An ACIA contains the hardware to interface RS232 signals to the microprocessor. Since true RS232 signals are not TTL-compatible, buffers are used to bring these into TTL/CMOS levels.
The ACIA also provides error checking and synchronisation functions.
Tx data register Control register Status register Rx data registerbut has only one register select line (i.e. 2 addresses only). This is because the first two registers are WRITE-only and the last two registers are READ-only.
In passing, note that normal instructions such as INC, DEC and SHIFT will not work with this kind of register usage since when you read or write, you are accessing a different register. Only LOAD and STORE instructions are valid for the registers above.
There is no RESET signal on the 6850. A Master Reset command has to be given to the Control Register.
The end of transmission of a character can be detected by a bit in the Status Register, SR1 which is otherwise called TRDE - Transmit Data Register Empty.
Once a character has been moved from the Tx Data Register to the ACIA's Tx shift register, the TDRE flag is cleared. So another character can be queued whilst one is being transmitted.
When the Rx Data Register is read, then the RDRF flag is automatically cleared, although the data remains in the register until overwritten by a new received character.
|set data word|
|Clock divider /|
A write to CR1,0 of '11' causes a Master Reset which clears all registers (except SR bit 3).
The number of bits, parity selection and number of STOP bits are selected by CR2,3,4.
CR5,6 set the Tx interrupt options and CR7 sets the Rx interrupt option.
DCD indicates when a modem carrier is present. This
signal is output by the modem.
CTS is an input to the ACIA which inhibits the transmission of new data.
FE is set if a framing error has occurred. This means that an expected STOP bit was not received.
OVRN is set if an overrun error occurred. This means that a new character was received in the Rx data register before the previous character was read by the microprocessor. Data is lost in such circumstances.
PE shows that a Parity error occurred.
IRQ shows if the 6850 is sending an interrupt signal. This flag is cleared when the Control Register is read.
These flags are automatically reset when the next character arrives or when data is read from the 6850.
RTS is generated by the ACIA when the Rx shift register is full and the data has not been read by the microprocessor. This signal may be used to prevent a remote modem from transmitting further data.
Xon/Xoff is a software protocol which tells a remote system to
stop sending data. This can be used in place of RTS/CTS hardware
signals. However, owing to the time taken to transmit the Xoff
character itself, the receiver must have some buffering capability.
Xoff may be sent from the keyboard by typing CTRL-S and Xon by typing CTRL-Q.