Radio Shack TRS-80 Service Manual page 64

PORT AND BIT ASSIGNMENTS
PORT E8H
OUTPUT: MASTER RESET
INPUT: MODEM STATUS REGISTER
An output to this port (and data), performs a master reset to the UART and enables the control register
load enable bit. The following table details the bit definitions for an input from port EBH.
DATA BIT
07
06
OS
D4
03
02
01
00
PORT E9H
OUTPUT: BAUD RATE LOAD
INPUT: NOT USED
FUNCTION
Clear To Send, Pin 5 08·25
Data Set Ready, Pin 6 08·25
Carrier Detect. Pin 8 08-25
Ring Indicator, Pin 22 08-25
Not
Used
Not Used
Not Used
Receiver Input. UART Pin 20 oB-25
An output to this port loads the Baud rate generator with a code which corresponds to the desired receive
and
transmit Baud rate
as
outlined in the BRG Programming Table. The low order nibble of the data
output to this port determines the receiver Baud rate. while the high order nibble determines tne trans-
mit Baud rate.
PORT EAH
OUTPUT: UART AND MODEM CONTROL
INPUT: UART STATUS
An output to this port loads the UART Control register if the enable bit for this function is set (01 port
E8H ~
11.
The UART Control register is five bits wid!'! (07 .
03)
leaving three bits for modem control
(02 .
00).
Three morc modem mnlrol bits were added by allowing software to enable or disable the
UART Control register. The tables below summarize the bit allocations with the UART Control register
enabled and disabled.
PORT EAH OUTPUT BlTSWITH UART CONTROL REGISTER ENABLED
DATA BIT
07
06
OS
D'
03
02
01
00
FUNCTION
Even Parity Enable. 1
-=
even.
Q ""
odd
Word Length Select 1
Word Length Select 2
Stop' Bit Select. 1 =
two
stop bits. G - one stop bit
Parity Inhibit, 1 - disable parity
Break IJ '" disable transmit data Icontinous space}
Data Terminal Ready, Pin 20 08-25
Request To Send, Pin 4 OB-25
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