Radio Shack TRS-80 Service Manual page 62

RS-232C TECHNICAL DESCRIPTION
The RS-232C option board for the Model III computer
supports asynchronous serial transmissions and COflforms
to the EIA RS-232C standards
at
the input· output
interface (Pli. The heart of the board is the TRl602
Asyndlrooous Receivertrransmittef. It performs the job
of con'o'llrting the parallel byte date from the CPU to a
serial data stream including start, stop, and paritY bits.
For a more detailed description of how this LSI circuit
per-forms these functions, refer to the TRl602 data
sheets and application notes. The transmit and reoeive
clock rates that the TRl602 needs are supplied by the
Baud rate generator (B R194111. This circuit takes the
5.0688 MHz supplied
by
the CPU board and the
programmed information received from the CPU over
the data bus and divides the bolsic clock rate to provide
two clocks. The rates available
from
the BRG go from
50 Baud to 19200 Baud. See the BRG table for tlte
complete list.
BRG PROGRAMMING TABLE
TRANSMIT OR RECEIVE SUPPa ATED BY
NIBBLE LOADED BAUD RATE l6X CLOCK FREQUENCY
SETCOM
GH 50
0.8 kHz
YO'
lH 75
1.2 kHz
,
..
2H 110
1.76 kHz
,
. .
3H 1345
2.1523kHz
YO'
4H 150
2.4 kHz
YO'
5H
300 4.8 kHz
,
..
6H
600 9.6 kHz.
,
..
7H
1200 19.2 kHz
,
..
6H 1600
28.8 kHz
,
. .
9H
2Q00
32.081 kHz
,
. .
AH
2400
38.4 kHz
,
. .
BH 3600
57.6 kHz
,
. .
CH 4800
76.8 kl-lz
,
. .
OH 7200
115.2 kHz
,
. .
EH
9600
113.6 kHz
YO'
FH
19,200 316.8 kHz
YO'
55