NorthStar HSIO-4 User's & Technical Manual page 87

TRANSMIT BUFFER/
CONVERTER
PIN IDENTIFICATION
(CONT.)
8 2 5 1 IN T E R F A C E J T O 8 0 8 0
i 1 AIM U A n U O T ö 1 tIV I B U S
North Star HSIO-4
The Transmit Buffer/Converter receives parallel data from the Data Bus Buffer via the
internal data bus, converts parallel to serial data, inserts the necessary characters or
bits needed fo r the programmed communication format and outputs composite serial
data on the TxD output.
PIN
NO. SYMBOL
Transmit Control Logic
15 TxRDY Transmitter Ready
18 TxE
Transmitter Empty
9 Transmitter Clock
19 TxD Transmitter Data
---------------------------------------------
A0
)
\
'______ £ ■
CIO CS
82
NAME
The Transmit Control Logic accepts and outputs
all external and internal signals necessary for
serial data transmission.
Transmitter Ready signals the processor that the
transmitter is ready to accept a data character.
TxRDY can be used as an interrupt or may be
tested through the Status information for Polled
operation. Loading a character from the pro­
cessor automatically resets TxRDY.
The Transmitter Empty output signals the
processor that the USART has no further char­
acters to transmit. TxE is automatically reset
upon receiving a data character from the pro­
cessor. In half-duplex, TxE can be used to signal
end of a transmission and request the processor
to "turn the line around." The TxEn bit in the
command instruction does not effect TxE.
In the Synchronous mode, a "one" on this out­
put indicates that a Sync character or charac­
ters are about to be automatically transmitted
as "fillers" because the next data character has
not been loaded.
The Transmitter Clock controls the serial charac­
ter transmission rate. In the Asynchronous
mode, the TxC frequency is a multiple of the
actual Baud Rate. Two bits of the Mode Instruc­
tion select the multiple to be lx , 16x, or 64x
the Baud Rate. In the Synchronous mode, the
TxC frequency is automatically selected to
equal the actual Baud Rate.
Note that for both Synchronous and Asynchro­
nous modes, serial data is shifted out of the
USART by the falling edge of TxC.
The Transmit Control Logic outputs the
composite serial data stream on this pin.
a d d r e s s bus
CONTROL BUS
I/O R I/O w
DATA BUS
8
\ Z
c ______ I
D ; - Do RD WR
8251
User/Technical Manual
FUNCTION
02
RESET
(TTL)
_______;
RESET CLK
\
\
\