Questions have been asked frequently about the cost of the cable
to link between personal computers and TI Graphics Calculators.
A number of people have wanted details to build their own cable,
perceiving it to be merely a matter of matching the RS-232 port TX
and RX lines to the proper pins on the calculator plug.
Texas Instruments wants the interconnection of the Graphics
Calculator to other devices to be as simple and cost effective as
possible. For this reason, the link port on the calculator is
not designed like RS-232. A RS-232 type port requires accurate
timing and involves circuitry that consumes significant power.
The calculator's link ports have two data lines and a ground,
using simple CMOS digital levels in a self clocking protocol that
does not require any precise timing, but can operate well above
9600 bits per second. This allows the linking between two
calculators to be accomplished at minimal cost and power drain.
In line with this strategy, our first link cable (the black one),
utilized relatively simple transistor circuitry and did not even
use the RX and TX lines of the RS-232 port. Instead, it used
the RS-232 "control" lines (2 for inputs and 2 for outputs) to
operate the data lines. The reason we had to change away from
this approach is that many Macintosh computers do not have all
the necessary control lines available on their serial port
connections. To make a MAC compatible connection we had to
communicate through RX and TX and this required putting an eight-
bit, crystal controlled oscillator, PIC microcontroller in the
cable. This microcontroller communicates with the PC at a
standard baud rate and converts each bit into the self clocking
protocol to communicate to the calculator. The power for the
cable is derived from the PC and doesn't impose significant
drain from the calculator. Overall, this approach provides a
reasonable cost link without any cost burden to the calculator
user who does not use a link to personal computers.
Due to the specialized components and also because some parts of
the design are necessary to ensure compliance with FCC radio
frequency emission regulations, it is not practical for individual
users to build their own cables on a general basis. We hope this
explanation helps you understand why this is.