I used to use 4N28 optoisolators and a few resistors and diodes for 20 ma
current loop designs. This was a common type of interface back when the
old Teletype printers were being converted to all electronic printers.

In the 20ma receive portion, a resistor value is selected to draw the 20
mils, and the led portion of the opto across that resistor with another
selected resistor in series (or parallel depending on the current
created) to protect the led. The transistor portion of the isolator would
interface to your TTL circuit.

To transmit the 20 mils, drive the led in another 4N28 from your TTL
output. Select a value of resistor to create your 20 mils and put in
series with the transistor output of he opto. If you desire a non
polarized output, just short a diode bridge circuit with the opto
transistor.

Sure there's probably single chip that will do the same, but sometimes
using discretes are simpler and easier to troubleshoot and repair,
especially when static electricity gets ahold of the interface. It will
all depend on your quantity of production units.
Rick

Tony K=FCbek wrote:

> Hi,
> I have some designs for an 20mA current loop i/f that we use on some
> of our products, however these are designed for TTL i/f as they are
> internal.
> Now I need to make an external version, my questions are:
> Are there any designs available on the net ?
> Are there any 'driver chips' available ?
>
> I need to cater for both active and passive loops.

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