Sun SBus card and provided an X-window-system-compatible writing and display surface . This same design
was used inside our first wall-sized displays, the liveboards, as well . Our later untethered pad devices,
the XPad and MPad, continued the system design principles of X-compatibility, ease of construction, and
flexibility in software and hardware expansion .
As I write, at the end of 1992, commercial portable pen devices have been on the market for two
years, although most of the early companies have now gone out of business . Why should a pioneering re-
search lab be building its own such device ? Each year we ask ourselves the same question, and so far three
things always drive us to continue to design our own pad hardware .
First, we need the right balance of features; this is the essence of systems design . The commercial
devices all aim at particular niches, and so balance their design to that niche . For research we need a
rather different balance, all the more so for pervasive computing . For instance, can the device communi-
cate simultaneously along multiple channels ?Does the O .S support multiprocessing ?What about the poten-
tial for high-speed tethering ? Is there a high-quality pen ?Is there a high-speed expansion port sufficient for
video in and out ? Is sound in/ out and ISDN available ? Optional keyboard ? Any one commercial device
tends to satisfy some of these, ignore others, and choose a balance of the ones it does satisfy that optimize
its niche , rather than ubiquitous computing-style scrap computing . The balance for us emphasizes commu-
nication, ram, multi-media, and expansion ports .
Second, apart from balance are the requirements for particular features . Key among these are a pen
emphasis, connection to research environments like Unix, and communication emphasis .A high-speed ( >
64kbps) wireless capability is built into no commercial devices, nor do they generally have a sufficiently
high speed port to which such a radio can be added . Commercial devices generally come with DOS or Pen-
point, and while we have developed in both, they are not our favorite research vehicles because of lack of
full access and customizability .
The third thing driving our own pad designs is ease of expansion and modification . We need full hard-
ware specs, complete 0 .S . source code, and the ability to rip-out and replace both hardware and software
components . Naturally these goals are opposed to best price in a niche market, which orients the documen-
tation to the end user, and which keeps price down by integrated rather than modular design .
We have now gone through three generations of Pad designs . Six scratchpads were built, three
XPads, and thirteen MPads, the latest . The MPad uses an FPGA for almost all random logic, giving ex-
treme flexibility . For instance, changing the power control functions, and adding high-quality sound, were
relatively simple FPGA changes . The MPad has built-in both IR (tab compatible) and radio communica-
tion, and includes sufficient uncommitted space for adding new circuit boards later . It can be used with a
tether that provides it with recharging and operating power and an ethernet connection . The operating sys-
tem is a standalone version of the public-domain Portable Common Runtime developed at PARC
[Weiser 89] .
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