Commodore Plus 4 - Page 391

The Printer 379
This is a routine to close all the files and print ERROR on the screen:
. 2080 20 E7 FP JSR $FF$7 Close all the files and reset I/O
channels.
. 2083 A2 05 LDX #$05 Counter for message.
. 2085 BD F8 20 LDA $20F8,X Get a byte of message.
. 2088 20 D2 FF JSR $FFD2 Send to the screen.
. 208B CA DEX Decrement the counter.
. 208C 10 F7 BPL $2085 If not done, go on.
. 208E 00 BRK Stop processing.
These are the data for the error message:
>20F8 52 4F 52 52 45 OD 00 00 -1R0RRE...1
Programming the Serial Bus Occasionally it is desirable to program directly to
the serial bus. The family of operating subroutines used for this purpose consists
of LISTN (SFFB1), SECND ($FF93)5 CIOUT($FFA8), and UNLSN ($FFAE).
Programming at this level eliminates the file structure used with OPEN and
CLOSE. Error checking must be done using the status variable for device not
present, and such. When this is done depends to a large extent on what assump
tions the particular application allows. When in doubt, check for an error.
To Send Data to a Printer File
Here is an outline of a procedure:
1. Set the status variable ($90) to zero.
2. Load .A with the device number of the printer (and store in the current device
number at $AE if this has not been done previously). Call LISTN ($FFB1).
Check for a device not present error in the status variable.
3. Load .A with the secondary address you want to send ORed with $60 Call
SECND ($FF93).
4. Send the data through .A to the printer using CIOUT ($FFA8).
5. Call UNLSN ($FFAE).
Example: This program commands the printer to listen and sends to it whatever the user
types on the keyboard until a carriage return is received. It uses device number 4
and a secondary address of 7 (which means upper/lower case on the MPS-801). If
these are not appropriate, change them.