RAM module build - part 1

Summary

In this instructional video, the presenter constructs a computer memory using two 74LS189 SRAM chips, which provide 16, 4-bit words. To achieve 16 bytes, two chips are necessary. The script explains how to connect the chips, inverters, and a 74LS245 tri-state buffer to ensure proper data output. The presenter demonstrates hooking up power, connecting outputs to inverters for correct data inversion, and using LEDs to visualize memory data. Address lines are connected to enable chip selection, and a write enable signal is used to program the memory. The video concludes with a test of the memory setup, showing successful data writing and reading.

Takeaways

• 💾 The video discusses the use of 74LS189 SRAM chips for computer memory, requiring two chips to achieve 16 bytes of storage.
• 🔁 The output data from the 74LS189 is in complement form, necessitating the use of 74LS04 inverters to correct the output.
• 🔌 Power connections are made to the chips, with 5 volts and ground identified for proper setup.
• 🔗 The outputs of the 74LS189 chips are connected to the inverters to re-invert the data, ensuring accurate output.
• 💡 LEDs are used to visually monitor the outputs of the inverters, providing a means to see the memory contents.
• 🛠 The 74LS245 is introduced as an 8-bit tri-state buffer to control data flow to the bus, with the direction pin set for output.
• 🔑 Address lines (A0-A3) are connected across both memory chips to enable simultaneous addressing.
• ✏️ A write enable signal is connected to both chips, allowing data to be written into memory when the signal is active low.
• 🔍 The video demonstrates testing the memory by writing and reading data at different addresses, verifying the memory's functionality.
• 🔧 Future videos will cover connecting address and data lines to registers and using DIP switches for computer programming.

Q & A

What is the purpose of the 74LS189 SRAM chip in the computer memory?

The 74LS189 SRAM chip is used for memory in the computer, providing 16, 4-bit words of storage. Two of these chips are needed to achieve 16 bytes or 16 8-bit words of memory.

Why are inverters used with the 74LS189 SRAM chip?

The output data from the 74LS189 is the complement of the stored data, which means it needs to be inverted to match the stored values. Inverters are used to re-invert the outputs so that the correct data is obtained.

How many inverters are used from each 74LS04 chip, and what is the total number of inverters available on one chip?

Each 74LS04 chip has 6 inverters, but only 4 of them are used from each chip. This means a total of 8 inverters are used from two 74LS04 chips.

What is the function of the 74LS245 in the memory setup described in the script?

The 74LS245 is an 8-bit tri-state buffer used to control whether the output of the memory chips is actually going out on the data bus. It is set to always transfer data from the bottom 8 pins to the top 8 pins.

How are the address lines connected between the two 74LS189 chips?

The address lines A0, A1, A2, and A3 are tied together between the two 74LS189 chips so that both chips are addressed simultaneously, with one chip handling the high 4 bits and the other the low 4 bits.

What is the purpose of the chip select pin on the 74LS189, and how is it connected?

The chip select pin on the 74LS189 is active low and is used to enable or disable the chip. In the setup, it is connected to ground to always keep the chips enabled.

Why do the outputs of the memory chips appear as random data when powered up?

When the memory chips are powered up, they contain random data, often referred to as 'garbage,' because they have not yet been written to. Writing to the memory locations will then set the data stored in those locations.

What is the write enable signal, and how does it affect the memory chips?

The write enable signal is an active low signal that, when low, allows data to be written into the memory chips. When high, it prevents writing to the chips.

How are the data inputs connected to the memory chips, and why is this order important?

The data inputs are connected directly to the memory chips, and it is crucial to maintain the correct order to ensure that the bits are written and read in the correct sequence. Incorrect ordering can lead to data errors.

What is the next step in the computer memory setup after the memory chips are tested?

The next step is to hook up the address lines to an address register and the data lines to a data input, which will be controlled by DIP switches or other means for programming the computer.