8-bit inverting bus transceiver — 15 mA / 48 mA output drive
The SN74LS640-1N is an 8-bit inverting transceiver from Texas Instruments' 74LS low-power Schottky family, with 3-state outputs that let it drive or isolate a bidirectional data bus. The 15 mA output-high and 48 mA output-low current ratings set the fan-out limit: at 5 V TTL levels, each output can drive up to 10 standard LS loads without external buffers, making it a direct fit for 8-bit microprocessor buses, memory interfaces, or peripheral backplanes where inversion is needed on the data direction.
Pin-compatible alternative — SN74LS645-1N non-inverting version
The closest peer is the SN74LS645-1N, which is identical in every parametric except logic type: the 645 is a non-inverting transceiver, while the 640 inverts. Both share the same 20-DIP package, 4.75-5.25 V supply, 15 mA / 48 mA drive, and 0-70 °C temperature range. A board laid out for the SN74LS640-1N accepts the SN74LS645-1N without any wiring change — the pinout is the same; only the data polarity on the bus differs. If the system requires non-inverted data, the 645 is a drop-in swap.