11-23
ASYNCHRONOUS SERIAL I/O UNIT
11.3.4 Transmit Buffer Register (TBR
n
)
Write the data words to be transmitted to TBRn. Use the interrupt control or DMA units or pollthe serial line status register (LSRn) to determine whether the transmit buffer is empt y.

Figure 11-13. Transmit Buffer Register (TBR

n
)
Transmit Buffer
TBR0, TBR1
(write only) Expanded Addr:
ISA Addr:
Reset State:
TBR0 TBR1
F4F8H F8F8H
03F8H 02F8H
XXH XXH
7 0
TB7 TB6 TB5 TB4 TB3 TB2 TB1 TB0
Bit
Number Bit
Mnemonic Function
7–0 TB7:0 Transmit Buffer Bits:
These bits make up the next data wo rd to be transmitted. The transmitter
loads this word into the tran smit shift register. The transmit shift register
then shifts the bits out, along with the asynchronous com munication bits
(start, stop, and parity). The data bits a re shifted out least-significant bit
(TB0) first.
NOTE: The transmit buffer register shares an address port with other SIO re gisters. You must clear
bit 7 (DLAB) of LCR
n
before you can write to th e transmit buffer register.