Xerox WorkCentre 385 Service Manual page 149

ENGINE CONTROLLER
• Message Communication
The print interface uses CnPMSG and CnEMSG
to transmit and receive 8-bit message, CnPBSY
and CnEBSY to indicate the direction of data
transfer and COMCLK to pace data transmissions.
PIFC does not employ handshaking, but asserts
CnPBSY and CnEBSY before the actual data
transmission to provide sufficient time for the logic
to prepare for the subsequent data.
COMCLK remains inactive until either CnPBSY or
CnEBSY is asserted and then goes through eight
periods for 8-bit data transmission or reception.
Three registers, TBR (Transmit Buffer Register),
RBR (Receive Buffer Register), are used for
message communication. The TBR and RBR
contain the 8-bit command to be transmitted to the
printer engine through the CnEMSG pin and the 8-
bit engine message received for the printer engine
through the Cnemsg pin, respectively. The CMOD
contain a transmit enable bit (TX) to make
CnPSBY signal active, a read-only status bit (RX)
to indicate the Cnebsy signal status and 5-bit
prescaler value used to generate COMCLK clock.
In message reception, the RX bit is cleared when
a low-to-high transition occurs on CnEBSY, and at
the meantime an interrupt signal INT_BUSY is
posted to indicate that one-byte engine message
has been received by PIFC.
WorkCentre 385
COMCLK
CnPBSY
Printer
CnPMSG
Interface
Controller
CnEBSY
CnEMSG
12/98
www.printcopy.info
7 - 15
Laser
Printer
Engine
Circuit Description