Intel 8080 III. Bi-Directional Bus Driver, IV. Interrupting Input Port, BI-DIRECTIONAL BUS Driver

SCHOTTKY BIPOLAR 8212

III. Bi-Directional Bus Driver

Apair of 8212'swired (back-to-back) can be used as a symmetrical drive, bi-directional bus driver. The devices are controlled by the data bus input control which is connected to D81 on the first 8212 and to D82 on the second. One device is active, and acting as a straight through buffer the other is in 3-statemode. This is a very useful circuit in small system design.

IV. Interrupting Input Port

This use of an 8212 is that of a system input port that accepts a strobe from the system input source, which in turn clears the service request flip-flop and interrupts the processor. The processor then goes through a service routine, identifies the port, and causes the device selection logic to go true - enabling the system input data onto the data bus.

V. Interrupt Instruction Port

The 8212 can be used to gate the interrupt instruc- tion, normally RESTART instructions, onto the data bus. The device is enabled from the interrupt acknowledge signal from the microprocessor and from a port selection signal. This signal is normally tied to ground. (081 could be used to multiplex a variety of interrupt instruction ports onto a com- mon bus).

BI-DIRECTIONAL BUS DRIVER

	STB
DATA---~	8212	1 ---- ' --- ......... OATA
BUS		BUS
DATA BUS
CONTROL	GND
(o= L - R)
(I = R - L)

STB

8212

		GND
		INTERRUPTING INPUT PORT
		DATA
	INPUT	BUS
	STROBE
		STB
	SYSTEM
	INPUT
	SYSTEM
	RESET
PORT	{
SELECTION		TO PRIORITY CKT
(DS1.DS2)		-------- ' _ - ... - (ACTIVE LOW)
		OR .
		TO CPU
		INTERRUPT INPUT

INTERRUPT INSTRUCTION PORT

DATA

BUS

STB

RESTART INSTRUCTION (RST 0 - RST 7)

(DSI) PORT SELECTION -	........~
INTERRUPT ACKNOWLEDGE -	... -- _

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