CDP1857C pdf datasheet(4-Bit B

The CDP1857C is a 4-bit CMOS non-inverting bus separator
designed for use in CDP1800-series microprocessor systems. It can
be controlled directly by a 1800-series microprocessor without the
use of additional components.
The CDP1857 is designed for use as a bus buffer or separator
between the 1800-series microprocessor data bus and I/O devices.
It provides a chip-select (CS) input signal which, when high (1),
enables the bus-separator three-state output drivers. The direction
of data flow, when enabled, is controlled by the MRD input signal.
In the CDP1857, when MRD = 1, it enables the three-state bus drivers
(DB0-DB3) and transfers data from the DATA-IN lines onto the
data bus. When MRD = 0, it disables the three-state bus drivers
(DB0-DB3) and enables the three-state data output drivers (DO0-
DO3), thus, transferring data from the data bus to the DATA-OUT terminals.