REV.-A
r-
7.8 Serial
Controller
82C51
The 82C51 controls serial data transfers between the RS-232C interface and the
option
unit. Its operation modes vary speed
of
tronsfer, parity, and character length, etc., modes are con-
trolled by a control
word
which
is
written
in the 82C51 by the main
CPU.
Data are transferred one
byte at a time. The
transmit
and receive clock signals are supplied from the gate array
GAH40M.
Figure 7
-17
is a
block
diagram illustrating 82C51 signal
flow
to
and from external connectors.
Data bus
(GAH40D)
RESET
(2.45 MHz)
ClK
(ABO)
C/O
rRD
(GAH40M)
~WR
lcs
[DSR
(RS-232C)
~
DTR
CTS
lRTS
A
<
..
,
,--------------,
lA A
...
l-byte
transmit
1
Data bus
buffer
(with
a
,0"
buffer
..
...
paralle'to
I
serial conversion
I
feature)
I
I
I~
I
I
\ I
I'
A
..
Read/write
Transmit
,
I'
control
..
0"
control
I
I'
...
circuit
,
I
,
..
I
I
l'
r--
1-byte
receive -
,
""-
buffer
(with
a
I
I
serial
to
paral-
I
...
lei conversion
~
feature
I'
~
i
I
I
..
MODEM
A
I
"
control
...
~
I
I
Receive
,
control
I
I
L
______________
~
Fig.
7-17
82C51
Serial
Controller
Block
Diagram
L;.
...
...
,
L
....
TXD (RS-232C)
TXD Option
TXRDY}
TXE
Not
used
TXC (GAH40M)
RXD
(RS-232C)
RXD
(Option)
RXRDY}
RXC
SYNDET/BD (Not used)
The signals
to
the RS-232C interface are converted
to
the ±
8V
(RS-232C) levels between 82C51
and the interface. The Carrier Detect
(CD)
signal,
which
is
not
included in the above block dia-
gram, is connected
to
the interrupt port
of
GAH40M
to
interrupt the main
CPU.
The 82C51 itself can support
both
synchronous mode (BI-SYNC) and asynchronous mode (Start-
Stop system: includes
start/stop
bits); however, in
this
CPU,
TXE and SYNDET/BD are
not
con-
nected
for
SYNC character control signals.
When
an
asynchronous system is used, processing on the SYNC character, etc. needs
to
be sup-
ported
by
the application software.
The 82C51 cannot simultaneously process transmit/receive data
to
and from the RS-232C inter-
face and option unit. Thus, the GAH40M AUX signal is provided
to
allow
an
external control
for
enabling and disabling the RS-232C transmit/receive lines.
7-36
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