What is the output-tolerant function?
The output circuitry of CMOS logic ICs has a totem-pole configuration consisting of a pair of p-channel and n-channel MOSFETs as shown below. Typically, CMOS logic ICs have a parasitic diode between an output and power supply.
The parasitic diode turns on when voltage is applied to an IC when it is off or to an IC with a three-state output (such as the 74LCX245) when it is on. The IC might be destroyed by the resulting large current.
Even when a voltage higher than the supply voltage (VCC) is applied to the output, current does not flow into an IC with an output configuration that does not have this parasitic diode. Such an IC has an output-tolerant function.
It is easy to distinguish whether an IC has an output-tolerant function.
Let’s look at the datasheets for ICs of the 74VHC and 74LCX Series.
See the bus I/O voltage shown in the Operating Ranges table.
The datasheet of the 74VHC245 shows VIO＝０ to VCC. This means that the output pin is tolerant of only up to VCC. Therefore, the 74VHC245 does not have an output-tolerant function.
In contrast, the datasheet of the 74LCX245 shows VIN＝０to 5.5 V, meaning that up to 5.5 V can be applied to the bus I/O pin regardless of VCC. Therefore, the 74LCX245 has an output-tolerant function.