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IBM-AUSTRIA - PC-HW-Support 30 Aug 1999 |
IEEE 1284-1994 EPP Mode
IEEE 1284 EPP-Enhanced Parallel Port Mode
The Enhanced Parallel Port protocol was originally developed by Intel®, Xircom
and Zenith Data Systems, as a means to provide a high performance parallel port link that
would still be compatible with the standard parallel port. This protocol capability was
implemented by Intel® in the 386SL chipset (82360 I/O chip). This was prior to the
establishment of the IEEE 1284 committee and the associated standards work.
The EPP protocol offered many advantages to parallel port peripheral
manufactures and was quickly adopted by many as an optional data transfer method. A
loose association of around 80 interested manufacturers was formed to develop and
promote the EPP protocol. This association became the EPP Committee and was
instrumental in helping to get this protocol adopted as one of the IEEE 1284 advanced
modes.
Since EPP capable parallel ports were available prior to the release of the 1284
standard, there is a small deviation between the pre-1284 EPP ports and 1284 EPP
protocol. This will be made clearer later.
The EPP protocol provides four types of data transfer cycles:
- Data Write Cycle
- Data Read Cycle
- Address Write Cycle
- Address Read Cycle
Data cycles are intended to be used to transfer data between the host and the
peripheral. Address cycles may be used to pass address, channel, or command and control
information. These cycles can be viewed as just two different data cycles. The developer
may use and parse the address/data information in any method that makes sense for a
particular design. Table 1 describes the EPP signals and their associated SPP signals.
Table 1 -- EPP Signal Definitions
| SPP Signal |
EPP Signal
Name |
In/Out |
EPP Signal Description |
| nSTROBE |
nWRITE |
Out |
Active low.
Indicates a write operation High for a read cycle. |
| nAUTOFEED |
nDATASTB |
Out |
Active low. Indicates a Data_Read or Data_Write operation
is in process. |
| nSELECTIN |
nADDRSTB |
Out |
Active low. Indicates an Address_Read or Address_Write
operation is in process. |
| nINIT |
nRESET |
Out |
Active low. Peripheral reset. |
| nACK |
nINTR |
In |
Peripheral interrupt.
Used to generate an interrupt to the host. |
| BUSY |
nWAIT |
In |
Handshake signal. When low it indicates that is OK to start
a cycle (assert a strobe), when high it indicates that it is
OK to end the cycle (de-assert a strobe). |
| D[8:1] |
AD[8:1] |
Bi-Di |
Bi-directional address/data lines. |
| PE |
user
defined |
In |
Can be used differently by each peripheral |
| SELECT |
user
defined |
In |
Can be used differently by each peripheral. |
| nERROR |
user
defined |
In |
Can be used differently by each peripheral. |
Figure 1 is an example of a Data_Write cycle. The CPU signal nIOW is shown just to
emphasize that this entire handshake occurs within a single I/O cycle.
Data Write cycle phase transistions:
- Program executes an I/O write cycle to port 4 (EPP Data Port)
- The nWrite line is asserted and the data is output to the parallel port
- The data strobe is asserted, since nWAIT is asserted low
- The port waits for the acknowledge from the peripheral (nWAIT deasserted)
- The data strobe is deasserted and the EPP cycle ends
- The ISA I/O cycle ends
- nWAIT is asserted low to indicate that the next cycle may begin
One of the most important features to note here is that the entire data transfer
occurs within one ISA I/O cycle. The effect is that by using the EPP protocol for data
transfer, a system can achieve transfer rates from 500K to 2M bytes per second. In this
fashion, parallel port peripherals can operate at close to the same performance levels as an
equivalent ISA plug-in card. The ability to get this level of performance from a parallel
port attached device is one of the major features of the EPP protocol. With interlocked
handshakes, the data transfer will happen at the speed of the slowest of the interfaces, the
host adapter or the peripheral device. This 'speed adaptive' property is transparant to
both the host and peripheral. All 1284 transfer modes are implemented with interlocking
handshakes.
Interlocking refers to the criteria that each control signal transition is acknowledged
by the opposite side of the interface. In the above diagram, nDataStrobe can be asserted
because nWAIT is low, nWAIT deasserts in response to nDataStrobe be asserted,
nDataStrobe deasserts in response to nWAIT being deasserted, and finally nWAIT asserts
in response to nDataStrobe being deasserted. In this way the peripheral can control the
setup time required for its operation. This is done in the following manner: the setup time
is the time from the assertion of nDataStrobe to the deassertion of nWAIT, the peripherals
controls this time. Interlocking also has the advantage of making the transfer cycle
independent of the cable length. The Nibble, Byte, EPP and ECP modes all have
interlocked protocols.
As previously mentioned, the pre-1284 EPP devices deviated from the 1284
protocol. At the start of the cycle, the nDataStrobe or nAddrStrobe would assert
regardless of the state of the nWAIT signal. This means that the peripheral could not hold
off the start of a cycle by having nWAIT deasserted. This is sometimes referred to as EPP
1.7, in reference to a Xircom proposal version 1.7. This is the version that Intel®
implemented in the original 82360 I/O controller. A 1284 EPP compatible peripheral will
work properly with an EPP 1.7 version host adapter, but an EPP 1.7 peripheral may not
operate properly with a 1284 compliant host. Figure 2 is an example of an Address_Read cycle.
EPP Register Interface
The simplest software view of EPP is that of an extension to the register definitions
for the standard parallel port. As shown earlier, the SPP consists of three registers, offset
from the port's base address: Data port, Status port, and Control port. The most common
EPP implementations expand this to use ports not defined by the SPP. See table 2.
Table 2 -- EPP Register Definitions
| Port Name |
Offset |
Mode |
Read/Write |
Description |
| SPP Data Port |
+0 |
SPP/EPP |
W |
Standard SPP data port. No autostrobing. |
| SPP Status Port |
+1 |
SPP/EPP |
R |
Reads the input status lines on the interface. |
| SPP Control Port |
+2 |
SPP/EPP |
W |
Sets the state of the output control lines. |
| EPP Address Port |
+3 |
EPP |
R/W |
Generates an interlocked address read or write cycle. |
| EPP Data Port |
+4 |
EPP |
R/W |
Generates an interlocked data read or write cycle. |
| Not Defined |
+5 to +7 |
EPP |
N/A |
Used differently by various implementations.
May be used for 16 and 32 bit I/O. |
By generating a single I/O write instruction to 'base_address + 4', the EPP
controller will generate the necessary handshake signals and strobes to transfer the data
using an EPP Data_Write cycle. I/O instructions to the base addresses, ports 0 through 2,
will cause behavior exactly as that as to a standard parallel port. This guarantees
compatibility with standard parallel port peripherals and printers. Address cycles are
generated when read or write I/O operations are generated to 'base_address + 3'.
Ports 5 through 7 are used differently by various hardware implementations. These
may be used to implement 16 or 32-bit software interfaces, or used for configuration
registers, or not used at all. For example, the FarPoint Communications F/PortPlus card
has only an 8-bit data interface, but can be accessed using 32-bit I/O, for EPP data
operations. The ISA controller will intercept the 32- bit I/O and actually generate 4 fast 8-
bit I/O cycles. The first cycle will be to the addressed I/O port using byte 0 (bits 0-7), the
second cycle will be to port+1 using byte 1, then port+2 using byte 2 and finally port+3
using byte 3. These additional cycles are generated by hardware and are transparant to the
software. The total time for these four cycles will be less than 4 independent 8 bit cycles.
For example, the F/PortPlus card (from FarPoint Communications) maps 4 I/O ports
(offset 4 to 7) to the internal EPP Data register. This enables the software to use 32-bit
I/O operations for EPP data transfer. Address cycles are still limited to 8-bit I/O.
The ability to transfer data to or from the PC by the use of a single instruction is
what enables EPP mode parallel ports to transfer data at ISA bus speeds. Rather than
having the software implement an I/O intensive software loop, a block of data can be
transferred with a single REP_IO instruction. Depending upon the host adapter port
implementation and the capability of the peripheral, an EPP port can transfer data from
500K bytes to nearly 2M bytes per second. This data transfer rate is more than enough to
enable network adapters, CD ROM, tape backup and other peripherals to operate at nearly
ISA bus performance levels.
The EPP protocol and current implementations provide a high degree of coupling
between the peripheral driver and the peripheral. What this means is the software driver is
always able to determine and control the state of communication to the peripheral at any
given time. Intermixing of read and write operations as well as block transfers are readily
done. This type of coupling is ideal for many register-oriented or real-time controlled
peripherals such as network adapters, data acquisition, portable hard drives, and other
devices.
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