25 pin D-SUB male connector at the DTE (Computer).
25 pin D-SUB female connector at the DCE (Modem).
EIA-530 (also known as RS-530 or TIA/EIA-530-A in its 1992 revision) is a balanced differential serial interface standard finalised in 1987 that was designed to replace the unwieldy 37-pin RS-449 (EIA-449) whilst retaining full RS-449 functionality in the familiar 25-position DB-25 connector already common on RS-232 equipment. Category I circuits employ RS-422 (TIA/EIA-422-B) balanced differential signalling, with each signal carried on a twisted pair (e.g. TD+ and TD−) rather than a signal-plus-ground arrangement as in RS-232; this allows data signalling rates from 20 kbit/s up to a nominal 2 Mbit/s, with timing leads capable of reaching 4 Mbit/s. Category II circuits — Local Loopback (pin 18), Remote Loopback (pin 21), and Test Mode (pin 25) — use RS-423 unbalanced single-ended signalling. The standard was widely used for synchronous WAN connections to leased-line modems, DSU/CSU units, and similar DCE equipment.
| Pin | Name | Dir | Description | Circuit | Paired with |
|---|---|---|---|---|---|
| 1 | - | Shield | 18 | ||
| 2 | TxD | OUT | Transmitted Data | BA | 14 |
| 3 | RxD | IN | Received Data | BB | 16 |
| 4 | RTS | OUT | Request To Send | CA | 19 |
| 5 | CTS | IN | Clear To Send | CB | 13 |
| 6 | DSR | IN | Data Set Ready | CC | 22 |
| 7 | SGND | - | Signal Ground | Ground | 21 |
| 8 | DCD | IN | Data Carrier Detect | CF | 10 |
| 9 | IN | Rtrn Receive Sig. Elmnt Timing | DD | 17 | |
| 10 | IN | Rtrn DCD | CF | 8 | |
| 11 | OUT | Rtrn Transmit Sig. Elmnt Timing | DA | 24 | |
| 12 | IN | Rtrn Transmit Sig. Elmnt Timing | DB | 15 | |
| 13 | IN | Rtrn CTS | CB | 5 | |
| 14 | OUT | Rtrn TxD | BA | 2 | |
| 15 | IN | Transmit Signal Element Timing | DB | 12 | |
| 16 | IN | Rtrn RxD | BB | 3 | |
| 17 | IN | Receive Signal Element Timing | DD | 9 | |
| 18 | LL | OUT | Local Loopback | LL | 1 |
| 19 | OUT | Rtrn RTS | CA | 4 | |
| 20 | DTR | OUT | Data Terminal Ready | CD | 23 |
| 21 | RL | OUT | Remote Loopback | RL | 7 |
| 22 | IN | Rtrn DSR | CC | 6 | |
| 23 | OUT | Rtrn DTR | CD | 20 | |
| 24 | OUT | Transmit Signal Element timing | DA | 11 | |
| 25 | IN | Test Mode | TM |
Implementation Options
| DCE | DTE |
|---|---|
| RS530 | RS232 |
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
| 4 | 4 |
| 5 | 5 |
| 6 | 6 |
| 7 | 7 |
| 8 | 8 |
| 9 | - |
| 10 | - |
| 11 | - |
| 12 | - |
| 13 | - |
| 14 | - |
| 15 | 15 |
| 16 | - |
| 17 | 17 |
| 18 | - |
| 19 | - |
| 20 | 20 |
| 21 | - |
| 22 | - |
| 23 | - |
| 24 | - |
| 25 | - |
Signals
- TxD (pin 2 / pin 14) — Transmitted Data differential pair (BA+/BA−); DTE output carrying serial data to the DCE.
- RxD (pin 3 / pin 16) — Received Data differential pair (BB+/BB−); DCE output carrying serial data to the DTE.
- RTS (pin 4 / pin 19) — Request To Send differential pair (CA+/CA−); DTE asserts to indicate readiness to transmit.
- CTS (pin 5 / pin 13) — Clear To Send differential pair (CB+/CB−); DCE grants permission to transmit.
- DSR (pin 6 / pin 22) — Data Set Ready differential pair (CC+/CC−); DCE indicates it is powered and ready.
- SGND (pin 7) — Signal Ground (single wire); common voltage reference for the RS-423 Category II circuits.
- DCD (pin 8 / pin 10) — Data Carrier Detect differential pair (CF+/CF−); DCE indicates a valid carrier is present.
- Receive Signal Element Timing (pin 17 / pin 9) — DD+/DD− differential pair; DCE-sourced receive clock provided to the DTE for clock recovery on received data.
- Transmit Signal Element Timing, DCE-sourced (pin 15 / pin 12) — DB+/DB− differential pair; DCE-sourced transmit clock provided to the DTE.
- Transmit Signal Element Timing, DTE-sourced (pin 24 / pin 11) — DA+/DA− differential pair; DTE-sourced transmit clock, used when the DTE generates its own clock (e.g. synchronous terminal timing).
- DTR (pin 20 / pin 23) — Data Terminal Ready differential pair (CD+/CD−); DTE indicates it is ready to operate.
- LL / Local Loopback (pin 18) — Category II (RS-423 unbalanced); DTE commands the DCE to loop its transmit signals back locally for testing.
- RL / Remote Loopback (pin 21) — Category II (RS-423 unbalanced); DTE commands the DCE to loop signals back at the remote end for end-to-end testing.
- Test Mode (pin 25) — Category II (RS-423 unbalanced); DCE indicates it is in a test condition.
- Shield (pin 1) — Cable shield/chassis ground, paired with pin 18 in the connector wiring.
Notes
- EIA-530 was finalised in 1987; the TIA/EIA-530-A revision followed in 1992.
- The standard was designed as a compact replacement for RS-449 (EIA-449, DB-37), providing equivalent functionality in the DB-25 connector already prevalent on RS-232 equipment.
- Category I circuits (all signals except pins 18, 21, and 25) use RS-422 balanced differential signalling: minimum differential output voltage 2 V across a 100 Ω load, ±200 mV receiver sensitivity, and ±7 V common-mode range.
- Category II circuits (pin 18 Local Loopback, pin 21 Remote Loopback, pin 25 Test Mode) use RS-423 unbalanced single-ended signalling referenced to pin 7 (Signal Ground).
- Each Category I signal is a twisted pair; the “Rtrn” (return) pins carry the inverted (−) leg of the differential pair. Recommended cable is 24 AWG twisted pair with 100 Ω characteristic impedance and 16 pF/ft shunt capacitance; Cat 5 Ethernet cable is a compatible alternative.
- Maximum cable distance for non-tailored applications is approximately 60 m (200 ft); on tailored networks distances up to 1 km are achievable at reduced data rates.
- Data signalling rates: 20 kbit/s to a nominal 2 Mbit/s; timing leads are specified to 4 Mbit/s.
- The DTE presents a male DB-25 connector; the DCE presents a female DB-25 connector.
- EIA-530 signal functions correspond to ITU-T V.24 interchange circuit definitions (circuit BA = TxD, BB = RxD, CA = RTS, CB = CTS, CC = DSR, CD = DTR, CF = DCD, DA/DB/DD = timing circuits, LL/RL/TM = test circuits).
References
- AN-1031: TIA/EIA-422-B Overview (Rev. B) — Texas Instruments
- AN-917: Popular Connector Pin Assignments for Data Communications — Texas Instruments
- Overview of Serial Communication Standards — Sealevel Systems
- ITU-T Recommendation V.24: List of Definitions for Interchange Circuits Between DTE and DCE — International Telecommunication Union