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From: daver@hpsgpa.UUCP
Newsgroups: comp.sys.hp
Subject: Re: HP-28C (HP82240A)
Message-ID: <2790003@hpsgpa.HP.COM>
Date: Tue, 24-Mar-87 20:25:06 EST
Article-I.D.: hpsgpa.2790003
Posted: Tue Mar 24 20:25:06 1987
Date-Received: Sat, 28-Mar-87 04:35:48 EST
References: <44700002@uicsrd>
Organization: HP Singapore IC Design Ctr
Lines: 33

>.....................................................  One could
>always rely on the calculator's operating system to protect the user
>from entering invalid operations and reporting errors with reasonably
>meaningful messages.  This security has all but been discarded with
>the 28C and its printer.  The 28 happily blasts (or more accurately
>oozes) out data into thin air hoping there is something out there
>ready to receive it.  It has no way of knowing if the data has been
>received correctly, or indeed at all.
>
>This might be slightly more acceptable if there was some physical
>connection between the calculator and its printer as this would
>eradicate the possibility of two calcs trying to talk to the same
>printer; it would also eliminate the problem of paper and the like
>blocking the dataflow - a common problem on my desk.  At least they
>should have added a checksum at the end of each line of output so the
>printer could verify what it had received.

A printer is an interesting device from this point of view, since it provides
instant feedback via the user.  The printer was designed to be reliable and
inexpensive.  The IR range was deliberately limited to avoid the problem of
interference between two calculators being used on adjacent desks.  Each byte
of data has an additional 4 bits of error-correction code which will correct
single-bit and many double-bit errors and detect most other errors.  In
addition, there are several levels of redundancy in the encoding of individual
bits to make the printer as immune as possible to outside interference.
Accidentally placing paper between the calculator and printer is equivalent to
accidentally pulling out the cable between a wired calculator and printer.  The
printer was designed to be easy to use and inexpensive (compare the price to
the older HP printers), and it is.  It is also capable and reliable, in the HP
tradition.

Dave Rabinowitz
hplabs!hp-pcd!daver