Radio Shack TRS-80 Service Manual page 65

PORT EAH OUTPUT BITS WITH UAAT CONTROL REGISTER DISABLED
DATA BIT
07
06
OS
04
03
02
01
""
OATA81TS
07
06
OS
04
03
02
01
OIl
FUNCTION
Not Used
Not Used
secondary unassigned, Pin 18 08-25
Secondary Transmit Data, Pin 14 08-25
Secondary Request To Send, Pin 19 08·25
Break"
=
di$3b1e Tran!ifTlit Data (continous space)
Data Terminal Ready, Pin 20 OB·25
ReQUest To Send, Pin 4 OB-25
PORT EAH INPUT BITS
FUNCTION
Data Received,
1 •
condition true
Transmitter Holding register empty, , . condition true
Overrun error, 1" condition true
Framing error, 1" condition true
Parity error, 1. condition true
Not
Used
Not Used
Not Used
58
PORT E8H
OUTPUT: TRANSMITTER HOLDING REGISTER
INPUT: RECEIVER HOLDING REGISTER
An output to this port loads the UART Transmitter Holding register with a word to be transmitted,
as soon 8S the last word loaded in the holding register is transmitted. This register should never be loaded
until the Transmitter Holding register empty bit (port EAH) is true.
An
input from this
port
reads the
last word received from the UART received data holding register _This register should not be read until
the data received bit
(port
EAH) is true.