Technical Reference Guide www.hp.com 5-11
Input/Output Interfaces
5.5 Parallel Interface
Systems covered in this guide may include a parallel interface for connection to a peripheral
device with a compatible interface, the most common being a printer. The parallel interface
function is integrated into the SCH5317 I/O controller component and provides bi-directional
8-bit parallel data transfers with a peripheral device. The parallel interface supports three main
modes of operation:
■ Standard Parallel Port (SPP) mode
■ Enhanced Parallel Port (EPP) mode
■ Extended Capabilities Port (ECP) mode
These three modes (and their submodes) provide complete support as specified for an IEEE 1284
parallel port.
5.5.1 Standard Parallel Port Mode
The Standard Parallel Port (SPP) mode uses software-based protocol and includes two
sub-modes of operation, compatible and extended, both of which can provide data transfers up to
150 KB/s. In the compatible mode, CPU write data is simply presented on the eight data lines. A
CPU read of the parallel port yields the last data byte that was written.
The following steps define the standard procedure for communicating with a printing device:
1. The system checks the Printer Status register. If the Busy, Paper Out, or Printer Fault signals
are indicated as being active, the system either waits for a status change or generates an error
message.
2. The system sends a byte of data to the Printer Data register, then pulses the printer STROBE
signal (through the Printer Control register) for at least 500 ns.
3. The system then monitors the Printer Status register for acknowledgment of the data byte
before sending the next byte.
In extended mode, a direction control bit (CTR 37Ah, bit <5>) controls the latching of output
data while allowing a CPU read to fetch data present on the data lines, thereby providing
bi-directional parallel transfers to occur.
The SPP mode uses three registers for operation: the Data register (DTR), the Status register
(STR) and the Control register (CTR). Address decoding in SPP mode includes address lines A0
and A1.
5.5.2 Enhanced Parallel Port Mode
In Enhanced Parallel Port (EPP) mode, increased data transfers are possible (up to 2 MB/s) due
to a hardware protocol that provides automatic address and strobe generation. EPP revisions 1.7
and 1.9 are both supported. For the parallel interface to be initialized for EPP mode, a negotiation
phase is entered to detect whether or not the connected peripheral is compatible with EPP mode.
If compatible, then EPP mode can be used. In EPP mode, system timing is closely coupled to
EPP timing. A watchdog timer is used to prevent system lockup.
Five additional registers are available in EPP mode to handle 16- and 32-bit CPU accesses with
the parallel interface. Address decoding includes address lines A0, A1, and A2.
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