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1.3. Registers - Standard Parallel Port (SPP) Table 4 Port Data
Note 1 : If the Port is Bi-Directional then Read and Write Operations can be performed on the Data Register. The base address, usually called the Data Port or Data Register is simply used for outputting data on the Parallel Port's data lines (Pins 2-9). This register is normally a write only port. If you read from the port, you should get the last byte sent. However if your port is bi-directional, you can receive data on this address. See Bi-directional Ports for more detail. Table 5. Port Status
The Status Port (base address + 1) is a read only port. Any data written to this port will be ignored. The Status Port is made up of 5 input lines (Pins 10,11,12,13 & 15), a IRQ status register and two reserved bits. Please note that Bit 7 (Busy) is a active low input. E.g. If bit 7 happens to show a logic 0, this means that there is +5v at pin 11. Likewise with Bit 2. (nIRQ) If this bit shows a '1' then an interrupt has not occurred. Table 6. Port Control
The Control Port (base address + 2) was intended as a write only port. When a printer is attached to the Parallel Port, four "controls" are used. These are Strobe, Auto Linefeed, Initialize and Select Printer, all of which are inverted except Initialize. The printer would not send a signal to initialize the computer, nor would it tell the computer to use auto linefeed. However these four outputs can also be used for inputs. If the computer has placed a pin high (e.g. +5v) and your device wanted to take it low, you would effectively short out the port, causing a conflict on that pin. Therefore these lines are "open collector" outputs (or open drain for CMOS devices). This means that it has two states. A low state (0v) and a high impedance state (open circuit). Normally the Printer Card will have internal pull-up resistors, but as you would expect, not all will. Some may just have open collector outputs, while others may even have normal totem pole outputs. In order to make your device work correctly on as many Printer Ports as possible, you can use an external resistor as well. Should you already have an internal resistor, then it will act in Parallel with it, or if you have Totem pole outputs, the resistor will act as a load. An external 4.7k resistor can be used to pull the pin high. I wouldn't use anything lower, just in case you do have an internal pull up resistor, as the external resistor would act in parallel giving effectively, a lower value pull up resistor. When in high impedance state the pin on the Parallel Port is high (+5v). When in this state, your external device can pull the pin low and have the control port change read a different value. This way the 4 pins of the Control Port can be used for bi-directional data transfer. However the Control Port must be set to xxxx0100 to be able to read data, that is all pins to be +5v at the port so that you can pull it down to GND (logic 0). Bits 4 & 5 are internal controls. Bit four will enable the IRQ (See Using the Parallel Ports IRQ) and Bit 5 will enable the bi-directional port meaning that you can input 8 bits using (DATA0-7). This mode is only possible if your card supports it. Bits 6 & 7 are reserved. Any writes to these two bits will be ignored. |
Free Software Lesson 1 Lesson
4
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