Quatech RS-422/485, MPA-200/300 user manual RR0, RR1, RR2, RR3, RR6, RR7, RR8

Example 3: Write data into the transmit buffer of channel A.

mov	dx,base	; load base address
out	dx,al	; write data in ax to buffer

Example 4: Read data from the receive buffer of channel A.

		mov	dx,base	; load base address
		in	al,dx	; write data in ax to buffer
Table 2	SCC read register description.
RR0		Transmit, Receive buffer statuses and external status
RR1		Special Receive Condition status, residue codes, error conditions
RR2		Modified Channel B interrupt vector and Unmodified Channel A
			interrupt vector
RR3		Interrupt Pending bits
RR6		LSB of frame byte count register
RR7		MSB of frame byte count and FIFO status register
RR8		Receive buffer
RR10		Miscellaneous status parameters
RR12		Lower byte of baud rate time constant
RR13		Upper byte of baud rate time constant
RR15		External/Status interrupt information

The SCC can perform three basic forms of I/O operations: polling, interrupts, and block transfer. Polling transfers data, without interrupts, by reading the status of RR0 and then reading or writing data to the SCC buffers via CPU port accesses. Interrupts on the SCC can be sourced from the receiver, the transmitter, or External/Status conditions. At the event of an interrupt, Status can be determined, then data can be written to or read from the SCC via CPU port accesses. For block transfer mode, DMA transfers accomplish data transfers from the SCC to memory or from memory to the SCC, interrupting the CPU only when the Block is finished. Further information on these subjects are found in the chapters titled INTERRUPTS, and DIRECT MEMORY ACCESS.

The SCC incorporates additional circuitry supporting serial communications. This circuitry includes clocking options, baud rate generator (BRG), data encoding, and internal loopback. The SCC may be programmed to select one of several sources to provide the transmit and receive clocks. These clocks can be programmed in WR11 to come from the RTXC pin, the TRXC pin, the output of the BRG, or the transmit output of the DPLL. The MPA-200 uses the TRXC pin for its transmit

7 Quatech Inc., MPA-200/300 Manual