Lesson 6. Bus buffers: 74HC244 and 74HC245

After this lesson the library has the 74HC244 and the 74HC245 — the chips any computer bus relies on — and the canvas has a four-wire bus: two devices talk on it in turn, a third one listens or answers.

6.1. The idea: a bus and the third state

In a computer the processor, memory and devices sit on the same wires — the bus. If two outputs drive different values onto one wire, that is a short circuit. So everyone who sits on the bus has an output with a third state: only the one that is enabled talks, the others are disconnected. The 74HC244 has eight such outputs, in two groups of four with their own OE.

6.2. The idea: a buffer is a double inversion

Inside the 74HC244 every input is inverted twice: the value is the same, but the output is now strong and goes one way only — the bus does not "leak" back into the input. That is a buffer. And the third state is a switch at the output, as in the 74HC595.

6.3. The idea: both ways — two 74HC244

Memory on a bus is both read and written: the data goes one way, then the other. The 74HC245 passes both ways: DIR chooses the direction. Inside are two 74HC244 facing each other, and only one is ever enabled. A chip made of chips — like the 74HC390 and the 74HC595.

6.4. Building

The same project as in lesson 5: the library has all the chips of lessons 1–5.

Step 1. Chip 74HC244 — 2 × 4 buffers with a third state

Package pins from the datasheet: 1 1OE, 2 1A0, 3 2Y0, 4 1A1, 5 2Y1, 6 1A2, 7 2Y2, 8 1A3, 9 2Y3, 11 2A3, 12 1Y3, 13 2A2, 14 1Y2, 15 2A1, 16 1Y1, 17 2A0, 18 1Y0, 19 2OE; 10 — GND, 20 — VCC. Two groups of four: 1A0–1A3 → 1Y0–1Y3 and 2A0–2A3 → 2Y0–2Y3, each with its own enable — 1OE, 2OE (active 0). Inside, three 74HC04: every input is inverted twice — that makes a strong buffer that drives the signal one way only — plus two inversions of OE; at the output — a Transmission gate.

Ctrl+G → name 74HC244. Legs: inputs 1OE, 1A0, 1A1, 1A2, 1A3, 2A3, 2A2, 2A1, 2A0, 2OE; outputs 2Y0, 2Y1, 2Y2, 2Y3, 1Y3, 1Y2, 1Y1, 1Y0.

Step 2. Chip 74HC245 — 8 bidirectional buffers

Package pins from the datasheet: 1 DIR, 2 A0, 3 A1, 4 A2, 5 A3, 6 A4, 7 A5, 8 A6, 9 A7, 11 B7, 12 B6, 13 B5, 14 B4, 15 B3, 16 B2, 17 B1, 18 B0, 19 OE; 10 — GND, 20 — VCC. DIR = 1 — data goes A → B, DIR = 0 — B → A; OE = 1 — both sides disconnected (active 0). Inside — two 74HC244, one per direction, and a 74HC04 and 74HC00 decide which one is enabled. Pins A and B work both ways.

Ctrl+G → name 74HC245. Legs: inputs DIR, A0, A1, A2, A3, A4, A5, A6, A7, OE; outputs B7, B6, B5, B4, B3, B2, B1, B0.

Step 3. Devices X and Y

Eight switches: X0–X3 — device X, Y0–Y3 — device Y.

Step 4. Who talks

SEL chooses who is on the bus: 0 — X, 1 — Y; OFF = 1 — nobody. The 74HC244 enable acts on a zero, so oeX = SEL OR OFF, oeY = NOT SEL OR OFF (74HC04 and 74HC32 from lesson 1).

Step 5. 74HC244 onto the bus

The outputs of both groups go to the same wires BUS0–BUS3. That is the bus: only the enabled group talks, the other is disconnected — the third state.

Step 6. Bus resistors

When nobody talks, the resistors keep 0 on the bus.

Step 7. A third device through a 74HC245

DIR = 1 — bus → lamps K0–K3. DIR = 0 — the device switches KS0–KS3 through resistors → K → 74HC245 → bus. Here the resistor is the "weak" output of the switch: when the 74HC245 drives K itself, it is stronger and there is no conflict.

Step 8. Unused 74HC245 pins — through resistors

On a 74HC245, pins A and B are sometimes inputs and sometimes outputs: at DIR = 1 the chip drives B, at DIR = 0 — A. An output straight to GND is a short circuit (the editor will highlight it). So the unused A4–A7, B4–B7 go to ground through a resistor: it holds 0, and when the pin becomes an output, there is no conflict.

Step 9. Display and lamps

Display (bus) ⚙: width 4, DEC, bus BUS. Lamps K0–K3 are what the third device sees.

6.5. Check

1. Set the switches: X = 5 (X0 and X2), Y = 9 (Y0 and Y3), KS = 12 (KS2 and KS3). DIR = 1, OFF = 0.
2. SEL = 0 — the bus display shows 5, lamps K show 5: X talks, the third device listens.
3. SEL = 1 — 9 on the bus: Y talks.
4. OFF = 1 — 0 on the bus: nobody talks, the resistors pull the wires to ground.
5. Keeping OFF on, set DIR = 0 — 12 on the bus: now the third device talks, through the 74HC245 the other way.

The main bus rule: only one talks. If you turn OFF off at DIR = 0, two will drive the bus — a short circuit (in the simulator the wire turns red). On a real board chips burn this way.

Lesson 6 paste (Cmd+I)

On top of lesson 5.

# Lesson 6 — 74HC244 and 74HC245: bus buffers; a shared 4-wire bus — whoever is enabled talks.# Paste: Cmd+I on top of lesson 5. chip 74HC244 {  portIn(1OE)  portIn(1A0)  portIn(1A1)  portIn(1A2)  portIn(1A3)  portIn(2A3)  portIn(2A2)  portIn(2A1)  portIn(2A0)  portIn(2OE)  portOut(2Y0)  portOut(2Y1)  portOut(2Y2)  portOut(2Y3)  portOut(1Y3)  portOut(1Y2)  portOut(1Y1)  portOut(1Y0)  74HC04(1A=1A0, 1Y=n1A0, 2A=1A1, 2Y=n1A1, 3A=1A2, 3Y=n1A2, 4Y=n1A3, 4A=1A3, 5Y=n2A0, 5A=2A0, 6Y=n2A1, 6A=2A1)  74HC04(1A=2A2, 1Y=n2A2, 2A=2A3, 2Y=n2A3, 3A=n1A0, 3Y=b1A0, 4Y=b1A1, 4A=n1A1, 5Y=b1A2, 5A=n1A2, 6Y=b1A3, 6A=n1A3)  74HC04(1A=n2A0, 1Y=b2A0, 2A=n2A1, 2Y=b2A1, 3A=n2A2, 3Y=b2A2, 4Y=b2A3, 4A=n2A3, 5Y=e1, 5A=1OE, 6Y=e2, 6A=2OE)  tgate(b1A0, e1, 1Y0)  tgate(b1A1, e1, 1Y1)  tgate(b1A2, e1, 1Y2)  tgate(b1A3, e1, 1Y3)  tgate(b2A0, e2, 2Y0)  tgate(b2A1, e2, 2Y1)  tgate(b2A2, e2, 2Y2)  tgate(b2A3, e2, 2Y3)} chip 74HC245 {  portIn(DIR)  portIn(A0)  portIn(A1)  portIn(A2)  portIn(A3)  portIn(A4)  portIn(A5)  portIn(A6)  portIn(A7)  portIn(OE)  portOut(B7)  portOut(B6)  portOut(B5)  portOut(B4)  portOut(B3)  portOut(B2)  portOut(B1)  portOut(B0)  74HC04(1A=OE, 1Y=nOE, 2A=DIR, 2Y=nDIR, 3A=GND, 4A=GND, 5A=GND, 6A=GND)  74HC00(1A=nOE, 1B=DIR, 1Y=oAB, 2A=nOE, 2B=nDIR, 2Y=oBA, 3A=GND, 3B=GND, 4A=GND, 4B=GND)  74HC244(1OE=oAB, 2OE=oAB, 1A0=A0, 1Y0=B0, 1A1=A1, 1Y1=B1, 1A2=A2, 1Y2=B2, 1A3=A3, 1Y3=B3, 2A0=A4, 2Y0=B4, 2A1=A5, 2Y1=B5, 2A2=A6, 2Y2=B6, 2A3=A7, 2Y3=B7)  74HC244(1OE=oBA, 2OE=oBA, 1A0=B0, 1Y0=A0, 1A1=B1, 1Y1=A1, 1A2=B2, 1Y2=A2, 1A3=B3, 1Y3=A3, 2A0=B4, 2Y0=A4, 2A1=B5, 2Y1=A5, 2A2=B6, 2Y2=A6, 2A3=B7, 2Y3=A7)} input(X0)input(X1)input(X2)input(X3)input(Y0)input(Y1)input(Y2)input(Y3)input(SEL)input(OFF)74HC04(1A=SEL, 1Y=nSEL, 2A=GND, 3A=GND, 4A=GND, 5A=GND, 6A=GND)74HC32(1A=SEL, 1B=OFF, 1Y=oeX, 2A=nSEL, 2B=OFF, 2Y=oeY, 3A=GND, 3B=GND, 4A=GND, 4B=GND)74HC244(1OE=oeX, 2OE=oeY, 1A0=X0, 1Y0=BUS0, 1A1=X1, 1Y1=BUS1, 1A2=X2, 1Y2=BUS2, 1A3=X3, 1Y3=BUS3, 2A0=Y0, 2Y0=BUS0, 2A1=Y1, 2Y1=BUS1, 2A2=Y2, 2Y2=BUS2, 2A3=Y3, 2Y3=BUS3)resistor(BUS0, GND)resistor(BUS1, GND)resistor(BUS2, GND)resistor(BUS3, GND)input(DIR)input(KS0)input(KS1)input(KS2)input(KS3)resistor(KS0, K0)resistor(KS1, K1)resistor(KS2, K2)resistor(KS3, K3)74HC245(DIR=DIR, OE=GND, A0=BUS0, A1=BUS1, A2=BUS2, A3=BUS3, A4=UA4, A5=UA5, A6=UA6, A7=UA7, B0=K0, B1=K1, B2=K2, B3=K3, B4=UB4, B5=UB5, B6=UB6, B7=UB7)resistor(UA4, GND)resistor(UA5, GND)resistor(UA6, GND)resistor(UA7, GND)resistor(UB4, GND)resistor(UB5, GND)resistor(UB6, GND)resistor(UB7, GND)display{bits=4, base="DEC", busSource="BUS"}()led(K0)led(K1)led(K2)led(K3)