120
Digital Electronics
• LOW-level input voltage, V IL . This is the maximum voltage level applied at the input that is
recognized as a legal LOW level for the specified family. For the standard TTL family, an input
voltage of 0.8 V is a legal LOW logic state.
• HIGH-level output voltage, V OH . This is the minimum voltage on the output pin of a logic function
when the input conditions establish logic HIGH at the output for the specified family. In the case of
the standard TTL family of devices, the HIGH level output voltage can be as low as 2.4 V and still
be treated as a legal HIGH logic state. It may be mentioned here that, for a given logic family, the
V OH specification is always greater than the V IH specification to ensure output-to-input compatibility
when the output of one device feeds the input of another.
• LOW-level output voltage, V OL . This is the maximum voltage on the output pin of a logic function
when the input conditions establish logic LOW at the output for the specified family. In the case of
the standard TTL family of devices, the LOW-level output voltage can be as high as 0.4 V and still
be treated as a legal LOW logic state. It may be mentioned here that, for a given logic family, the
V OL specification is always smaller than the V IL specification to ensure output-to-input compatibility
when the output of one device feeds the input of another.
The different input/output current and voltage parameters are shown in Fig. 5.3, with HIGH-level current
and voltage parameters in Fig. 5.3(a) and LOW-level current and voltage parameters in Fig. 5.3(b).
It may be mentioned here that the direction of the LOW-level input and output currents shown in
Fig. 5.3(b) is applicable to logic families with current-sinking action such as TTL.
Figure 5.3 (a) HIGH-level current and voltage parameters and (b) LOW-level current and voltage parameters.
Digital Electronics
• LOW-level input voltage, V IL . This is the maximum voltage level applied at the input that is
recognized as a legal LOW level for the specified family. For the standard TTL family, an input
voltage of 0.8 V is a legal LOW logic state.
• HIGH-level output voltage, V OH . This is the minimum voltage on the output pin of a logic function
when the input conditions establish logic HIGH at the output for the specified family. In the case of
the standard TTL family of devices, the HIGH level output voltage can be as low as 2.4 V and still
be treated as a legal HIGH logic state. It may be mentioned here that, for a given logic family, the
V OH specification is always greater than the V IH specification to ensure output-to-input compatibility
when the output of one device feeds the input of another.
• LOW-level output voltage, V OL . This is the maximum voltage on the output pin of a logic function
when the input conditions establish logic LOW at the output for the specified family. In the case of
the standard TTL family of devices, the LOW-level output voltage can be as high as 0.4 V and still
be treated as a legal LOW logic state. It may be mentioned here that, for a given logic family, the
V OL specification is always smaller than the V IL specification to ensure output-to-input compatibility
when the output of one device feeds the input of another.
The different input/output current and voltage parameters are shown in Fig. 5.3, with HIGH-level current
and voltage parameters in Fig. 5.3(a) and LOW-level current and voltage parameters in Fig. 5.3(b).
It may be mentioned here that the direction of the LOW-level input and output currents shown in
Fig. 5.3(b) is applicable to logic families with current-sinking action such as TTL.
Figure 5.3 (a) HIGH-level current and voltage parameters and (b) LOW-level current and voltage parameters.