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- 2073 Chaitra · 1+3 marks
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RS-232C handshaking signals
Short answer
TXD carries data from DTE to DCE, RXD carries data from DCE to DTE. RTS requests to send, CTS grants permission; DTR indicates DTE ready, DSR indicates DCE ready, providing hardware flow control.
Describe the functions of the RS-232C signals used in handshaking (DSR, DTR, RTS, CTS, TXD, RXD).
RS-232C connects a DTE (data terminal equipment, e.g. computer) to a DCE (data communication equipment, e.g. modem). Handshaking signals control the flow so that data is sent only when the other side is ready.
| Signal | DB-25 pin | DB-9 pin | Direction (from DTE) | Function |
|---|---|---|---|---|
| TXD | 2 | 3 | Output | Transmitted data, serial data from DTE to DCE |
| RXD | 3 | 2 | Input | Received data, serial data from DCE to DTE |
| RTS | 4 | 7 | Output | Request To Send: DTE asks DCE for permission to send |
| CTS | 5 | 8 | Input | Clear To Send: DCE is ready to receive; DTE may send |
| DSR | 6 | 6 | Input | Data Set Ready: DCE is powered on and ready |
| DTR | 20 | 4 | Output | Data Terminal Ready: DTE is ready to communicate |
| GND | 7 | 5 | - | Signal ground |
Handshake sequence (DTE sending to a modem)
- DTE raises DTR; DCE raises DSR (both ready).
- DTE raises RTS to request to send.
- DCE (when it can accept data) raises CTS.
- DTE sends data on TXD; data coming from the DCE arrives on RXD.
- After sending, DTE drops RTS and DCE drops CTS.
DTE DCE
DTR (20) ------------> DTR : "I am ready"
DSR (6) <------------ DSR : "I am ready"
RTS (4) ------------> RTS : "may I send?"
CTS (5) <------------ CTS : "yes, send"
TXD (2) ------------> data
RXD (3) <------------ data
Hardware flow control (RTS/CTS) prevents buffer overflow when a device is slower than the sender.
✓ Answer checked by Claude Sonnet 5.5
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