7

RD/WE - Read/Write Enable (Input):

This signal defines whether the selected register is to be

read from or written to. In 1800 systems use MRD if MDU's

are addressed as I/O devices, MWR is used if MDU's are

addressed as memory devices.

RA2, RA1, RA0 - Register Address (Input):

These input signals define which register is to be read from

or written to. It can be seen in the “CONTROL TRUTH

TABLE” that RA2 can be used as a chip enable. It is identical

to the CE pin, except only CE controls the three-state

C.O./O.F. on the most significant MDU. In 1800 systems use

N lines if MDU's are used as I/O devices, use address lines

or function of address lines if MDU's are used as memory

devices.

Bus 0 - Bus 7 - Bus Lines:

Three-state bi-directional bus for direct interface with

CDP1800 series and other 8-bit microprocessors.

See Pin 6.

See Pin 5.

Cl- Carry In (Input):

This is an input for the carry from the next less significant

all others it must be connected to the CO pin of the next less

significant MDU.

CN1, CN0 - Chip Number (Input):

These two input pins are wired high or low to indicate the

MDU position in the cascaded chain. Both are high for the

most significant MDU regardless of how many CDP1855

MDU's are used. Then CN1 = high and CN0 = low for the

next MDU and so forth.

Power supply line.

Power supply line.

CONTROL TRUTH TABLE

INPUTS (NOTE 1)

RESPONSE

CE

RA2

(N2)

RA1

(N1)

RA0

(N0)

RD/WE

(MRD)

STB

(TPB)

0

XXXXX

No Action (Bus Floats)

X

0

XXXX

No Action (Bus Floats)

11001

X

X to Bus

Increment Sequence

Counter When STB

and RD = 1

11011

X

Z to Bus

11101

X

Y to Bus

11111

X

Status to Bus

110001

Load X from Bus

Increment Sequence

Counter

110101

Load Z from Bus

111001

Load Y from Bus

111101

Load Control Register

1

1

X

X

0

0

No Action (Bus Floats)

NOTE:

1. ( ) = 1800 System Signals. 1 = High Level, 0 = Low Level, X = High or Low Level.

CDP1855, CDP1855C