74HC245 bus transceiver: pinout, DIR and OE, bus fights and the 3.3 V question

The chip is eight buffers that can face either way. DIR sets the direction for all eight at once; there is no per-channel choice. OE is active low: with it low the chip drives whichever side DIR names, and with it high every A and B pin goes high-impedance, so the two buses are disconnected.

Used one way only, the chip is an eight-line buffer: tie DIR to VCC, OE to ground, feed the A side and take the B side out. That is the common case on hobby boards, where it sits between a controller and LEDs, a long ribbon cable or another board.

Two current figures matter and they do not quite agree. The absolute maximum is 35 mA per output and 70 mA through the VCC or ground pin, while the design notes in the same sheet allow 25 mA per output and 75 mA for the part. Design to the lower pair: 25 mA per output and no more than 70 mA for all eight together.

TI gives the input thresholds at 4.5 V and 6 V: 3.15 V and 1.35 V, then 4.2 V and 1.8 V. Both pairs are 0.7 and 0.3 of VCC, so on a 5 V supply a high must be at least 3.5 V and a low at most 1.5 V. A signal goes through in 26 ns at worst (up to 85 °C, 50 pF load), fast enough for anything a hobby bus carries.

A common plan is to power a 74HC245 from 5 V and drive its A side from a 3.3 V controller. On 5 V the chip needs 3.5 V for a high. A 3.3 V output reaches 3.3 V at best, 0.2 V short, before any load or temperature drift pulls it lower. It often works on the bench and then fails on a warm day or a different chip.

The other direction is worse. Run the HC245 on 3.3 V so its outputs suit the controller, and its inputs may not go above VCC; a 5 V signal pushes current into the input clamp diodes and up into the 3.3 V rail. TI's own application drawing shows the part between 5 V and 3.3 V systems, but its input-voltage row (0 to VCC) rules out both arrangements.

Both relatives use the same 20-pin layout, so a board laid out for a 74HC245 takes either one. Neither works in both directions at once across the two voltages; a bus that has to turn round between 3.3 V and 5 V needs a dual-supply translator instead.

The datasheet warns about bus contention: two outputs enabled onto the same wire, one pulling high and one pulling low. TI's typical output figures at 4.5 V (4.3 V at 6 mA out, 0.17 V at 6 mA in) put the output resistances near 33.3 Ω high and 28.3 Ω low. On 5 V:

These are typical figures; a real pair of chips will differ, but not by enough to bring the current under the limit. A fight that lasts as long as a program runs keeps both outputs well past their rating the whole time.

On a bus that carries data both ways, change DIR only while OE is high. Take OE high, wait 50 ns for the outputs to let go (the SN74's worst case to 85 °C at 4.5 V), change DIR, then take OE low again; the new side drives within 58 ns. The whole turn takes about 108 ns, a couple of instructions on most controllers.

While OE is high nothing drives the bus, so the lines float. Pull-ups or pull-downs on the bus hold them at a known level; TI's layout notes point out that disabling the outputs does not disable the inputs, so a floating transceiver pin still needs a level.

Standing still, the chip draws at most 80 µA (to 85 °C). Switching costs more: each transceiver has a power dissipation capacitance of 40 pF. With all 8 lines toggling at 1 MHz on 5 V that is 40 pF × 5 V × 1 MHz × 8 = 1.6 mA, or 8 mW, before the current that charges the wiring on the outputs. Fit 100 nF from VCC to ground right at pin 20.

• For a one-way buffer, tie DIR to VCC and OE to ground, and feed the A side.
• Pull OE up with 10 kΩ on anything that powers up with the controller, so nothing drives before the code is ready.
• Change DIR only with OE high. A DIR change with OE low makes a short fight on every line.
• Unused A or B pins on the input side must not float. Tie them to ground or VCC; OE does not switch off the inputs.
• For 3.3 V to 5 V use a 74HCT245 on 5 V; for 5 V to 3.3 V a 74LVC245A on 3.3 V. Both drop into the same footprint.
• Keep each output under 25 mA and all eight under 70 mA; eight LEDs at 6 mA each come to 48 mA.

A 74HC245 from another maker should drop into the same socket. The HCT and LVC parts below share the pinout but have different input levels and supply ranges, so check those before fitting one in place of the HC245.

1. Wire VCC, ground and 100 nF at pin 20. Tie DIR to VCC and OE to ground.
2. Put an LED and 470 Ω from B1 to ground. Tie A2 to A8 to ground.
3. Take A1 to VCC: the LED lights. Take A1 to ground: it goes out.
4. Take OE to VCC with A1 high. The LED should go out; B1 is now high-impedance.
5. With OE low again, measure B1 with the LED lit. Near 4.3 V or above is normal at about 6 mA.

The logic levels lesson explains the 0.7 VCC and 0.3 VCC thresholds and the HCT input levels, and the 74HC595 is the usual next step when eight outputs have to come from fewer pins.

Use the SN74HC245N as a one-way buffer on a 5 V board: DIR to VCC, OE pulled up with 10 kΩ and driven low by the controller, 100 nF at pin 20, and every unused input tied down. On a two-way bus, change DIR only with OE high. For 3.3 V to 5 V fit a 74HCT245 instead, and for 5 V down to 3.3 V a 74LVC245A.

Shops list it as 74HC245 or SN74HC245N in DIP-20, and as 74HC245D or SN74HC245DW for boards. Check the letters after 74: HC, HCT and LVC parts look the same and are sold side by side. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.