Quatech DS-102 user manual Line Status Register bit definitions

LINE STATUS REGISTER

This register is located at I/O address [base+5]. It is used to provide various types of status information concerning the data transfer. As Figure 22 shows, the Line Status Register indicates several types of errors, an empty transmit buffer, a ready receive buffer, or a break on the receive line.

BIT	DESCRIPTION
7	FFRX --- Error in RCVR FIFO (16550 FIFO mode only):
	Always logic 0 in 16450 or 16550 non-FIFO mode.
	Indicates one or more parity errors, framing errors, or break indications in the
	receiver FIFO. FFRX is reset by reading the line status register.
6	TEMT --- Transmitter empty:
	Indicates the transmitter holding register or FIFO (16550) AND the transmitter
	shift register are empty and are ready to receive new data. TEMT is reset by
	writing a character to the transmitter holding register.
5	THRE --- Transmitter holding register empty:
	Indicates the transmitter holding register or FIFO (16550) is empty and it is
	ready to accept new data. THRE is reset by writing data to the transmitter
	holding register.
4	BI --- Break interrupt:
	Indicates the receive data input has been in the spacing state (logic 0) for longer
	than one full word transmission time. In 16550 FIFO mode, only one zero
	character is loaded into the FIFO and transfers are disabled until the serial data
	input goes to the mark state (logic 1) and a valid start bit is received.
3	FE --- Framing error:
	Indicates the received character had an invalid stop bit. The stop bit following
	the last data or parity bit was a 0 bit (spacing level).
2	PE --- Parity error:
	Indicates that the received data does not have the correct parity.
1	OE --- Overrun error:
	Indicates the receive buffer was not read before the next character was received
	and the character is destroyed. In 16550 FIFO mode, indicates the receive FIFO
	is full and another character has been shifted in. The character in the shift
	register is destroyed but is not transferred to the FIFO.
0	DR --- Data ready:
	Indicates data is present in the receive buffer or FIFO (16550). DR is reset by
	reading the receive buffer register or receiver FIFO.
	Figure 22 --- Line Status Register bit definitions

Bits BI, FE, PE, and OE are the sources of receiver line status

interrupts. The bits are reset by reading the line status register. In 16550 FIFO mode, these bits are associated with a specific character in the FIFO and the exception is revealed only when that character reaches the top of the FIFO.

Quatech DS-102 User's Manual 21