE) in a null modem connection, with and without handshaking. ## Short answer Without handshaking, only TXD, RXD, and GND are crossed, and local loops emulate ready signals; flow control relies on software. With handshaking, RTS/CTS and DTR/DSR lines are cross-connected to allow hardware flow control, preventing overrun. ## Answer A **null modem** connection joins two DTEs (e.g. two PCs) directly by a cable (or adapter) that crosses the transmit/receive and handshake lines, so each DTE sees the other as a DCE. No modem is needed. #### 1. Null modem without handshaking (three-wire) Only TXD, RXD and ground are used. TXD of each is connected to RXD of the other; the handshake pins at each connector are shorted locally, so the software always sees "ready". In the sketches, the number after each arrow is the pin on DTE B. ```text DTE A DTE B TXD 2 ---------------------------> 3 RXD RXD 3 <--------------------------- 2 TXD GND 7 --------------------------- 7 GND (local loops at each end: RTS 4 - CTS 5 ; DTR 20 - DSR 6 - DCD 8) ``` Communication: A sends on TXD and B receives on RXD; B sends on its TXD and A receives. Both directions work together (full duplex). The sender cannot know whether the receiver is ready, so software flow control (XON/XOFF) or a low speed is required, otherwise data may be lost when the receiver is slow. #### 2. Null modem with handshaking (full handshake) Control lines are cross-connected so the hardware tells each side when the other is ready. ```text DTE A DTE B 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 ``` Sequence: A raises DTR, so B sees DSR and DCD high (A is on-line). When A is ready to receive, it raises RTS; B sees this on its CTS input and may send. If A's buffer becomes full, A drops RTS, B's CTS goes low and B stops sending until A is ready again. The same happens in the other direction (B's RTS to A's CTS). In this way hardware flow control prevents overflow. | Point | Without handshaking | With handshaking | |---|---|---| | Wires | 3 | 7 or more | | Flow control | Software (XON/XOFF) | Hardware (RTS/CTS, DTR/DSR) | | Reliability at high speed | Lower | Higher | | Use | Simple, short links | File transfer, fast links | ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is a null modem? A. A cable joining two DTEs by crossing transmit/receive and handshake lines B. A modem without a phone line C. A type of DCE D. A USB hub Answer: A. It makes each DTE see the other as a DCE, with no modem needed. (Key point: Null modem purpose) 2. In three-wire null modem, what is used? A. TXD, RXD and ground, with handshake pins looped locally B. All 25 pins C. Only TXD D. RTS and CTS only Answer: A. Handshake pins are shorted at each end so software always sees ready. (Key point: Three-wire) 3. What flow control is needed without hardware handshaking? A. Hardware RTS/CTS B. Software XON/XOFF or low speed C. None D. DTR only Answer: B. The sender cannot know if the receiver is ready, so software flow control or low speed is required. (Key point: Flow control without handshake) 4. With full handshake, how is overflow prevented? A. RTS/CTS and DTR/DSR cross-connected so a side can stop the other B. Using parity C. Extra stop bits D. Ignoring CTS Answer: A. If the receiver's buffer is full it drops RTS, so the sender's CTS goes low and sending stops. (Key point: Full handshake) ## Related in Instrumentation II - [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× - [Master-slave 8085 bidirectional communication via 8255](https://www.bctengineer.com/question.md?key=semester_5%2Finstrumentation_ii%3Abidirectional-8085-master-slave) — 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× - [Why hydration of cement takes place](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Ahydration-of-cement-why) — Civil Engineering Materials, chapter 5 - [ADSL operation, modem connection and topology](https://www.bctengineer.com/question.md?key=semester_8%2Ftelecommunications%3Aadsl-modem-topology) — Telecommunications, chapter 8, asked 3× - [Two wattmeter method connection diagram](https://www.bctengineer.com/question.md?key=semester_1%2Fbasic_electrical_engineering%3Atwo-wattmeter-connection-diagram) — Basic Electrical Engineering, chapter 8, asked 2×