Retro computing field guide · Part 3
How a 74HC245 shares an 8-bit bus The 74HC245 is a gatekeeper for eight data wires. It can pass data in either direction—or disconnect both sides so another chip can use the bus.
See the MC74HC245AN Why put a chip between two buses? On a simple computer, several devices may connect to shared data wires. If two active outputs drive different values at once, they fight. The 74HC245 is an 8-bit bidirectional bus transceiver : its A1–A8 pins face one bus, and B1–B8 face another. Each A pin pairs with the same-numbered B pin. It does not store a byte or decide which address is selected; control logic tells it when to pass data.
Direction chooses which side drives the other. Output enable decides whether the chip drives either side at all. Two control inputs, three states /OE high: disconnected The outputs are high impedance , often written Hi-Z. That means the chip is electrically quiet on both buses—not that it sends a logic 0.
/OE low, DIR high: A → B Whatever logic levels appear on A1–A8 are reproduced on B1–B8. The A side is read as input and the B side is driven.
/OE low, DIR low: B → A Data travels the other way. The B side is read as input and the A side is driven.
The slash in /OE means active low : a low level enables the outputs. DIR is not active-low; high means A to B. The eight channels are non-inverting, so a 1 remains a 1.
Find the pins Top view of the through-hole 20-pin package, notch at the pin-1 end. Pin numbers and function table follow the manufacturer datasheet. A concrete read example Suppose a selected ROM is on bus B and the CPU is on bus A. The address decoder selects the ROM for a read. Control logic sets DIR low, so data may travel from B to A, and brings /OE low only when the ROM is ready to drive the bus. The byte crosses the 74HC245 to the CPU. When that transaction ends, /OE goes high again, isolating the buses. Not a drop-in bus fix. A 74HC245 does not generate correct read/write timing or chip selects for you. Never let another device drive the destination bus at the same time. Keep control inputs at defined levels, add a nearby 0.1 µF supply bypass capacitor, and check supply voltage, logic thresholds, fan-out and timing for the exact parts in your circuit. HC input thresholds are not automatically suitable for every TTL/NMOS output.
The shop's MC74HC245AN is a 20-pin through-hole part. Check its live stock and the listing before ordering; quantity changes as parts sell. For the wider context, begin with the 6502 parts guide .
Next: a one-way buffer The 74HC245 is useful when data must travel in either direction. If a path only needs to carry eight signals one way, the SN74HCT541N guide shows a simpler alternative with two active-low output enables and TTL-compatible inputs. It cannot pass a read back in the opposite direction.