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:
Ra =
Rs$V
Vh
Rb =
Rs$V
V−Vh
which are found as the solutions to
Rs =
Ra$Rb
Ra+Rb
Vh = V
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.
Test results for 'tip' circuit
tip: LOW, about 0V
CTS: LOW, about 0V
Low
(Connect RTS to V-)
Yes
(Connect tip to Tx)
#4
tip: LOW, about 0V
CTS: LOW, about 0V
High
(Connect RTS to V+)
Yes
(Connect tip to Tx)
#3
tip: LOW, about 0.6V
CTS: LOW, about 0V
Low
(Connect RTS to V-)
No
(Connect tip to Vh)
#2
tip: HIGH, Vh
CTS: HIGH; about 9.3V
High
(Connect RTS to V+)
No
(Connect tip to Vh)
#1
Result at data line (Plug tip and CTS)State of RTS line
Calculator
asserting
low?
Case
Test results for 'ring' circuit
ring: LOW, about 0V
DSR: LOW, about 0V
Low
(Connect DTR to V-)
Yes
(Connect ring to Tx)
#4
ring: LOW, about 0V
DSR: LOW, about 0V
High
(Connect DTR to V+)
Yes
(Connect ring to Tx)
#3
ring: LOW, about 0.6V
DSR: LOW, about 0V
Low
(Connect DTR to V-)
No
(Connect ring to Vh)
#2
ring: HIGH; Vh
DSR: HIGH; about 9.3V
High
(Connect DTR to V+)
No
(Connect ring to Vh)
#1
Result at data line (Plug tip and CTS)State of RTS line
Calculator
asserting
low?
Case
5 - 11
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