Commodore Plus 4 - Page 395

The Modem and Other RS232 Devices 383
011= even parity receive and transmit
101 = mark parity bit transmit/no parity check
111= space parity bit transmit/no parity check
Errors associated with the RS232 port can be read from the status variable
(ST). The meanings of its bits are as follows:
Bit Meaning
0
1
2
3
4
5
6
7
parity error
framing error
receiver buffer overrun
receiver buffer empty
clear to send missing*
—
data set ready missing
(data set refers to the modem)
break detected*
♦From disassembling the operating system, it appears
that these bits will never be set.
Bits 2 and 3 reflect the status of the RS232 buffer. When bit 3 is set, no
character was received. When bit 2 is set, the buffer is full. Bit 6 may be set if
communication through the port is begun before it fully initializes.
The RS232 logic has a built-in capability to send and receive x-on (transmitting
on) and x-off (transmitting off) characters. It is enabled by storing the nonzero
x-on character in location 252 ($FC) and the nonzero x-off character in location
253 ($FD). When the RS232 buffer is almost full, the x-off character is sent to the
host computer. When it is almost empty again, the x-on character is sent.
Similarly, when the x-off character is received, the operating system holds off
sending data until an x-on character is received. Check the specifications of the
host computer you are communicating with to determine the proper values for
x-on and x-off.
As with all logical files, the RS232 port should be closed using
CLOSE file number
When the file is closed, the buffer area and its associated registers are cleared. To
check that everything has been received before this is done, you can check the
value found at 2003 (S07D3). This location contains the count for the RS232
buffer.