Developing new drivers

The ppbus system has been designed to support the development of standard and non-standard software:

Driver	Description It uses standard and non-standard parallel port accesses.
ppi	Parallel port interface for general I/O
pps	Pulse per second Timing Interface

Porting existing drivers

Another approach to the ppbus system is to port existing drivers. Various drivers have already been ported:

Driver	Description
lpt	lpt printer driver
lp	plip network interface driver

ppbus should let you port any other software even from other operating systems that provide similar services.

Description

The parallel port may operate in the following modes:

• Compatible (Centronics -- the standard parallel port mode) mode, output byte
• Nibble mode, input 4-bits
• Byte (PS/2) mode, input byte
• Extended Capability Port (ECP) mode, bidirectional byte
• Enhanced Parallel Port (EPP) mode, bidirectional byte

Compatible mode

This mode defines the protocol used by most PCs to transfer data to a printer. In this mode, data is placed on the port's data lines, the printer status is checked for no errors and that it is not busy, and then a data Strobe is generated by the software to clock the data to the printer.

Many I/O controllers have implemented a mode that uses a FIFO buffer to transfer data with the Compatibility mode protocol. This mode is referred to as “Fast Centronics” or “Parallel Port FIFO mode”.

Nibble mode

The Nibble mode is the most common way to get reverse channel data from a printer or peripheral. When combined with the standard host to printer mode, a bidirectional data channel is created. Inputs are accomplished by reading 4 of the 8 bits of the status register.

Byte mode

In this mode, the data register is used either for outputs and inputs. All transfers are 8-bits long. Channel direction must be negotiated when doing IEEE 1248 compliant operations.

Extended Capability Port mode

The ECP protocol was proposed as an advanced mode for communication with printer and scanner type peripherals. Like the EPP protocol, ECP mode provides for a high performance bidirectional communication path between the host adapter and the peripheral.

ECP protocol features include:

• Run_Length_Encoding (RLE) data compression for host adapters (not supported in these drivers)
• FIFO's for both the forward and reverse channels
• DMA or programmed I/O for the host register interface.

Enhanced Parallel Port mode

The EPP protocol was originally developed as a means to provide a high performance parallel port link that would still be compatible with the standard parallel port.

The EPP mode has two types of cycle: address and data. What makes the difference at hardware level is the strobe of the byte placed on the data lines. Data are strobed with nAutofeed, addresses are strobed with nSelectin signals.

A particularity of the ISA implementation of the EPP protocol is that an EPP cycle fits in an ISA cycle. In this fashion, parallel port peripherals can operate at close to the same performance levels as an equivalent ISA plug-in card.

At software level, you may implement the protocol you wish, using data and address cycles as you want. This is for the IEEE 1284 compatible part. Peripheral vendors may implement protocol handshake with the following status lines: PError, nFault and Select. Try to know how these lines toggle with your peripheral, allowing the peripheral to request more data, stop the transfer and so on.

At any time, the peripheral may interrupt the host with the nAck signal without disturbing the current transfer.

Mixed modes

Some manufacturers, like SMC, have implemented chip sets that support mixed modes. With such chip sets, mode switching is available at any time by accessing the extended control register. All ECP-capable chip sets can switch between standard, byte, fast centronics, and ECP modes. Some ECP chip sets also support switching to EPP mode.

Background

This standard is also named “IEEE Standard Signaling Method for a Bidirectional Parallel Peripheral Interface for Personal Computers”. It defines a signaling method for asynchronous, fully interlocked, bidirectional parallel communications between hosts and printers or other peripherals. It also specifies a format for a peripheral identification string and a method of returning this string to the host.

This standard is architecture independent and only specifies dialog handshake at signal level. One should refer to architecture specific documentation in order to manipulate machine dependent registers, mapped memory or other methods to control these signals.

The IEEE 1284 protocol is fully oriented with all supported parallel port modes. The computer acts as master and the peripheral as slave.

Any transfer is defined as a finite state automate. It allows software to properly manage the fully interlocked scheme of the signaling method. The compatible mode is supported “as is” without any negotiation because it is the default, backward-compatible transfer mode. Any other mode must be firstly negotiated by the host to check it is supported by the peripheral, then to enter one of the forward idle states.

At any time, the slave may want to send data to the host. The host must negotiate to permit the peripheral to complete the transfer. Interrupt lines may be dedicated to the requesting signals to prevent time consuming polling methods.

If the host accepts the transfer, it must firstly negotiate the reverse mode and then start the transfer. At any time during reverse transfer, the host may terminate the transfer or the slave may drive wires to signal that no more data is available.

Implementation

IEEE 1284 Standard support has been implemented at the top of the ppbus system as a set of procedures that perform high level functions like negotiation, termination, transfer in any mode without bothering you with low level characteristics of the standard.

IEEE 1284 interacts with the ppbus system as little as possible. That means you still have to request the ppbus when you want to access it, and of course, release it when finished.

Chip set, ppbus and device layers

First, there is the chip set layer, the lowest of the ppbus system. It provides chip set abstraction through a set of low level functions that maps the logical model to the underlying hardware.

Secondly, there is the ppbus layer that provides functions to:

1. share the parallel port bus among the daisy-chain like connected devices
2. manage devices linked to ppbus
3. propose an arch-independent interface to access the hardware layer.

Finally, the device layer represents the traditional device drivers such as lpt(4) which now use an abstraction instead of real hardware.

Parallel port mode management

Operating modes are differentiated at various ppbus system layers. There is a difference between a capability and a mode. A chip set may have a combination of capabilities, but at any one time the ppbus system operates in a single mode.

Nibble mode is a virtual mode: the actual chip set would be in standard mode and the driver would change its behavior to drive the right lines on the parallel port.

Each child device of ppbus must set its operating mode and other parameters whenever it requests and gets access to its parent ppbus.

The boot process

ppbus attachment tries to detect any PnP parallel peripheral (according to Plug and Play Parallel Port Devices draft from (c)1993-4 Microsoft Corporation) then probes and attaches known device drivers.

During probe, device drivers should request the ppbus and try to determine if the right capabilities are present in the system.

Bus request and interrupts

ppbus reservation via a bus request is mandatory not to corrupt I/O of other devices. For example, when the lpt(4) device is opened, the bus will be “allocated” to the device driver and attempts to reserve the bus for another device will fail until the lpt(4) driver releases the bus.

Child devices can also register interrupt handlers to be called when a hardware interrupt occurs. In order to attach a handler, drivers must own the bus. Drivers should have interrupt handlers that check to see if the device still owns the bus when they are called and/or ensure that these handlers are removed whenever the device does not own the bus.

Micro-sequences

Micro-sequences are a general purpose mechanism to allow fast low-level manipulation of the parallel port. Micro-sequences may be used to do either standard (in IEEE 1284 modes) or non-standard transfers. The philosophy of micro-sequences is to avoid the overhead of the ppbus layer for a sequence of operations and do most of the job at the chip set level.

A micro-sequence is an array of opcodes and parameters. Each opcode codes an operation (opcodes are described in microseq(9)). Standard I/O operations are implemented at ppbus level whereas basic I/O operations and microseq language are coded at adapter level for efficiency.

GPIO interface

Pins 1 through 17 of the parallel port can be controlled through the gpio(4) interface, pins 18 through 25 are hardwired to ground. Pins 10 through 13 and pin 15 are input pins, the others are output pins. Some of the pins are inverted by the hardware, the values read or written are adjusted accordingly. Note that the gpio(4) interface starts at 0 when numbering pins.