The parallel port consists of eight data lines, four control lines, five status lines, and eight ground lines. In normal usage, the lines are controlled by the host computer software and the peripheral device following a protocol such as IEEE Standard 1284 - 1994. The protocol defines procedures for transferring data such as handshaking, returning status information, and so on. The table shows the functions of each pin of the Parallel Port. more »
A low on this
line indicates a Write, High indicates a Read.
Line. Interrupt occurs on Positive (Rising) Edge.
handshaking. A EPP cycle can be started when low, and finished
in EPP Handshake
in EPP Handshake
indicates Data transfer
used in EPP Handshake
indicates Address transfer
Parallel Port is configured in bidirectional mode. As the parallel port operates on TTL logic levels while the FPGA follows the CMOS logic, level translation has to be performed. For this all the data lines and the associated control and status signals are sent through a circuit that is designed for this purpose.
The Parallel Port is operated in BiDirectional mode, and EPP is emulated in software. See Software implementation for more details.
An interfacing circuitry is needed to ensure proper communication between the computer and the FPGA. The computer parallel port works at TTL logic while the FPGA works on CMOS logic. As these two different families have to be interfaced for harmonious working, level transceivers are used.
As eight bits of data is sent at a time, an octal transceiver is used for the parallel communication. 74LS641 is one such octal transceiver that allows bi-directional asynchronous communication between the two devices.
The 74LS641 is an octal transceiver that is available as a 20 pin DIP IC. It operates on an optimal supply voltage of 5V. The IC has two data buses A and B thus there are eight channels. The data on the data buses move from either A to B or from B to A depending on the control exerted on the direction pin. Twelve signals were used between the computer and the FPGA. Of this, the eight data lines are bidirectional. The remaining four are unidirectional signals. The IC has a direction pin, which controls the direction of transfer.
The open collector configuration of the IC makes it ideal for two way communication. Pull up resistors were used to ensure that signals are pulled up to the appropriate values so that they are properly recognized and interpreted.
The maximum voltage on the parallel port pins is 5V while the highest voltage on the FPGA pins should not exceed 3.3V. As the Xilinx Spartan 3 series is powered by 3.3V, the pins of the transceiver that go to the FPGA are connected to a voltage of 3.3V through pull up resistors of 2.2kilo ohms. On the other hand, the signals from the FPGA have a maximum of 3.3V and hence the data bus on the IC that feeds the pins on the computer parallel port are pulled up to 5V. A 10 kilo ohm potentiometer is used to create a 3.3V needed for the signals on the FPGA side.
74LS641 has an open collector configuration, which means that the user can control the output voltage on the IC pins. Open collector circuits can be used to interface different families of devices that have different operating logic voltage levels.
According to the open collector configuration, the output essentially acts as either an open circuit (no connection to anything) or a short circuit to ground. This is then coupled to an external pull-up resistor, which sets a high voltage to the output when the transistor is open. When any transistor connected to this resistor is closed, the output is forced to 0 volts. So the pull-up resistor need not be connected to a voltage similar to that of the chip supply (Vcc); a lower or higher voltage can be used instead. Open collector circuits are therefore sometimes used to interface different families of devices that have different operating logic voltage levels.
IC1 – 74LS641
IC2 – 74LS641
IC3 – 74LS33
C1 – DB25 connector
R – 2.2K?
74LS641 is an octal transciever for converting 5 to 3.3v and back.
74LS33 is a 3.3 Voltage Regulator.
DB25 is the parallel port male connector to attach to the Parallel Port.
2.2k Resistors are pull up resistors.