Checking The Bd Pulse - Canon LBP-CX Series Service Manual

a. Introduction
The multiplexer and error detection circuitry are
explained in this section.
When the operating program in ROM tells the
microprocessor to read a certain signal (both the
ROM program and the microprocessor are in the
microcomputer chip), a select code (MPXCD 1-3) is
input to the multiplexer (IC215, IC219), which acts
as an electronic switch by connecting one of its 8
inputs to pin 13 and 14 of the microcomputer chip.
It is not necessary for the microcomputer to check
these input signals continually, so each one is switch-
ed through the multiplexer only when necessary.
The microprocessor checks the input signals
received from the multiplexers (IC215, IC219)
for the prescribed voltage level, and also to make
sure that the prescribed voltage level is reached
within a set time limit.
If this check detects a malfunction, the micro-
processor sets the READY signal (RDY) FALSE to
inform the external device that malfunction has
occurred. In response to this, the external device
transmits a status requestcommand(via the STATUS/
COMMAND signal (SC) line) to the DC controller.
Upon receipt of this command, the DC controller
encodes the nature of the malfunction (as status
information) in the
SC
signal, and turns off READY/
WAIT display on the display panel sends it to the
external device.
b. Checking the BO pulse
Each time the reflected laser beam reaches home
position (the stationary mirror), it lights a beam
position sensor (optic fiber). The optic fiber transmits
the laser beam to the photodiode at the H. sync
generator, which generates an INTERNAL BEAM
DETECT pulse (IBD). Accordingly, if the scanning
mirror is rotating constantly at the correct speed,
IBD pulses are generated at the correct frequency.
All the frequency detection circuits for IBD
pulses are contained in one custom IC. The custom IC
is a collection of integrated logic circuits users may
need.
From
H. syn
IBD
the
c generator
40
From t
microc
(IC208
he
VMSK
10
omputer
-62)
I
I
38
osc
I
CLOCK
IC210
IC213
~
E
0
...
"'
::J
(.)
BORDY
15
BOERA
16
j
to the
multiplexer
(IC215)
Figure 3-35 Frequency detection circuit for IBD
pulses
A block diagram of the frequency detection
circuit is shown in Figure 3-35. The circuit in Figure
3-3 5 checks whether the IBD frequency is correct.
If correct, the circuit generates a TRUE BEAM
DETECT READY signal (BDRDY); if not, a FALSE
BDRDY.
If VMSK and BDRDY go FALSE during print-
ing, custom IC213 outputs a TRUE BEAM DETECT
ERROR signal (TRUE BDERR). Microprocessor
IC208 receives the TRUE BDERR signal through
multiplexer IC215, and sets the READY signal
(RDY) FALSE. At the same time, the microproces-
sor uses the STATUS/COMMAND signal (SC) to send
a PRINT CHECK REQUEST to the external device.
When RDY is FALSE, the external device will
receive status information from the DC controller
via the SC signal. The external device then knows
that the DC controller
is
in a PRINT CHECK
REQUEST state.
Additionally, if BDRDY stays FALSE for at least
one second, the external device interprets (via the SC
signal) this to mean that a malfunction has occurred
in the BD generator on the DC controller. The
READY
fW
AIT indicator goes out.
When the READY/WAIT indicator goes out,
the printer stops printing, goes through a LAST
ROTATION cycle, then halts. Further printing
operations are inhibited.
BDERR from custom IC213 becomes FALSE
after one page has been printed. The BD error status
(where BDERR is TRUE) is maintained by micro-
computer IC208, and is not released unless the
printer is switched OFF, then ON again, or until the
external device sends a DATA RETRANSMISSION
REQUEST RELEASE command to the DC control-
ler via the STATUS/COMMAND (SC) signal line.
The timing chart in Figure 3-36 shows normal
BD generation.
3-32
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©
BY CANON INC. CANON LBP-CX SERIES RBl.2 JULY 1985 PRlllTm Ill JAPAN (IMPRIME AU JAPOll)