Texas Instruments TI-89 Tip List page 153

The voltage source V may be a power supply or even a 9V battery. The two resistor Ra and Rb set
voltage Vh, and simulate the 10K source impedance of the calculator data lines. For a desired source
impedance Rs and output voltage Vh, use these equations to find Ra and Rb:
Rs$V
Ra =
Vh
which are found as the solutions to
Rs = Ra$Rb
Ra+Rb
For example, if we use a 9V battery, and want a 5V output voltage with 10K source impedance, then
Rs = 10K, V = 9 and Vh = 5, and we find Ra = 18K and Rb = 22.5K. The nearest standard 5%
tolerance values are 18K and 22K, which results in an output voltage of 4.95V and a source
impedance of 9.9K, which is close enough. It is simpler and just as effective to use two 20K resistors
for Ra and Rb.
The 'tip' and 'ring' sections of the circuit are tested separately, following cases 1 to 4 as described
above. The tables below show the correct results, as well as my measured results when using a 9V
battery. Note that V+ is the positive terminal of the battery (or power supply), and V- is the negative
terminal.
Calculator
Case asserting
#1
(Connect tip to Vh)
#2
(Connect tip to Vh)
#3
(Connect tip to Tx)
#4
(Connect tip to Tx)
Calculator
Case asserting
#1
(Connect ring to Vh)
#2
(Connect ring to Vh)
#3
(Connect ring to Tx)
#4
(Connect ring to Tx)
Rs$V
Rb =
V−Vh
Vh = V
Test results for 'tip' circuit
State of RTS line
low?
No High
(Connect RTS to V+)
No
(Connect RTS to V-)
Yes High
(Connect RTS to V+)
Yes
(Connect RTS to V-)
Test results for 'ring' circuit
State of RTS line
low?
No
(Connect DTR to V+)
No
(Connect DTR to V-)
Yes
(Connect DTR to V+)
Yes
(Connect DTR to V-)
Rb
Ra+Rb
Result at data line (Plug tip and CTS)
tip: HIGH, Vh
CTS: HIGH; about 9.3V
Low tip: LOW, about 0.6V
CTS: LOW, about 0V
tip: LOW, about 0V
CTS: LOW, about 0V
Low tip: LOW, about 0V
CTS: LOW, about 0V
Result at data line (Plug tip and CTS)
High ring: HIGH; Vh
DSR: HIGH; about 9.3V
Low ring: LOW, about 0.6V
DSR: LOW, about 0V
High ring: LOW, about 0V
DSR: LOW, about 0V
Low ring: LOW, about 0V
DSR: LOW, about 0V
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