Serial Ports - An introduction to serial port technology
Credit for this information should go to Mark Clarkson.
Access.bus is the name of a new proposal for slow serial communications.
Typical usages: keyboards, mice, modems and printers. Also, the new Super
VGA specification DDC2 calls for Access.bus compatible monitors.
Cables: 4-wire, with a connector like a RJ-11 phone jack at each end.
Devices: Most devices will have 2 sockets, for in and out, so devices can
be daisy-chained. In theory, Access.bus will support up to 125 peripherals
- over a total cable distance of 8 m - from a single jack in the back of
your computer. The implication is enormous: Using today's technology, you'd
need 125 ports, 125 interrupts...
Intelligence (at long last): Every Access.bus peripheral will contain a
microcontroller, which can identify the device to the bus, and can pass
data along to the next peripheral in the chain.
Hot plugging: This means Access.bus devices can be disconnected and
connected without powering down the PC and without reconfiguring the
Speed: Up to 125 Kb/sec.
P1394 is the name of a new proposal for fast serial communications.
Connectors: Similar to the above (but presumably incompatible).
Devices: Up to 63 per port. Up to 1022 FireWire buses can be somehow
bridged together. Also, each FireWire-compatible device will ideally have
its own 64-bit ID number. This means the OS will know exactly what type of
device it is talking to. Thus the OS should be able to load the correct
device driver automatically.
Hot plugging: As above.
Speed: 100 Mb/sec to 400 Mb/sec. But taking protocol and overhead into
account, this leaves 5 Mb/sec to 20 Mb/sec of live data on the move.
See my articles ``Parallel Ports'' and ``SCSI Technology''.
'Seriously Serial', Byte, August 1994, p 117, by Mark Clarkson.
This article was written by Ron Savage in 1997.