RS-232 was created for one purpose, to interface between Data Terminal Equipment (DTE) and Data Communications Equipment (DCE) employing serial binary data interchange. So as stated the DTE is the terminal or computer and the DCE is the modem or other communications device. The current designation of the standard is TIA-232-F; its interchange circuits follow ITU-T V.24 and its electrical levels follow ITU-T V.28. A logic 1 (mark) is a negative voltage (−3 to −15 V) and a logic 0 (space) a positive voltage (+3 to +15 V); the maximum signalling rate is 20 kbit/s.
(At the DTE)
25 PIN D-SUB MALE at the DTE (Computer)
25 PIN D-SUB FEMALE at the DCE (Modem)
| Pin | Name | ITU-T | Dir | Description |
|---|---|---|---|---|
| 1 | GND | 101 | - | AA: Protective Ground |
| 2 | TXD | 103 | OUT | BA: Transmit Data |
| 3 | RXD | 104 | IN | BB: Receive Data |
| 4 | RTS | 105 | OUT | CA: Request to Send |
| 5 | CTS | 106 | IN | CB: Clear to Send |
| 6 | DSR | 107 | IN | CC: Data Set Ready |
| 7 | GND | 102 | - | AB: Signal Ground |
| 8 | CD | 109 | IN | CF: Carrier Detect |
| 9 | - | - | - | RESERVED - Positive test |
| 10 | - | - | - | RESERVED - Negative test |
| 11 | STF | 126 | OUT | Select Transmit Channel |
| 12 | S.CD | 122 | IN | SCF: Secondary Carrier Detect |
| 13 | S.CTS | 121 | IN | SCB: Secondary Clear to Send |
| 14 | S.TXD | 118 | OUT | SBA: Secondary Transmit Data |
| 15 | TCK | 114 | IN | DB: Transmission Signal Element Timing |
| 16 | S.RXD | 119 | IN | SBB: Secondary Receive Data |
| 17 | RCK | 115 | IN | DD: Receiver Signal Element Timing |
| 18 | LL | 141 | OUT | Local Loop Control |
| 19 | S.RTS | 120 | OUT | SCA: Secondary Request to Send |
| 20 | DTR | 108 | OUT | CD: Data Terminal Ready |
| 21 | RL | 140 | OUT | Remote Loop Control |
| 22 | RI | 125 | IN | CE: Ring Indicator |
| 23 | DSR | 111/112 | OUT | CH/CI: Data Signal Rate Selector |
| 24 | XCK | 113 | OUT | DA: Transmit Signal Element Timing |
| 25 | TI | 142 | IN | Test Indicator |
Notes
- Direction is DTE (Computer) relative DCE (Modem)
- Do not connect SHIELD(1) to GND(7)
HISTORY
RS-232 was originally adopted in 1962 by the Electronic Industries Association (EIA). The standard evolved over the years and in 1969 the third revision (RS-232C) was to be the standard of choice of PC makers. In 1987 a fourth revision was adopted (RS-232D also known as EIA-232D). In most part of this new revision, they added 3 additional test lines. In this document we will see several parts of the original RS-232C standard and mostly the ones used in the PC world. Most equipment using RS-232 serial ports use a DB-25 type connector even if the original documents didn’t specify a specific connector, many PCs today use DB-9 connectors since all you need in asynchronous mode is 9 signals. But take note that the document does specify the amount of pins and there assignment, 20 affected to different signals, three are reserved and two are not affected. Normally the male connector is on the DTE side and the female connector is on the DCE side even if this is not always the case.
Signals
Each interchange circuit has a two-letter name: the first letter gives its class — A common, B data, C control, D timing — and a leading S marks a secondary-channel circuit.
- AA — Protective Ground (pin 1): Bonds the cable shield to the equipment frame; not used as a signal return.
- AB — Signal Ground (pin 7): Common reference for every other circuit; must be present for communication.
- BA — Transmitted Data (pin 2): Data from the DTE to the DCE; held at mark (logic 1) when idle.
- BB — Received Data (pin 3): Data from the DCE to the DTE.
- CA — Request To Send (pin 4): Asserted by the DTE to prepare the DCE to receive data.
- CB — Clear To Send (pin 5): Asserted by the DCE when it is ready to accept data from the DTE.
- CC — Data Set Ready (pin 6): Asserted by the DCE to indicate it is powered and ready.
- CF — Data Carrier Detect (pin 8): Asserted by the DCE when it detects a carrier from the remote device.
- CD — Data Terminal Ready (pin 20): Asserted by the DTE to bring the DCE onto the line; dropping it disconnects.
- RL — Remote Loopback (pin 21): Requests the remote DCE to enter a loopback test.
- CE — Ring Indicator (pin 22): Asserted by the DCE while an incoming call ring is present.
- CH / CI — Data Signal Rate Selector (pin 23): Selects between two DCE signalling rates (CH DTE-sourced, CI DCE-sourced).
- DA / DB — Transmitter Signal Element Timing (pins 24 / 15): Transmit clock; DA is supplied by the DTE, DB by the DCE, only one at a time.
- DD — Receiver Signal Element Timing (pin 17): Receive clock from the DCE, marking when the DTE samples Received Data.
Macintosh to RS-232 cable
Signal correspondence for building a cable between a Macintosh serial port and a standard RS-232 device:
| Mac name | RS-232 name |
|---|---|
| RxD | RD |
| TxD | TD |
| Ground | SGND |
| HSKi | CTS |
| HSKo | RTS |
| DTR | |
| GPi | DCD |
| RI | |
| DSR |
Notes
- There are two ground pins on the Mac, 4 and 8
- Both RTS and DTR are wired to pin 1 on the Mac
- Not all Macs support DCD via pin 7
- RI and DSR are not supported on the Mac
- DSR is not supported on the RJ-45
References
- Interface Circuits for TIA/EIA-232-F (Texas Instruments, SLLA037A) — the electrical specification, voltage levels and the DTE-male / DCE-female connector convention.
- ITU-T Recommendation V.24 — list of definitions for interchange circuits — the circuit names and numbers (103 TXD, 104 RXD, 105 RTS …) used in the pin table.
- EIA/TIA-232-E interface standard (Datapro reproduction) — the DB-25 pin-assignment table and the mark/space voltage definitions.