CS-480A, Lab # 3 Simple I/O Ports 9-25-02 and 10-3-02 Due Date: 10-10-02 In this lab you will learn how to interface simple I/O ports to the 8086 microprocessor. Specifically you will design simple 4-bit input and output ports for the SDK-86. These will be implemented on a prototyping board that is connected via ribbon cables to 8086 signal lines on the SDK-86. Once you have set up your I/O ports, you will connect the input port to four dip switches, and the output port to four Light Emitting Diodes (LEDs). You will write software that will continually read the state of the DIP switches and turn on the LEDs corresponding to the switches that are in the "on" position. The various 8086 bus signals are available on connectors J1, J2, and J3 on the SDK-86. These signals may be brought out to a prototyping board using ribbon cables. All the existing I/O addresses decoded and used by the SDK-86 board require that the address bits A5-A10 be high. Therefore, for our port address decoding it is sufficient to use address 00h for both the input and output ports and require a single address bit (say A7) be low. (This simple decoding scheme means that our I/O ports will respond not only to address 00h but to any I/O address in which A7 is 0. The RD* and WR* signals from the microprocessor may be used to distinguish input bus cycles from output bus cycles, and the MEM/IO* signal can be used to differentiate I/O bus cycles from memory bus cycles. These signals can be combined suitably using NAND/NOR gates to select the input or output port. You will need to devise the decoding circuits that do that selection. Figure 1 shows the pin numbers corresponding to each 8086 bus signal on SDK-86 connectors J1, J2, and J3, and what pin numbers they are mapped to on the prototyping board connectors P1, P2, and P3. Figure 1. SDK-86 Breadboard Connectors A 4-bit tristate buffer (e.g., the 74126 quad tristate buffer) can be used as the input port. On the SDK-86 prototyping board wire up an 8-pin set of DIP switches as shown in Figure 2. You will only use four of the eight switches for this experiment. The enable signals to the buffer will come from the circuit you devised to select the input port in response to a port 0 input action. Be sure to conect +5 V and ground leads from P2 on the SDK-86 to the 74126 tristate buffer. Pinouts for the 74126 are given in Figure 3. Figure 3. Pinouts for the 74126 tristate buffer A 4-bit latch (e.g., the 7475 quad transparent D Latch) can be used as the output port. In a different area of the breadboard, wire up four LEDs to the 7475 latch as shown in Figure 4. If your packet does not contain a 7407 hex buffer, you should use a 7417, which is equivalent. Be sure that you get the polarity of the LEDs right. (The flat side of the LED with the shorter of the two wires is the cathode--the side that corresponds to the vertical line and NOT the triangle on the LED schematic symbol.) This is very important, since if you wire the LEDs backwards, they could literally blow up on you!! The E12 and E23 signals to the latch will come from the circuit you devised to select the output port in response to a port 0 output action. Be sure to conect +5 V and ground leads from P2 on the prototype board to the 7475 latch. Pinouts for the 7475 are given in Figure 5 and those of the 7407/17 in Figure 6. Figure 5. Pinouts for the 7475 Quad D Latch Figure 6. Pinouts for the 7407 Quad Hex Buffer The software 1. Write a program that will turn off all of the LEDs, and then continually read the switches and send its bit pattern to the output port, thereby displaying it on the LEDs. Note from the circuit diagrams that an input bit will be low when a switch is closed, and that an LED will be lit when its bit from the output port is also low. If things are working properly, every time a switch is changed, the corresponding LED should reflect the new state of the switch. 2. Write another program that does different things when different switches are closed. Specifically: if the switch that's connected to D0 is closed, all the LEDs should be turned off; if the one connected to D1 is closed, all the LEDs should be turned on; if the one connected to D2 is closed, the LEDs should simultaneously blink on and off; if the one connected to D3 is closed, the D0 LED should be lit and turned off, then the D1 LED, then the D2, LED, then the D3 LED. This action should continue until a different switch is closed. For this part you may assume that only one switch at a time is closed. Demonstrate to the lab instructor that everything is working correctly. Lab Report Your lab report should contain the detailed circuit diagrams of your I/O ports and decoding logic as well as an assembled listing (.LST) of your program.