Internal Memo
Dragon I/O Subsystem Design Summary
The architecture of the I/O subsystem (IOS) falls naturally into two pieces about the main bus, called the SloBus. These are: the Backplane side, where the IOBridge and the disk controller have connections to the DynaBus and to the disk interface, respectively and the External side, where the remaining peripheral controllers on the SloBus are linked to their respective cable connectors. The IOS is controlled primarily by the IOBridge, a custom VLSI chip. Activity on the SloBus is handled by the SloArbiter. With the exception of the display controller, all the I/O controllers interface to the SloBus. These devices include an Ethernet controller, an integrated disk controller, the expansion bus controller and serial devices, viz., the SCC, keyboard and, teledebugging interface.
There are three main sub-buses that interface to the SloBus. A multiplexed Intel bus which supports both the 80186 microprocessor and 82586 Ethernet controller. A 16-bit DataBus which supports the serial data devices and the XBus from the SloBus expansion unit. The subsystem design can be broken down into the following main blocks.
Blocks
Local Memory and Control
64KB (or so) of local EPROM and 1-4MB RAM are provided. The local RAM (SIPS?) will contain the buffer descriptors for the Ethernet controller thereby avoiding long access times via the IOBridge. The EPROM will contain the initial boot code and a simple monitor to allow debugging operations.
SloBus
The SloBus provides the common communication backbone for the IO subsystem. The SloBus comprises 34 lines of address, 16 data lines, 10 lines for control signals, an 8 line interrupt bus, and 10 discrete signals for SloBus arbitration. The SloBus propagates the 8MHz IOS clock derived from the 80186.
IOBridge
Performs the address translation between the Dyna and Slo buses. Allows the Dragon processors to directly address a specific I/O device on the SloBus.
Microprocessor
The IOS contains a resident microprocessor. Its primary role is to provide initialization of the Dragon system and its peripherals and thereafter become dormant. A secondary role is to manage the DBus during debugging.
Ethernet Controller
The Intel 82586 Data Link Controller manages network frame transmission and reception. The main functions of the this controller are: executing commands which come directly from the DynaBus via the IOBridge and receiving serial data from the SEEQ 8023A Serial Ethernet Interface.
Integrated Disk Controller
The integrated disk controller provides the capability for transactions with any combination of up to four floppy and hard disk drives. The Am 9580A supports the ST506 standard, while the plug compatible Am 9890 supports ESDI in addition.
Serial Communcations Controller
Provides both RS232C DTE and DCE channels for a modem and a printer, respectively. (MOSART?, SCSI?)
Keyboard Controller
Provides the interface for keyboard control. The mouse controller and speaker are incorporated in the Xerox product keyboard. The keyboard controller also provides control of the maintenance panel LEDs.
Teledebug Interface
A 20 pin socket provides a parallel communication link to the Xerox standard "Song Board". The main chip on the Song board is an Intel 8255A-5 PIA.
Expansion Unit
Translates between signals on the SloBus and those which conform to a commercial bus standard. An ideal situation would be to allow a jumpered configuration to select between a 62-pin IBMpc connector and a Multibus connector. Should also include a wire-wrap area to accomodate board customization. Several schemes are under investigation.
Schedule
Complete by November '86
Board level schematics.
Simulation of SloBus arbitration.
Add slave device to SloBus.
Add master device e.g. Ethernet controller.
Complete by December '86
PCB design and capture of IOS.
Complete by January '87
Wire-wrap or stitch-weld IOS board.
Complete by March '87
Debug prototype IOS board.
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