Re: TTL to HTL signal conversion: How to

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Lars Levin"
<lars.levin@s...> wrote:

> Hello Group!
>
> I have a couple of old servodrives with step/direction
> inputs, but they use HTL signals, logic high is 24v
> instead of 5v.
> I want to be able to connect these to the parallell port,
> and therefor need some converter.
> Is there anybody that have some schematics or ideas for
> a preferably opto isolated 5v to 24v and 24v to 5v converter.
>
> Best Regards!
> Lars Levin

The opto part is easy. Find an opto that operates in the 10 ma on
the emitter side. This is basically turning on an LED with current
from the PC. The series resistor with the LED should let the
suggested current from the spec sheet flow through the LED when the
applied voltage is 5V. Using 5V from the PC or from a separate +5
with it's ground common to the PC ground, connect the +5 to the anode
side of the opto. One side of the series resistor connects to the
cathode of the opto and the other end to the pin on the parallel port
of the PC for that signal. This is an "active low" configuration so
that a low on the pin turns on the emitter LED.

Now that you have that side working, the other side should be an npn
transistor. Tie the emmiter to the logic ground on the servodrive.
tie the collector to +24VDC that is ground referenced to the
servodrive logic ground through a resistor who's value will limit the
current to about 70% of the max rating of the opto's transisor (from
the spec sheet. This will "pull up " the output of the collector to
+24. At this point you should have a nice clean inverted signal on
the collector of the opto NPN that swings from ground to +24. You
will need a scope or LED with a 2K series resistor (poor man's logic
probe) to see the signal. Use slow pulse settings to test.

You need one opto for each signal (Step and Direction). Several
mfg's make dual and quad opto's in 14Pin DIPS.

The other approach (without isolation) is to find a driver IC like
the ULN2803 with open collector outputs and logic inputs. The 2803
will take TTL/CMOS inputs and 24VDC on the outputs and drive up to
500ma per transistor. There are 8 inverters in each package. YOu
will need to have pull-up resistors on each output to +24DC since
they are open collector. One of the SIP resistor packs where 8
resistors of the same value are tied to a common point work really
good for pull-ups.

You might want to check with the Sound Logic guy (he monitors this
list) and see if his breakout card which has the interface IC's like
the 2803 can be modified to work with the +24VDC.