Overview

The 74HC245 (SN74HC245) is an octal 3-state non-inverting bidirectional bus transceiver manufactured using high-speed Silicon-Gate CMOS technology. Designed for asynchronous 8-bit two-way data communication between data buses, it features 8 channels (A1–A8 and B1–B8), a direction control pin (DIR), and an active-low output enable pin (/OE or CE).

It is widely used in retro-computing, microcontroller development, LED matrix driving, and logic-level conversion circuits to isolate buses or buffer high-capacitance bus lines.

Quick reference

Supply Voltage (VCC)2.0 V to 6.0 V DC
Logic FamilyHigh-Speed CMOS (74HC) / TTL-Compatible (74HCT)
Bus Width8 Bits (Octal)
Max Output Drive Current±35 mA\pm 35\text{ mA} per pin
Propagation Delay13 ns13\text{ ns} typical at VCC=4.5VVCC = 4.5\text{V}
Package20-pin DIP / SOIC-20 / TSSOP-20

Pinout (DIP-20 Package)

text
             ┌───┴───┐
       DIR 1 │ 1  20 │ VCC
        A1 2 │ 2  19 │ /OE (Output Enable)
        A2 3 │ 3  18 │ B1
        A3 4 │ 4  17 │ B2
        A4 5 │ 5  16 │ B3
        A5 6 │ 6  15 │ B4
        A6 7 │ 7  14 │ B5
        A7 8 │ 8  13 │ B6
        A8 9 │ 9  12 │ B7
       GND 10│10  11 │ B8
             └───────┘
PinNameDescription
1DIRDirection Control: High (11) = A data to B bus; Low (00) = B data to A bus
2–9A1–A8Side A 8-bit bidirectional data pins
10GNDSystem ground reference (0 V)
11–18B8–B1Side B 8-bit bidirectional data pins
19/OEOutput Enable (Active Low): Low (00) = Outputs enabled; High (11) = High-Impedance (Z)
20VCCPositive supply voltage (+2V to +6V DC)

Function Table

Control InputsBus Operation
/OEDIRData Path
Low (LL)Low (LL)B data to A bus (B→AB \rightarrow A)
Low (LL)High (HH)A data to B bus (A→BA \rightarrow B)
High (HH)X (Don't Care)High-Impedance state (ZZ) on both A and B buses

Specifications

ParameterSymbolMinTypMaxUnitConditions
Supply VoltageVCCV_{CC}2.05.06.0VDC
High-Level Input VoltageVIHV_{IH}3.15——VVCC=4.5VV_{CC} = 4.5\text{V}
Low-Level Input VoltageVILV_{IL}——1.35VVCC=4.5VV_{CC} = 4.5\text{V}
Propagation Delay (A→BA \rightarrow B)tPDt_{PD}—1322nsVCC=4.5V,CL=50pFV_{CC} = 4.5\text{V}, C_L = 50\text{pF}
Output Drive CurrentIOUTI_{OUT}——±35\pm 35mAVCC=6.0VV_{CC} = 6.0\text{V}
Quiescent Supply CurrentICCI_{CC}——8.0µAVIN=VCC or GNDV_{IN} = V_{CC}\text{ or GND}

Typical Application (5V to 3.3V Unidirectional Buffer / Level Shifter)

When powered from VCC=3.3VV_{CC} = 3.3\text{V}, the 74HC245's inputs on Side A are 5V-tolerant in 74LVC245 or 74AHCT245 variants. For standard 74HC245 powered at 3.3V, use voltage divider resistors or 74LVC245 for 5V-to-3.3V translation.

text
       5V MCU / Bus                             3.3V Target Device
   [Arduino D2..D9]                             [Display / SD Card]
         │                                               │
     5V Signal A1..A8 ──► [Side A: Pins 2-9]           3.3V Signal B1..B8 ──► [Side B: Pins 18-11]
                           74LVC245 / 74HC245
                           [Pin 1: DIR] ◄── 3.3V / GND (Set Direction)
                           [Pin 19: /OE] ◄── GND (Enable Output)

Common mistakes

  • Leaving /OE or DIR pins floating: CMOS inputs must never be left floating. Unconnected /OE or DIR pins float between high and low logic states, causing high supply current draw, erratic bus output, or high-Z oscillations.
  • Bus contention: Enabling the outputs (/OE = LOW) when both Side A and Side B are simultaneously driven by external outputs of different voltage levels creates short-circuit conditions between bus drivers, damaging the 74HC245 or microcontrollers.
  • Assuming 74HC245 powered at 5V recognizes 3.3V logic high: When powered from 5V5\text{V}, a standard 74HC245 requires VIH≥3.15VV_{IH} \ge 3.15\text{V} (which is 0.7×VCC0.7 \times V_{CC}). A 3.3V microcontroller GPIO output (3.0V…3.3V3.0\text{V} \dots 3.3\text{V}) is barely on the threshold margin and can fail intermittently. Use 74HCT245 (TTL input levels, VIH≥2.0VV_{IH} \ge 2.0\text{V}) when driving a 5V transceiver from 3.3V logic.

Notes

  • 74HC245 vs 74HCT245 vs 74LVC245: 74HC245 features pure CMOS inputs (0.7×VCC0.7 \times V_{CC} threshold); 74HCT245 has TTL-compatible input thresholds (2.0V2.0\text{V} high threshold); 74LVC245 has 5V tolerant inputs even when powered from a 3.3V supply rail.

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