I have notes in various scattered places about the voltages that are used in various I/O connectors, but no convenient summary all in once place. So, here I go. When there is an “absolute maximum” voltage that is rated higher than the maximum signal voltage, I note it in the following list.
Vpp = volts peak to peak
- “Consumer line-out/in” audio, RCA jack and 3.5mm jack: 0.8 Vpp, absolute max 2 Vpp. Please note that line-out outputs can only drive high-impedance loads, such as a computer or amplifier, and therefore cannot drive speakers or headphones directly.
- Microphone audio input: 0.002828 Vpp or greater
- Composite video: 0.7 Vpp without blanking/sync signals, 1 Vpp with blanking/sync signals
- VGA video: 0.7 Vpp (i.e. differential signaling)
- Parallel port: 5 V
- PC Gameport: 5 V
- MIDI: 5 V
- PS/2 connector: 5 V
- USB: 5 V
- Ethernet: +/- 1 V (100BASE-TX), +/- 2 V (1000BASE-T)
- RS-232: At least +/- 5 V, +/- 10 V, +/- 12 V, +/- 15 V, absolute max +/- 25 V
- RJ11 Telephone: 48 V voice, 90 V ring
20191116/https://en.wikipedia.org/wiki/Line_level
20191116/https://en.wikipedia.org/wiki/Nominal_level
20191116/https://en.wikipedia.org/wiki/Decibel#Voltage
20191116/https://en.wikipedia.org/wiki/Output_impedance
20191116/https://en.wikipedia.org/wiki/MIDI
20191117/https://en.wikipedia.org/wiki/Ethernet_over_twisted_pair
20191117/https://en.wikipedia.org/wiki/Gigabit_Ethernet
Please note: Many of these figures are nominally expressed in dbV, and this measurement can be confusing. For this sake, this quick reference sheet can be greatly helpful.
0 dbV = 1 V_RMS = 1 V / sqrt(2)
V = sqrt(2 * 10^(dbV/10)) = sqrt(2) * 10^(dbV/20)
Vpp = 2 * V
The reason why we do 10^(dbV/20) is because decibels measures power,
not amplitude, so we must square the ratio difference in voltages, or
alternatively take the square root of dbV to convert to volts.
Multiplying by sqrt(2) is simply used to convert from
root-mean-square volts to volts.