The Metricom Ricochet SX is a wireless modem that connects to the Macintosh serial port via a proprietary 14-pin connector on the modem end and a mini-DIN-8 female connector on the Mac end. The Mac serial interface implements RS-422 balanced differential signalling governed by ANSI/TIA/EIA-422-B, though in this cable only the unbalanced (RS-232-compatible) lines are used — TxD− as transmit data and RxD− as receive data. The Macintosh Plus and all subsequent beige-era Macs use the mini-DIN-8 serial connector, with both the modem port and printer port controlled by a Zilog Z8530 Serial Communications Controller. The Ricochet SX uses a different proprietary modem-side connector than the earlier Ricochet SE model; the Mac-side wiring is otherwise equivalent for both.
Notes
- The Mac end of this cable is a mini-DIN-8 female connector.
- The modem end uses a proprietary 14-pin connector specific to the Ricochet SX; it is not interchangeable with the Ricochet SE modem connector.
- The Mac serial port operates in RS-422 unbalanced mode for this cable: TxD− carries transmit data and RxD− carries receive data; RxD+ is tied to GND to bias the differential receiver into RS-232-compatible operation.
- The Ricochet SX presents as a standard Hayes-compatible DCE modem to the Macintosh serial driver.
- Maximum throughput is constrained by the host Mac’s serial controller: approximately 19.2 kbps on 68000-based Macs, up to 57.6 kbps on 68020/030 models, and up to 115.2 kbps on 68040-based Macs.
Signals
- HSKo — Handshake output from Mac (Z8530 DTR); used for modem flow control.
- HSKi — Handshake input to Mac (DSR or external clock input).
- TxD− — Transmit data, negative/unbalanced line; carries serial data from Mac to modem in RS-232-compatible mode.
- TxD+ — Transmit data, positive line; not connected or tied to GND in unbalanced operation.
- RxD− — Receive data, negative/unbalanced line; carries serial data from modem to Mac.
- RxD+ — Receive data, positive line; tied to GND to bias the Mac differential receiver.
- GND — Signal ground (mini-DIN-8 pin 4).