Diamond Systems RUBY-MM-1612 user manual 82C55A Functional Description, Data Bus Buffer

82C55A

Functional Description

I/O

PA7-

Data Bus Buffer

This three-state bi-directional 8-bit buffer is used to interface the 82C55A to the system data bus. Data is transmitted or

POWER	+5V	GROUP A
SUPPLIES	GND
		CONTROL

GROUP A

PORT A

(8)

PA0

I/O

received by the buffer upon execution of input or output instructions by the CPU. Control words and status informa- tion are also transferred through the data bus buffer.

Read/Write and Control Logic

The function of this block is to manage all of the internal and external transfers of both Data and Control or Status words. It accepts inputs from the CPU Address and Control busses and in turn, issues commands to both of the Control Groups.

(CS) Chip Select. A “low” on this input pin enables the communcation between the 82C55A and the CPU.

(RD) Read. A “low” on this input pin enables 82C55A to send

BI-DIRECTIONAL
DATA BUS
	DATA
D7-D0	BUS
	BUFFER	8-BIT
		INTERNAL
		DATA BUS
RD	READ	GROUP B
WR
	WRITE
A1		CONTROL
	CONTROL
A0
	LOGIC
RESET
CS

PC7-

GROUP A PC4 PORT C

UPPER

(4) I/O

PC3-

GROUP B	PC0
PORT C
LOWER
(4)
	I/O
	PB7-
GROUP B	PB0
PORT B
(8)

the data or status information to the CPU on the data bus. In essence, it allows the CPU to “read from” the 82C55A.

(WR) Write. A “low” on this input pin enables the CPU to write data or control words into the 82C55A.

(A0 and A1) Port Select 0 and Port Select 1. These input signals, in conjunction with the RD and WR inputs, control the selection of one of the three ports or the control word register. They are normally connected to the least significant bits of the address bus (A0 and A1).

82C55A BASIC OPERATION

					INPUT OPERATION
A1	A0	RD	WR	CS	(READ)
0	0	0	1	0	Port A → Data Bus
0	1	0	1	0	Port B → Data Bus
1	0	0	1	0	Port C → Data Bus
1	1	0	1	0	Control Word → Data Bus

					OUTPUT OPERATION
					(WRITE)
0	0	1	0	0	Data Bus → Port A
0	1	1	0	0	Data Bus → Port B
1	0	1	0	0	Data Bus → Port C
1	1	1	0	0	Data Bus → Control

					DISABLE FUNCTION
X	X	X	X	1	Data Bus → Three-State
X	X	1	1	0	Data Bus → Three-State

FIGURE 1. 82C55A BLOCK DIAGRAM. DATA BUS BUFFER, READ/WRITE, GROUP A & B CONTROL LOGIC FUNCTIONS

(RESET) Reset. A “high” on this input initializes the control register to 9Bh and all ports (A, B, C) are set to the input mode. “Bus hold” devices internal to the 82C55A will hold the I/O port inputs to a logic “1” state with a maximum hold current of 400μA.

Group A and Group B Controls

The functional configuration of each port is programmed by the systems software. In essence, the CPU “outputs” a con- trol word to the 82C55A. The control word contains information such as “mode”, “bit set”, “bit reset”, etc., that ini- tializes the functional configuration of the 82C55A.

Each of the Control blocks (Group A and Group B) accepts “commands” from the Read/Write Control logic, receives “control words” from the internal data bus and issues the proper commands to its associated ports.

Control Group A - Port A and Port C upper (C7 - C4)

Control Group B - Port B and Port C lower (C3 - C0)

The control word register can be both written and read as shown in the “Basic Operation” table. Figure 4 shows the control word format for both Read and Write operations. When the control word is read, bit D7 will always be a logic “1”, as this implies control word mode information.

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