Comparison of Open-Collector, Totem-Pole, and Tri-State TTL Outputs
Feature Open-Collector TTL Totem-Pole TTL Tri-State TTL
Structure Uses only an NPN Uses NPN-PNP Similar to totem-pole,
transistor at the output transistors in a push- but includes an enable
with no internal pull- pull arrangement. control to enter a high-
up resistor. impedance state.
Output Can only pull output Can actively drive Can actively drive high
Voltage Levels low (0V), requires an both high (Vcc) and (Vcc), low (0V), or
external pull-up low (0V). enter a high-impedance
resistor for high state. (Hi-Z) state.
Speed Slower due to Faster switching Similar to totem-pole
external pull-up since both high and but depends on enable
resistor charging time. low states are signal timing.
actively driven.
Power Lower since only one Higher because both Power-efficient when in
Dissipation transistor conducts at transistors switch high-impedance mode
a time. actively. (reduces unnecessary
current flow).
Wired-AND Yes, multiple outputs No, connecting Yes, since the high-
Capability can be connected multiple outputs can impedance state allows
(wire-AND function). cause conflicts and multiple devices to
damage. share the same bus.
Current Limited by the Higher drive Similar to totem-pole,
Driving Ability external pull-up capability due to but depends on enable
resistor. active push-pull control.
design.
High- No No Yes, allows multiple
Impedance outputs to share a data
(Hi-Z) Mode? bus without
interference.
Applications Used in open-drain Common in high- Used in data buses,
buses (I²C), speed logic circuits multiplexing, memory
relay/motor control, where active drive is circuits (e.g.,
level shifting. needed. RAM/ROM access
lines).
• Open-collector TTL is ideal for interfacing with other voltages and wired-AND
logic applications.
• Totem-pole TTL is ideal for fast, high-speed digital circuits where strong output
drive is needed.
• Tri-state TTL is ideal for bus systems and multiplexing, as it can enter a high-
impedance state to avoid conflicts when multiple devices share a data line.
