Iso buffer serial to parallel converter

Posted on Feb 5, 2014

Using this circuit you will connect to a single parallel port electronic circuits requiring up to 96 digital output, optically insulated lines, 3 to 15V powered. The CD4094B consists of an 8-bit shift register and a 8-bit latch. Data is shifted serially through the shift register on the positive transition of the clock. The output of the last stag

Iso buffer serial to parallel converter
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e (QS) can be used to cascade several devices. The output of each stage of the shift register feeds a latch, which latches data on the negative edge of the strobe input. When strobe is high, data propagates through the latch to output gates. Total time required is about 50 us but using higher speed LPT limit values is set by optocouplers to about 20 us. You can connect to a single LPT up to six of these CD4094 chains using a common clock and strobe pin. Note that optocouplers are not high speed components. In following image, you can see a digital input signal to led pin (lower trace) and transistor output voltage: transition time is about 500 ns and maximum frequency clock is about 2 MHz. Using this simple cable there is a problem: LPT pin 2 and 3 should source current to light up to six led, one for each connector. A single LED require about 16 mA and some LPT can source no more then 20-30 mA. And if you use a portable PC current may be lower! So this connection work fine only using high current parallel port AND if you connect only a small number of chains. Otherwise you need an " active version " of cable. Maximum power supply current is about 300 mA but in normal use is 150 mA with six chains connected. If voltage power supply is greater the 10 V a suited heat sink is required for voltage regulator.

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