9 PIN D-SUB FEMALE at the Computer
The MSX RS-232C serial interface is an optional part of the MSX standard and was not built into the base specification of most MSX computers; it was typically supplied as a plug-in cartridge rather than a rear-panel port. The interface uses a DE-9 (9-pin D-sub) female connector at the computer in standard DTE orientation, following the EIA-574 convention in which pin 2 carries Receive Data and pin 3 carries Transmit Data — the reverse of the older DB-25 DTE assignment. Internally, implementations use an Intel 8251 USART paired with an 8253 or 8254 programmable interval timer for baud-rate generation; MSX BIOS version 1 supports a single adapter via I/O ports, whilst version 2 uses memory-mapped I/O to allow multiple adapters simultaneously. Some implementations connect only pins 1–8, leaving Ring Indicator on pin 9 unconnected.
| Pin | Name | Dir | Description |
|---|---|---|---|
| 1 | DCD | IN | Carrier Detect |
| 2 | RXD | IN | Receive Data |
| 3 | TXD | OUT | Transmit Data |
| 4 | DTR | OUT | Data Terminal Ready |
| 5 | GND | - | Signal Ground |
| 6 | DSR | IN | Data Set Ready |
| 7 | RTS | OUT | Request to Send |
| 8 | CTS | IN | Clear to Send |
| 9 | RI | IN | Ring Indicator |
Notes
- Direction is DTE (Computer) relative to DCE (Modem).
- The DE-9 DTE pin convention (pin 2=RXD, pin 3=TXD) is the inverse of the DB-25 DTE convention (pin 2=TXD, pin 3=RXD); both DB-25 and DE-9 variants exist across different MSX implementations.
- Pin 9 (RI) is not connected on many MSX RS-232C cartridge implementations.
- The RS-232C interface is an optional MSX standard feature; many MSX computers lack it entirely without an appropriate cartridge.
Signals
- DCD — Data Carrier Detect; input from modem indicating a carrier signal is present on the telephone line.
- RXD — Receive Data; serial data input from the remote DCE device to the MSX computer.
- TXD — Transmit Data; serial data output from the MSX computer to the remote DCE device.
- DTR — Data Terminal Ready; output asserting that the MSX computer is ready to communicate.
- GND — Signal Ground; reference potential for all signal lines.
- DSR — Data Set Ready; input from modem indicating it is connected and ready.
- RTS — Request to Send; output used for hardware flow control, requesting the DCE to prepare to receive data.
- CTS — Clear to Send; input from DCE indicating it is ready to accept data from the computer.
- RI — Ring Indicator; input from modem indicating an incoming call; not connected on many MSX implementations.