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Build my own link cable?

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.

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