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SN74HCT245 is a OCTAL BUS TRANSCEIVERS WITH 3-STATE OUTPUTS.

But i am not getting why we need it?

It basically transfers signal from port A to B or vice versa or isolate it.

Where we use it? Is it used as EPROM?

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    \$\begingroup\$ It's used wherever you need transmission or isolation between two buses. \$\endgroup\$ Commented Nov 23, 2014 at 7:12
  • \$\begingroup\$ Remember that there's an ENABLE input that will switch off the chip and isolate the two sides of it. That's very often the useful feature. \$\endgroup\$ Commented Nov 23, 2014 at 11:20

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The 74HCT245's obvious purpose is to achieve bidirectional transfer of bytes between two buses, but the TTL chip it is based on was designed to do more than that. The 74LS245 had high current outputs that could sink 25mA, and its inputs had hysteresis to clean up noisy bus signals. It was often used for connecting weak devices to a heavily loaded bus.

Here is an extreme example of where it was needed. The Texas Instruments T-99/4A home computer had a side expansion bus which peripherals could be connected to in 'daisy-chain' fashion. As more devices were plugged in the bus got longer, eventually reaching a length of several feet! This was a single bus, but without transceivers in each box it would not have worked reliably if at all.

Ti sidecar expansion

The partial circuit diagram is below is from the Ti-99/4A floppy disk drive controller. An LS245 is connected between the data bus and the TMS4732 ROM (and WD1771 controller chip, not shown). You can also see some of the address lines going through an LS244 octal 3-state line driver, which is permanently enabled to act as a buffer.

Ti FDC

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  • \$\begingroup\$ +1 for the TI reference/picture alone. My first computer and I have like 5 of them still. Love me some TI. :-) \$\endgroup\$
    – cbmeeks
    Commented May 15, 2015 at 19:22

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