The DEC Multia (Alpha AXP) and Universal Desktop Box (UDB) pack two independent RS-232 serial ports into a single DB-25 male connector. COM1 occupies the standard EIA-232 primary-channel pins, whilst COM2 uses the EIA-232E secondary-channel assignments (TXD on pin 14, RXD on pin 16, and so on). Both ports operate at conventional RS-232 voltage levels — not DEC-423 or EIA-423. A DEC splitter cable (part number BC25X-1F) is required to separate the two ports into individual connectors for use with standard serial peripherals.
25 PIN D-SUB MALE at the computer
| Pin | Port | Name | Dir | Description |
|---|---|---|---|---|
| 1 | n/c | Not connected | ||
| 2 | 1 | TXD | OUT | Transmit Data |
| 3 | 1 | RXD | IN | Receive Data |
| 4 | 1 | RTS | OUT | Ready To Send |
| 5 | 1 | CTS | IN | Clear To Send |
| 6 | 1 | DSR | IN | Data Set Ready |
| 7 | 1+2 | GND | - | Ground |
| 8 | 1 | DCD | IN | Data Carrier Detect |
| 9 | n/c | Not connected | ||
| 10 | n/c | Not connected | ||
| 11 | 2 | DTR | OUT | Data Terminal Ready |
| 12 | 2 | DCD | IN | Data Carrier Detect |
| 13 | 2 | CTS | IN | Clear To Send |
| 14 | 2 | TXD | OUT | Transmit Data |
| 15 | n/c | Not connected | ||
| 16 | 2 | RXD | IN | Receive Data |
| 17 | n/c | Not connected | ||
| 18 | n/c | Not connected | ||
| 19 | 2 | RTS | OUT | Ready To Send |
| 20 | 1 | DTR | OUT | Data Terminal Ready |
| 21 | n/c | Not connected | ||
| 22 | 1 | RI | IN | Ring Indicator |
| 23 | 2 | DSR | IN | Data Set Ready |
| 24 | n/c | Not connected | ||
| 25 | 2 | RI | IN | Ring Indicator |
Notes
- Direction is DTE (Computer) relative DCE (Modem).
- Pin 7 (GND) is shared between both ports; all other signals are independent.
- COM2 uses the EIA-232E secondary-channel pin assignments rather than a second standard DB-25 wiring; a standard serial cable plugged directly into this connector will reach COM1 only.
- The DEC BC25X-1F splitter cable breaks the shared DB-25 out into two separate connectors, one for each port.
- Both ports use standard RS-232 voltage levels; this connector is not related to DEC-423 or EIA-423 signalling used on DEC MMJ ports.
Signals
- TXD — Transmit Data; serial data output from the DTE (computer) to the DCE (modem or peripheral).
- RXD — Receive Data; serial data input from the DCE to the DTE.
- RTS — Ready To Send; hardware flow-control output asserted by the DTE to request permission to transmit.
- CTS — Clear To Send; hardware flow-control input; DCE asserts this to grant the DTE permission to transmit.
- DSR — Data Set Ready; input indicating the DCE is powered on and ready.
- GND — Signal Ground; common reference for all RS-232 voltage levels; pin 7 is shared by both COM1 and COM2.
- DCD — Data Carrier Detect; input asserted by the DCE when a carrier signal is detected on the line.
- DTR — Data Terminal Ready; output asserted by the DTE to signal readiness and to enable the DCE.
- RI — Ring Indicator; input asserted by the DCE when an incoming call ring signal is detected.