AR-PB2A (serv.man4). Cougar Connectivity Training Pre-course study guide (Glossary of Terms) - Sharp Copying Equipment Handy Guide. Page 11

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Parallel Communication
In Parallel communication, eight bits of data are simultaneously sent from a PC to an external device
such as a printer. There are five current parallel communication modes available within the market. 
Each of these are briefly explained below.
The AR5132 is capable of communicating in Compatibility Mode or Nibble Mode.  For the copier
to communicate in Nibble Mode, an IEEE 1284 compatible Centronics cable must be used, and the
PC must also be capable of communicating in this the mode.  If Nibble Mode cannot be used, both
the PC and the AR5132 will communicate in Compatibility Mode.
Compatibility Mode
One way direction only.  Machines with unlimited speed or an unlimited internal buffer
would work with this mode.  The PC will download data as quick as it can.  To control data
flow, a number of feedback signals are used to tell if data is pilling up, paper empty, printer
on etc.
Nibble Mode
Asymmetric two way data so that the printer can talk back. (Four bits for feedback).  Data
flows faster to printer than back to PC.  This is because the flow back to the PC is in four
bits.  Normally five signals are used for feedback back to PC.  In nibble mode four of these
are re-assigned so that the printer can feed the bi-directional data back.  In Nibble Mode,
the PC takes full control of the port.  This is the least attractive Bi-directional method.
Byte Mode
Full symmetrical two-way communication. (Eight bits used for feedback).  Special hardware
is required.  All eight data lines have in line buffers so that as soon as data is sent one way,
all eight lines can then be used for the reverse direction. A lot quicker than nibble mode.
Enhanced Parallel Port (EPP)
IBM, Xircom & Zenith Data Systems developed in 1991.
32 bit words are sent at once and then are converted / repackaged into 4 x 8bit (1 byte)
transfers.  Six signals are used for data control.  EPP makes the port become a true
expansion bus. Speed is increased up to 10 times faster than the previous three modes.
Extended Capabilities Port (ECP)
Hewlett Packard & Microsoft developed a universal expansion standard and ECP.  First
release Nov 1992.  As previous modes however ECP gives control as well as speed.  To
the PC transfers are moderated by closed loop handshaking.  Two extra additions over EPP
are: Fast two way communication,   Two way mode with data compression
ECP can determine what speed both devices can speak and then choose the best.  ECP
includes error detection and device handshaking. All bytes will get to their destination even if
the connection is temporarily disrupted.
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