Radio Shack 26-1145 - Timing Diagram

Port Addresses
The
Radio
Shack RS-232-C
Interface
is an 1/O-mapped
device which
uses four
port
addresses (E8H.
E9H. EAH
and
EBH)
for
communica-
tion with the CPU.
I/O-mapped
devices
in a Z-80 system
use the
lower
address
bits (A0-A7)
in conjunction
with
the IN*
and OUT*
signals
to address the desired
ports.
Figure
5 shows the
timing
associated with
this
addressing
scheme.
SYSTEM
CLOCK
A0-A7
IN*
DATA BUS
OUT*
DATA
BUS
T2
I
TW
PORTADRESS
OUT
J-
-GHh
1.8 Mhz
READ CYCLE
WRITE
CYCLE
FIGURE 5. I/O
TIMING
DIAGRAM
Decode
Logic
The block labeled DECODE LOGIC in Figure 4 provides eight
low-going
strobes that
are used
by
the RS-232-C.
These strobes
allow the CPU
to select any of the
registers and latches
in the
RS-232-C
Interface
for
an input
or
output
operation. Table 4
summarizes
the
I/O
port address allocation as implemented
in the
RS-232-C Interface
and the function
performed.
ADDRESS
OUT
(I/O PORT WRITE)
IN
(I/O PORT READ)
E8H
Master Reset
(Any
Data)
Modem
Status Register
E9H
Baud Rate
Select
Configuration Sense
Switches
EAH
UART
Control
Register
and
Handshake
Latch
UART
Status Register
EBH
Transmit Data
Register
Received
Data Register
TABLE 4. I/O
MAPPED MEMORY ALLOCATION
14