A serial to parallel converter

In this page you will find the description of a serial to parallel converter designed to connect a large number of digital output to a PC parallel port.

This isn't an "autonomous" circuit but should be used with other circuit, as a Micro-step driver or Unipolar step driver.

The problem

Often is required to connect a digital circuit to a PC; no problem if ten digital output are enough, no isolation is required, circuits are 5V powered...

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.

Drawback is a relative high time to update... Using high speed LPT and high speed opto-isolators throughput will be up to 500 kbyte/s. 

The idea

This design use CD4094B, a simple serial to parallel converter.

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 stage (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.

Three optocouplers are used for electrical insulation.

A simple schematic for iso-buffer

... and a real circuit.

Here a timing diagram, where you can see from top to bottom:

1. sixteen clock front
2. the data line, with some one and some zero 
3. the single strobe

Signals are inverted because optocouplers acts as inverter, see data sheet for more details.

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.

 

The software

You can write your-own code to generate timing using as reference data sheet and timing diagram in this page. 

Or you can use functions in VVIO project library.

A passive cable

To connect iso-buffer to LPT, you need a cable. You can see the simplest version in following schematic:

As sample, for 5x2 connector on the right, connections to do are:

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.

In following image, a prototype for a dual-chains connection: I think it work with almost all desktop parallel port.

Iso-Buffer

Using active cable you can:

• Use portable PC
• Use up to six chains
• Build safer circuits

As sample, for 5x2 connector on the right, connections are:

* trough inverter

Note: there is a double inversion so no software modification is required.

isobuffer.tgz file contains schematic and PCB required to connect up to six chains to a single LPT.

Components

C1	470uF - 16 Vl - Electrolytic capacitor
C2	4.7uF - Electrolytic capacitor
C3 C4 C5	220 nF - Capacitor
D1D2	1n4004 - 1A diode
D3	Led
JP1JP2 JP3 JP4JP5 JP6	Connector 5x2 pin, step 2.54mm
JP7	Connector 2 pin, step 5mm
P1	Connector DB25 male for printed board
R1	2K2 - 1/4W
U1 U2 U3	74F04 - Inverter
U4	7805 - Voltage regulator

Note

• Circuit is single layer but requires a lot of jumper...
• Power supply can vary form 8 to 15V and, for safety reasons, should be separated from motor power supply.
• 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. 
• You can use, with minor drawback three 7404 or 74LS04 in place of 74F04.
• If you use high speed LPT (as PCI Sunnix1888) optocouplers should be too slow: you can use SlowDown() function in VVIO library to slow down I/O

isobuffer.tgz contains:

licence.txt	License
schematic.pdf	Circuit schematic, in pdf format
bottom.pdf	Solder pcb side, in pdf format
assembly.pdf	Components layout, in pdf format

To build this circuit you need:

• This page !
• isobuffer.tgz
• VVIO - a library for I/O
• Components data sheets
• A circuit to connect to ISO-Buffer (see list at the top)

---

Iso-Buffer - Version 1.0 - August 2003
Copyright 2003, VincenzoV.net.

You need to read the license before to use this product

All trademark mentioned herein are property of their respective companies

You will find the original copy of this document on http://www.vincenzov.net.