SN74LS245N

The SN74LS245N from Texas Instruments is an octal bidirectional bus transceiver with 3-state outputs, designed for asynchronous two-way communication between data buses. The DIR (direction) pin controls data flow: DIR=HIGH transmits data from the A bus to the B bus, and DIR=LOW transmits from B to A. The OE (output enable) pin, when HIGH, places both A and B outputs in the high-impedance state, effectively disconnecting the transceiver from both buses. When OE is LOW, the selected direction is enabled. The 24mA output sink current provides the drive strength needed for heavily loaded bus lines with multiple TTL inputs. Input hysteresis improves noise margin on the control and data inputs, reducing susceptibility to noise on long bus traces. The N suffix denotes the PDIP-20 through-hole package.

The SN74LS245N contains eight bidirectional transceiver cells, each consisting of two non-inverting buffers with 3-state outputs connected back-to-back (one A-to-B, one B-to-A). The DIR input controls which set of buffers is active: DIR=HIGH enables the A-to-B buffers and disables the B-to-A buffers; DIR=LOW does the reverse. The OE input, when HIGH, overrides DIR and disables all 16 output buffers (both directions), placing both A and B ports in the high-impedance state. When OE is LOW, the direction selected by DIR is active. The A port and B port are symmetric; the device does not distinguish between them except by the DIR setting. In a typical microprocessor system, the SN74LS245N connects the CPU data bus to a peripheral bus. The CPU controls DIR based on the R/W signal, and OE is gated with the chip-select decode. When the CPU writes, DIR=HIGH and data flows from A (CPU side) to B (peripheral side). When the CPU reads, DIR=LOW and data flows from B to A. When the peripheral is not selected, OE=HIGH disconnects the transceiver from both buses, preventing bus contention.

SN74LS245N uses the PDIP-20 package: VCC at Pin 20, GND at Pin 10. Eight bidirectional data lines: A0-A7 on Pins 2-9 (side A), B0-B7 on Pins 18-11 (side B). Pin 1 = DIR (direction control: HIGH = A→B, LOW = B→A). Pin 19 = OE (Output Enable, active-low). When OE is HIGH, all outputs are in high-impedance state regardless of DIR. The two-enable architecture (DIR + OE) allows simple bidirectional bus control with just two control signals. This pinout is shared by 74HC245, 74HCT245, and 74LVC245 octal transceivers. For 3.3V systems, 74LVC245APW provides the same function in TSSOP-20. The A-side and B-side pins are symmetrically arranged for clean bus routing.

• CPU Bus Buffer: DIR=R/W signal; OE=chip select; 24mA drives multiple TTL loads on the data bus
• Backplane Driver: 3-state isolation allows multiple boards to share a common backplane bus
• Level Translation: A bus at 5V, B bus at 3.3V through current-limiting resistors (unidirectional per cycle)
• Bus Isolation: OE=HIGH isolates subsystem during debug or hot-swap; DIR selects read/write direction
• Memory Interface: Bidirectional buffer between CPU data bus and memory data pins; DIR from MEMR/MEMW

The 74245 is a Octal Bus Transceiver with 3-State Output. Family variants span from the original LS TTL (5V only) to modern LVC/AUP (down to 0.8V). HC and HCT versions offer 2-6V CMOS operation with HCT providing TTL-compatible input thresholds for mixed 5V systems. CD4000-series equivalents offer the widest 3-18V supply range at the cost of lower switching speed. DIP packages (N suffix) are through-hole; SOIC (D suffix) and TSSOP are surface-mount.