Part 21
Printer operations

Subjects covered...

	Parallel printers
	Serial printers
	LPRINT, LLIST
	FORMAT
	COPY

The +3 comes with an 8-bit Centronics parallel port and an RS232
serial port. Both are supported by built-in software enabling you to
use virtually any printer. These features are usable only in +3 BASIC
mode.

The printer must have either a Centronics compatible (parallel) or an
RS232 (serial) interface, and if you want to reproduce pictures of the
screen, then the printer must have an Epson compatible
quadruple-density bit-image graphics mode (ESC L n n).

Make sure you have the correct lead to connect the printer to the +3 -
if in doubt, consult your Sinclair dealer.

For further information about which printer and connecting lead to
purchase, together with details of the +3's PRINTER and RS232 socket
connections, see chapter 10 (Peripherals for your +3).


Parallel printers

When the +3 is first switched on it will assume that, if a printer is
present, it will be connected to the (parallel) PRINTER socket. The
hardware connection between computer and printer is relatively
straightforward - though you must make sure that you don't connect the
cable the wrong way up at the computer end (if the cable doesn't have
a locating 'key').

Once the connection has been made, the command...

	LPRINT "hello"

...should produce some printed output. If not, check the connection
and make sure that your printer is set to 'on line'.

Once you have got your printer to print, you may skip to the section
ahead entitled 'General printing'.


Serial printers

Unlike parallel printers, the connections between the +3 and a serial
(or RS232) printer will vary for different manufacturers' printers.
Make sure that your dealer has provided a lead suitable for connecting
your particular printer to the +3. A serial printer must be connected
to the +3's RS232 socket, and details of connections can be found in
chapter 10 (Peripherals for your +3).

The +3 always uses what is known as hardware flow control, or hardware
handshaking. This means that it will not transmit characters until
certain control signals from the printer have the right values. It is
therefore very important that connections are made to the control
lines of the +3 as well as the transmit and receive data lines. If
your printer does not support hardware handshaking then connect pins 4
and 5 of the +3's RS232 connector socket together. The drawback of not
using hardware handshaking is that the odd character may be lost when
transmitting a lot of data at high speed.

To get the +3 and the printer communicating with each other, they must
both use the same baud rate. The baud rate is the speed at which data
is transferred between computer and printer. Although it is possible
that your printer can be set to different baud rates, it'll probably
be easier to change the rate at the computer end. Somewhere in the
printer's operating manual, the baud rate will be specified - find
this out and then set the +3 to this rate, using the command...

	FORMAT LINE baud rate

For example...

	FORMAT LINE 300

(You won't need to do this if your printer normally uses 9600 baud, as
the +3 will assume this rate by default.)

As the +3 usually expects to be operating with a parallel printer, it
will be necessary to use the command...

	FORMAT LPRINT "R"

...before the +3 will successfully operate with a serial printer. (The
'R' in the above command is short for RS232.)

The command to set the +3 back to parallel (Centronics) mode is...

	FORMAT LPRINT "C"


General printing

Once you have everything set up, you can use three BASIC commands to
print things out. The first two, LPRINT and LLIST, are just like PRINT
and LIST except that they use the printer instead of the TV screen.
Note that the 'Print' option from +3 BASIC's edit menu has the same
effect as LLIST, but is included as an easier method of getting a
listing.

Try this program for example...

	10 LPRINT "This program..."'
	20 LLIST 40
	30 LPRINT '"...prints out the character set, ie..."'
	40 FOR n=32 TO 255
	50 LPRINT CHR$ n;
	60 NEXT n
	70 LPRINT

It's important to note that LPRINT and LLIST normally take care to
screen out any embedded colour codes (and their parameters) before
printing or listing anything. Embedded colour codes are a bit of a
hangover from the old 48K Spectrum - when included in a string they
set INK, PAPER and so on. Printers on the whole tend to use these
codes for completely different things like setting italics, underline,
etc., so it would be quite dangerous to send colour codes to the
printer and hope that nothing untoward happens. A side effect of this
is that the +3 will normally not be able to send escape control
sequences to the printer. For example, support your printer expects an
escape character (character 27) followed by '"x"; CHR$ (1)' to switch to
its NLQ mode; you would normally use the command...

	LPRINT CHR$ (27);"x"; CHR$ (1);
		"This is in Near Letter Quality"

However, in +3 BASIC, you must first issue the command...

	FORMAT LPRINT "U"

This command tells the +3 not to interpret characters as 'Spectrum
codes', but as ordinary unexpanded characters (the 'U' is short for
unexpanded). If the above command is not issued, then everything above
code 165 (see part 28 of this chapter) will be translated as one of
the +3's special words, or tokens. Likewise, almost everything below
code 32 will be screened out.

If you wish, you can instruct the +3 to interpret characters as
Spectrum codes by using...

	FORMAT LPRINT "E"

...(where 'E' stands for unexpanded). You'll need to do this if you're
going to use LLIST. The +3 starts off in expanded mode anyway, so
unless you've issued a 'FORMAT LPRINT "U"' command, you won't need to
use 'FORMAT LPRINT "E"'.

So, to summarise:

* If you want to send special characters (such as ESC) to your printer
(in order to use different styles of printing), issue the command....

	FORMAT LPRINT "U"

...before printing.

* If you are writing or modifying a program, and want to get a listing
on the printer, issue the command...

	FORMAT LPRINT "E"

...before listing the program.

The third BASIC statement used with a printer - COPY, prints out a
copy of the TV screen. To demonstrate, go into the small screen (by
selecting the 'Screen' option from the edit menu) and type in the
following command.

	FOR n=1 TO 20: PRINT n,: NEXT n

The numbers 1 to 20 will be printed in the top part of the screen. Now
type...

	COPY

The COPY command takes about 15-20 seconds to get started, so don't
panic if nothing appears to happen immediately. After a while, you'll
see a copy of the screen reproduced on the printer. (If all you get
from COPY is a lot of random characters on the printer, then it's
likely that your printer isn't fully compatible.)

You can always stop printing at any time by pressing the BREAK key.
Many printers have what's known as a buffer, which stores text before
printing. If your printer has a buffer, then pressing BREAK will not
stop the printer immediately (although the +3 will register the break
at once).

Note that if the COPY command is stopped by pressing the BREAK key,
the printer may be left in graphics mode (this will be indicated by
subsequent LPRINT statements) producing a mass of meaningless dots, or
printing each line of text partly over the previous line). In these
circumstances, switching the printer off then on again is the easiest
way to get things back to normal.

As well as the rather simple COPY command, which just produces a black
dot on the printer for each dot on the screen (whatever its colour may
be), there is an expanded version ('COPY EXP') which prints differing
combinations of dots depending on the colour of ink that was used on
the screen. To demonstrate, type in the following new program...

	10 FOR b=0 TO 1
	20 BRIGHT b
	30 FOR i=0 TO 6
	40 FOR c=0 TO 31
	50 PRINT INK i; i;
	60 NEXT c
	70 NEXT i
	80 NEXT b

...then switch to the bottom part of the screen (using the edit menu's
'Screen' option). Run the program (which displays twelve lines of
coloured numbers on the screen), then type in...

	COPY EXP

The printed output (or dump) from this command is slightly larger than
that from the standard COPY command - (EXP is short for expanded). The
command reproduces the coloured areas of the screen as different
densities of black dots on the printer. (All 24 lines of the screen
are reproduced.) Areas that have been printed with 'BRIGHT 1' will
appear lighter than areas printed normally (just as happens on the
screen).

The drawback of the 'COPY EXP' command is that it takes a longer time
to print (about 10 minutes), but is ideally suited to dumping graphic
pictures. The quicker COPY command, on the other hand, is a better bet
if you wish to dump text only.

If the screen display to be dumped is predominantly black, then it
will not only wear out your printer ribbon rather quickly, but also
will probably take longer to dump than a screen that has large areas
of white. To prevent this, the 'COPY EXP' command can be followed by
the word INVERSE, i.e....

	COPY EXP INVERSE

As the command suggests, the dump is printed in INVERSE (like a
photographic negative) so that all the dark areas of the screen are
printed-out light, and vice versa.

Note that INVERSE cannot be used after the simple COPY command - it
only works with 'COPY EXP'.

The dump produced by 'COPY EXP' and 'COPY EXP INVERSE' is designed to
fit a sheet of A4 paper; however, some printers will not print within
about an inch at either end of a sheet. If this problem occurs, then
it is possible to reduce the size of the dump slightly by using the
command...

	POKE 23419,8

This sets the number of 216ths inch used as a line feed at the end of
each pass of the print head. It is set to 9 when the +3 is first
switched on. Once set, it will not be changed even if the NEW command
is used. By reducing this value, each pass of the print head will
fractionally overlay the previous pass. As a consequence, the quality
of the dump reproduced will be degraded slightly.

If you try to use any of the printer commands when there isn't a
printer attached (or if the printer is off line), then the +3 will
stop dead while it patiently waits for the (non-existent) printer to
respond. In such a case, pressing BREAK twice will bring the +3 back
to life.

Try this...

	10 FOR n=31 TO 0 STEP -1
	20 PRINT AT 31-n,n; CHR$ ( CODE "0"+n);
	30 NEXT n

You will see a pattern of characters working down diagonally from the
top right-hand corner until it reaches the bottom of the screen, at
which point the program asks if you want to scroll.

Now change 'AT 31-n,n' in line 20 to 'TAB n'. The program will have
exactly the same effect as before.

Now change PRINT in line 20 to LPRINT. This time there will be no
pause to scroll (this does not occur with the printer).

Now change 'TAB n' back to 'AT 31-n,n' still using LPRINT. This time
you will get just a single line of symbols. The reason for the
difference is that the output from LPRINT is not printed straight
away, but is stored in the buffer until either one line's worth of
printer output has accumulated, or something else 'flushes' the
buffer. Hence, printing only takes place:

	1. When the buffer is full.

	2. After an LPRINT statement that does not end in a comma or
	   semicolon.

	3. When a comma, apostrophe or TAB item requires a new line.

	4. At the end of a program, if there is anything left
	   unprinted.

	5. When you set the printer off line (this depends on your
	   particular printer).

Number 3 above explains why our program with TAB works the way it
does. As for AT, the line number is ignored, and the LPRINT position
(like the PRINT position) is moved to the column number. An AT item
can never cause a line to be sent to the printer.


Exercises...

1. Make a printed graph of a sine wave by running the first (3 line)
program in part 17 of this chapter, then using COPY.

2. Run the program at the beginning of part 16 of this chapter and try
both a 'COPY EXP' and a 'COPY EXP INVERSE'.