EPP Details

EPP Handshaking
The most common use of the parallel port is to write data from a PC to a device. The EPP mode can provide support for reverse transfers of data or forward and reverse transfers of addresses. The handshaking for each of these varies only slightly so the focus in this section will be on a forward data write cycle.

When writing one byte from the PC to the peripheral, the host PC brings the nwrite signal low.
The PC then writes data to the data outputs and asserts ndstrobe. This is done only if the nwait signal is low.
The peripheral device brings nwait high to inform the host that it's ready to latch the data.
The host brings ndstrobe high and the data is latched by the peripheral.
When the peripheral is ready for another byte, it should bring nwait low. The nwrite signal will go high after nwait going low, and the write cycle would resume for the new byte.

A number of timing constraints for this mode could cause a transfer to time out if the contraints are exceeded. If the PC begins a write cycle and the perhipheral does not respond within 10 microseconds, a timeout bit (Status port bit 0) will be set. (The peripheral would respond by bringing nwait high). The method for clearing timeouts varies with the controller chip used by the PC.

For a PC to use EPP under Windows, a mode negotiation phase is necessary. This consisted of a series of handshaking signals where the PC and peripheral determine the mode that they will communicate. This was implemented in one version of the PPI module. Attempts to download under Windows using the default printer driver were not successful. This was the case even with mode negotiation implemented according to available references. After some investigation, the only document known to provide this information was the standard titled "1284-1994 IEEE Standard Signaling Method for a Bi-directional Parallel Peripheral Interface for Personal Computers." This document could only be purchased from the IEEE at a cost too high for the budget of this project.

Therefore, a device driver was written in C with the help of Dan Leder to download data to the PC. Since a PC running DOS could be set to run in EPP mode by the device driver, mode negotiation became unnecessary. Simply writing each byte of a file to the EPP data register would cause the appropriate handshaking signals to be generated by the parallel port controller.

Another problem encountered when attempting EPP transimissions was that excessive ringing was observed on the handshaking and data signals originating from the PC. This resulted in data transmissions that would not complete reliably. This problem was avoided at first by using counters to introduce delays to avoid the ringing. As well, all handshaking input signals were synchronized to the global clock in registers to reduce the chance of signal transitions violating setup and hold times.

The ultimate solution was to implement sampling at the clock frequency of control lines. Due to the fact that the clock had a period of ~40ns, and the parallel port interface operates at least an order of magnitude slower, the control lines were sampled and then once a transition detected, a delay was used to allow the ringing to subside.