Serial Interface Circuit - Tandy 102 Service Manual

Serial Interface Circuit
The serial interface circuit supports asynchronous data transmission/reception. The heart of this
circuit is the 6402 (UART). It performs the job of converting the parallel byte data from the CPU to a
serial data stream including start, stop and parity bits.
For a more detailed description of how this IC performs these functions, refer to Appendix C of this
manual.
Figure 4-14 shows the functional block diagram of the serial interface circuit. In this figure, the TO
signal, basic timing clock for the UART, defines the transmission/reception baud rate.
To transmit and receive the serial data from external devices, the RS232C signal selects either
MODEM or RS-232C interface. During the MODEM operation, the MODE signal switches either the
originate mode or answer mode for the MODEM IC.
The serial interface circuit is subdivided into the following circuits:
• RS-232C/MODEM Selection Circuit
• RS-232C Interface Circuit
• MODEM IC
• Transmission Filter Circuit
• Reception Filter Circuit
• MODEM Connector Circuit
RS232C
DT-R
CPU
PI0
R
M
D R
UART
TRO
RRI
OE.FE.PE.
M38 M TS
M13
CL/AS
M33
OORIRIrGo
RXCAR h ANS
RXM
TXM
M31
MODEM
CP/TL
M23
M4
M26
MODE
M24
DIR
ACP
DIR
ACP
CN6
U
N
:Y °
Er
RY2 RY3
L
CN4
TXMD
TL
TXM
RXM
RP
Figure 4- 14. Functional Block Diagram of the Serial Interface