25 pin D-SUB female connector at the computer.
25 pin D-SUB male connector at the cable.
The SPARCstation 4, 5, 10, 20, LX, Classic, Classic X, and Ultra Enterprise 450 family share a single 25-pin D-sub female connector that multiplexes two complete serial channels — Port A and Port B — onto one plug, a design used by Sun Microsystems from the early 1990s onwards. Both ports operate to the RS-423 (±5 V) or RS-232 (±12 V) standard, selectable by on-board jumpers; on the Enterprise 450 the relevant jumper is J3303. Port A supports both synchronous and asynchronous operation and carries full modem-control signals, whereas Port B is asynchronous only. Connecting a standard serial cable gives access to Port A alone; simultaneous use of both ports requires Sun’s serial-interface splitter Y-cable (Sun part number X985A).
| Pin | Name | Port | Dir | Description |
|---|---|---|---|---|
| 2 | TXD | A | OUT | Transmit Data |
| 3 | RXD | A | IN | Receive Data |
| 4 | RTS | A | OUT | Request to Send |
| 5 | CTS | A | IN | Clear to Send |
| 6 | DSR | A | IN | Data Set Ready |
| 7 | GND | - | - | System Ground |
| 8 | CD | A | IN | Carrier Detect |
| 11 | DTR | B | OUT | Data Terminal Ready |
| 12 | CD | B | IN | Carrier Detect |
| 13 | CTS | B | IN | Clear to Send |
| 14 | TXD | B | OUT | Transmit Data |
| 15 | TC | A | IN | Transmit Clock from DCE, usually not used |
| 16 | RXD | B | IN | Receive Data |
| 17 | RC | A | IN | Receive Clock from DCE, usually not used |
| 18 | TC | B | IN | Transmit Clock from DCE, usually not used |
| 19 | RTS | B | OUT | Request to Send |
| 20 | DTR | A | OUT | Data Terminal Ready |
| 21 | TCO | A | OUT | Transmit Clock from DTE, usually not used. |
| 25 | TCO | B | OUT | Transmit Clock from DTE, usually not used. |
Notes
- Direction is DTE (computer) relative to DCE (modem).
- Do not connect SHIELD (Pin 1) to GND (Pin 7).
- Both ports must operate in the same signalling mode simultaneously (both RS-423 or both RS-232); the mode is set by on-board jumpers (J0801/J0802 on the SPARCstation 10; J3303 on the Enterprise 450).
- Pin 21 is not connected. The Transmit Clock output for Port A is on Pin 24 (TCO A); Pin 25 carries the Transmit Clock output for Port B (TCO B).
- Simultaneous access to both Port A and Port B requires Sun’s Y-cable serial interface splitter (Sun part number X985A); a plain DB-25 cable connects to Port A only.
- Pin 21 corrected to N/C per Sun Microsystems SPARCstation 20 Service Manual, Table C-4 (previously listed in error as TCO A OUT).
Signals
- TXD — Transmit Data (asynchronous serial data output from DTE).
- RXD — Receive Data (asynchronous serial data input to DTE).
- RTS — Request to Send; flow-control output asserted by the DTE.
- CTS — Clear to Send; flow-control input from the DCE.
- DSR — Data Set Ready; indicates the DCE is ready (Port A only).
- GND — Signal Ground reference; shared by both ports.
- CD — Carrier Detect; asserted by DCE when a carrier is detected.
- DTR — Data Terminal Ready; output indicating the DTE is ready.
- TC — Transmit Clock input from DCE (synchronous mode, Port A); used for externally clocked synchronous operation — usually not connected in asynchronous use.
- RC — Receive Clock input from DCE (synchronous mode, Port A); similarly unused in asynchronous operation.
- TCO — Transmit Clock Output from DTE (Pins 24 and 25 for Ports A and B respectively); the DTE supplies a clock to the DCE in synchronous master mode — rarely used.