Texas Instruments TI-89 Tip List page 152

cases 1 and 2, the calculator is either in the idle state, or the 'other' data line is asserted low, so the
plug tip and CTS must follow the RTS line state. The BCC circuit implements these cases as follows.
Case 1:
RTS is at about +12V relative to serial port ground. The plug tip is at about 5V, relative to calculator
ground, so Q1's base is at about -7V relative to the serial port ground. This turns Q1 on which in turn
turns Q3 on, so the CTS line is only below RTS by Q3's Vce drop, or, essentially, at RTS, so CTS is
high.
Case 2:
RTS and TX are at the same voltage, so neither Q1 nor Q3 can turn on. D1 pulls the plug tip low, and
pull-down resistor R5 pulls CTS low.
Case 3:
The plug tip is low, so neither Q1 nor Q3 turn on. R5 pulls CTS low.
Case 4:
Neither Q1 nor Q3 can turn on. R5 pulls CTS low.
The deluxe BCC operates in much the same way, except that Q5 and Q6 form a push-pull output stage
to drive the CTS line.
Testing the cable
The simplest way to test the BCC is to plug it into you calculator and computer, and try it. Download
and install the GraphLink software from the TI website. From the Link menu, select the proper serial
port and choose the Windows only (black cable) option from the Cable type submenu. If you can
successfully complete the Get Screen operation from the Link menu, then the cable works. If the Get
Screen operation fails, immediately unplug the BCC from the calculator and the computer. While both
the computer serial port and calculator I/O port are fairly robust, you minimize the chance of damage,
to either one, by unplugging everything. If the operation fails, read the rest of this section, and see the
Troubleshooting section below.
You can also test the BCC before you connect it to the calculator and computer, with this simple test
circuit, a voltmeter and some test leads:
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