Commodore Plus 4 - Page 401

The Modem and Other RS232 Devices 389
Using an RS232 Device from Machine Language
The RS232 port is handled the same way in machine language as in BASIC. The
status variable is read with READSS.
Open
When the file is to be opened, the control and command registers (two characters)
must be stored in ascending order somewhere in memory. The following outlines
the operations required:
1. Load .A with the logical file number to use for the file, .X with $02 (the device
number of the RS232 port), and .Y with $FF (to signify no secondary address).
Call SETLFS ($FFBA).
2. Load .A with the length of the register information (usually 2), .X with the low
byte of the address where the registers are stored, and .Y with the high byte of
the address where the registers are stored. Call SETNAM ($FFBD).
3. Call OPEN ($FFC0).
4. Check for errors by examining the carry bit.
Close
Files can be closed individually with the CLOSE ($FFC3) routine:
1. Load .A with the logical file number of the file to be closed. Call CLOSE
($FFC3).
2. Check for errors by examining the carry bit.
All the files in use can be closed and the input and output channels reset to their
default devices by calling CLALL ($FFE7).
Print
To send information to the RS232 port, the port must have been opened. The
following outlines the operations required:
1. Load .X with the logical file number of the output file. Designate it as the
output channel by calling CHOUT ($FFC9).
2. Check the status byte for errors.