IBM System/370 Manual page 157

mechanism is self-adjusting and automatically rises in increments as
the stack of paper mounts. This insures that the stacker mechanism
is always the same distance above the top of the stack of forms. The
rate of rise during each increment is determined by the setting of
the stacker rate knob, which can be adjusted by the operator to produce
the best condition for the thickness of the forms being stacked. The
stacker also can be raised or lowered manually.
When forms are inserted, the printer platen automatically positions
itself close to the train cartridge in accordance with the thickness
of the forms. Thus, correct clearance between the platen and the
cartridge is achieved without operator intervention. Because of its
automatic forms thickness sensing, the 3211 is sensitive to forms with
a different degree of thickness at each edge. (For forms limitations,
see Form-Design Considerations-System Printers, GA24-3488.)
Forms control paper carriage tape loading and unloading by the
operator is eliminated by implementation of a tapeless carriage feature
for the 3211. Forms spacing and skipping are controlled by a program-
loaded forms control buffer (FCB) contained in the 3811 control unit.
The FCB contains 180 storage positions, each of which corresponds
to a print line, that is, a single space of the carriage. Thus, forms
up to 22.5 inches in length can be accommodated at 8 lines per inch
spacing (or 24 inches at 6 lines per inch). Up to twelve channel codes
(1-12), corresponding to the twelve channel positions of the paper
carriage tape used on a 1403 Printer, can be stored in the appropriate
buffer line positions to control carriage skipping. The FCB can be
considered to contain a storage image of a carriage control tape.
A carriage control address register is used to address the FCB
and maintain correct line position with respect to the form. This
register is incremented as space and skip commands are issued that
cause the form to advance. When a SKIP
TO
CHANNEL command is executed,
the carriage control address register is incremented and the form moves
until the channel specified is sensed in a line position in buffer
storage. If the requested channel number is not found in the buffer,
forms movement stops after address position 1 (line 1) has been sensed
twice. This prevents runaway forms skipping.
A flag in a buffer storage line position is used to indicate the
last line of the form for forms shorter than 180 lines. A flag bit
is also used in the first buffer storage position to indicate six or
eight lines per inch spacing. The FeB is loaded with the desired forms
spacing characters via a LOAD FCB command issued by a program. An
error indication is given if an end-of-page flag is not present or
if an invalid carriage code is loaded.
Serviceability features, in addition to those provided for the
1403 Printer, are incorporated into the design of the 3211. The fact
that a 3811 control unit controls only one 3211 Printer, instead of
multiple devices, permits offline repair of the malfunctioning printer
or control unit only, without the necessity of removing other
operational units from the system.
The 3811 control unit presents six bytes of sense information to
identify printer and control unit malfunctions instead of only one,
as is provided for the 1403. Certain errors (such as a parity check
in the print line buffer) that might be corrected by programmed retry
of the print operation are identified in the sense bytes, and carriage
motion is suppressed. This permits error recovery without operator
intervention if the retry is successful. The additional status data
presented can be stored for later analysis and should help speed the
diagnosis of hardware malfunctions.
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