Dictionary of Electronic Terms
"A" "B" "C", "D", "E", "F", "G", "H", "I", "J", "K", "L", "M",
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TTL Bus Transceiver ICs

74HC245 8-bit Bus Transceiver

A common 8 bit bus transceiver circuit package is shown above as a 54HC245; however TTL xx245 part will appear the same. For example a 74HC245 would have the same functional schematic, as would a 74HCT245, 74F245 and so on. The internal schematic would be changing but not the functional layout.

The quad 74HC243 shown below appears about the same; however the 54HC243 is only a 4-bit transceiver and the directional controls are a bit different. Note that a 54HC242 would look identical except that buffers would also be inverters.

74HC243 4-bit Bus Transceiver

A common bus transceiver truth table is shown below, but it does relate to a 54HC245 IC.

74HC245 8-bit Bus Transceiver Truth Table
54HC245 Truth Table

TTL Different Transceivers

54HC242; Quadruple inverting bus transceiver with 3-state outputs
54HC243; Quadruple non-inverting bus transceiver, 3-state outputs
54HC245; Octal non-inverting bus transceiver with 3-state outputs
54HC620 8-bit 3-state inverting bus transceiver
54HC623 Octal Inverting bus transceiver w/tristate output
54HC640; Octal inverting bus transceiver with three-state outputs
54HC643; Octal non-inverting or inverting bus transceiver with three-state outputs
54HC646; Octal non-inverting bus transceiver with 3-state outputs
54HC648; Octal inverting bus transceiver with three-state outputs
54HCT245; Octal non-inverting bus transceiver with three-state outputs and TTL-input voltage compatibility

These devices primarily have 3-state outputs which means that some place along the output path a Pull-up Resistor will be required. However a few of the devices are standard Totem-Pole Output devices.

The 54HCxx series parts are shown, but they are the same as the 74HCxx parts except for the temperature range. The 54HCxxx parts work to the military temperature range; Operating temperature range (TA) -55C to +125C

74HC646 8-bit Bus Transceiver schematic
54HC646 Functional Schematic

For additional internal IC schematics, also see the pages on;
Glue Logic IC Functions

Design Hint; always use a buffer or driver when driving signals off-card, and never drive signals going off-board with a controller or FPGA for example. The design risk of not using a IC driver is that noise is pushed into the controller IC which then effects all the I/O or functions of the controller and not just the signals having the issue.

At least when your using a buffer or driver to interface off-board there's a buffer between the controller and what ever happens at the board interface. The term buffer in this case refers both to the physical IC and some distance between the electrical disturbance at the board edge and the controller.

Other components that should be placed near the physical edge of the board are surge or transient protection, that way any surge is grounded as soon as it enters the card with out shorting out any copper traces or damaging any ICs.

PC motherboard

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