Part A
74LCX244MTC
onsemi
- Lifecycle
- Active
- Stock
- 5042 PCS
- Series
- Lógica - Buffers, Drivers, Receivers, Transceivers
Replacement verdict
Not a drop-in replacement
The 74LCX244MTC and 74LCX646TTR are not functionally interchangeable. The 74LCX244MTC is a quad buffer/line driver with 3-state outputs, while the 74LCX646TTR is an octal bus transceiver and register with dual enable and direction control. They differ in channel count, logic function, control architecture, and package footprint, so direct substitution is not possible without circuit redesign.
Different logic function
Different channel count
Different control architecture
Different package footprint
Parts at a glance
Part B
74LCX646TTR
STMicroeletrônica
- Lifecycle
- Active
- Stock
- 6706 PCS
- Package
- TSSOP-24
- Series
- Lógica - Buffers, Drivers, Receivers, Transceivers
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74LCX244MTC | 74LCX646TTR | Why it matters |
|---|---|---|---|
| Logic Function | Octal Buffer/Line Driver with 3-State Outputs | Octal Bus Transceiver/Register with 3-State Outputs | The core function differs: the 244 is a unidirectional buffer/driver, while the 646 is a bidirectional transceiver with registered outputs, so they are not functionally interchangeable. |
| Directionality | Unidirectional (single direction, dual enable) | Bidirectional (A-to-B or B-to-A selectable) | Bidirectional capability is essential for bus applications; a unidirectional buffer cannot replace a transceiver in bidirectional data paths. |
| Registered Outputs | No | Yes (D-type flip-flops on both A and B ports) | Registered outputs provide clocked data storage, which the 244 lacks; this affects timing and data-latching behavior in a system. |
| Supply Voltage Range | 2.0 V to 3.6 V | 2.0 V to 3.6 V | Both operate over the same low-voltage LCX range, so power supply compatibility is identical. |
| Output Type | 3-State | 3-State | Both support high-impedance output states for bus sharing, which is important for multi-device bus architectures. |
| Number of Channels | 8 (octal) | 8 (octal) | Matching channel count means both can handle the same bus width, which is a basic requirement for substitutability. |
| Control Inputs | Two output-enable inputs (1OE, 2OE) | Direction control (DIR), clock (CLKAB, CLKBA), and output enables (OEAB, OEBA) | The 646 requires more control signals for direction and clocking, so the surrounding circuit and pinout cannot be reused directly. |
| Package | TSSOP-20 | TSSOP-24 | Different pin counts and package sizes mean the two parts are not drop-in footprint compatible on a PCB. |
| Propagation Delay (typical) | Approximately 6.5 ns at 3.3 V | Approximately 7.0 ns at 3.3 V | Slight timing differences can affect setup/hold margins in high-speed synchronous designs. |
| Pin Count | 20 | 24 | A different pin count prevents direct pin-to-pin replacement and requires layout changes. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74LCX244MTC | 74LCX646TTR |
|---|---|---|
| Supplier | - | STMicroelectronics |
| Series | 74LCX | 74LCX |
| Part Status | Active | Obsolete |
| Logic Type | Buffer, Non-Inverting | Transceiver, Non-Inverting |
| Number of Elements | 2 | 1 |
| Number of Bits per Element | 4 | 8 |
| Output Type | 3-State | 3-State |
| Current - Output High Low | - | 24mA, 24mA |
| Voltage - Supply | 2V ~ 3.6V | 2V ~ 3.6V |
| Operating Temperature | -40°C ~ 85°C (TA) | -55°C ~ 125°C (TA) |
| Mounting Type | Surface Mount | Surface Mount |
| Base Product Number | 74LCX244 | - |
| Current - Output High, Low | 24mA, 24mA | - |
| Packaging | Tube | - |
Frequently asked questions
What are the primary functional differences between the 74LCX244MTC and the 74LCX646TTR?
The 74LCX244MTC is an octal buffer/line driver with 3-state outputs, featuring non-inverting buffers with two active-low output enables. The 74LCX646TTR is an octal bus transceiver and register with 3-state outputs, combining D-type flip-flops and bus transceiver functions with separate clock and enable controls for bidirectional data transfer.
Are the 74LCX244MTC and 74LCX646TTR pin-to-pin compatible?
No. The 74LCX244MTC is typically supplied in a 20-pin TSSOP package, while the 74LCX646TTR is supplied in a 24-pin TSSOP package. They have different pin counts and pin functions, so they are not pin-to-pin compatible.
Can the 74LCX244MTC and 74LCX646TTR operate at the same supply voltage range?
Yes. Both devices are part of the LCX family and operate over a supply voltage range of 2.0 V to 3.6 V, with typical operation at 3.3 V. They also feature 5 V tolerant inputs and outputs, allowing interfacing with 5 V logic systems.
What is the difference in data flow direction between the 74LCX244MTC and the 74LCX646TTR?
The 74LCX244MTC provides unidirectional data flow from input to output, with all eight buffers operating in the same direction. The 74LCX646TTR provides bidirectional data flow, with a direction control pin (DIR) that selects whether data flows from the A bus to the B bus or from the B bus to the A bus.
Do both the 74LCX244MTC and 74LCX646TTR have 3-state outputs?
Yes. Both devices feature 3-state outputs that can be placed in a high-impedance state. On the 74LCX244MTC, the outputs are controlled by two active-low output enable pins (1OE and 2OE). On the 74LCX646TTR, the outputs are controlled by the output enable (OE) and direction (DIR) pins, along with the clock and register enable functions.
Does the 74LCX646TTR include registers that the 74LCX244MTC lacks?
Yes. The 74LCX646TTR incorporates D-type flip-flops that allow data to be stored or registered on the A bus, the B bus, or both, depending on the configuration of the clock (CLK) and register enable (RE) pins. The 74LCX244MTC does not contain any registers or storage elements; it is purely a combinational buffer/driver.
Which device is more suitable for bidirectional bus interface applications?
The 74LCX646TTR is more suitable for bidirectional bus interface applications because it integrates a bus transceiver with direction control and registered outputs. The 74LCX244MTC is intended for unidirectional buffering or driving of signals and cannot directly support bidirectional data flow without external logic.
Are the 74LCX244MTC and 74LCX646TTR interchangeable in an existing design?
No. Due to differences in pin count, package size, internal logic functions, and control pin definitions, the two devices are not interchangeable. A direct substitution would require a complete redesign of the PCB layout and logic control circuitry, and must be validated against the manufacturer datasheets for the specific application.