parallel port relay control

[[email protected]]

I am trying to control a relay from a computer's parallel port using
the circuit labeled Fig B from this webpage
http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm. The
circuit works fine and I am able to control it with my software. But, I
have no control while the computer is booting, where the the data lines
can be set high or low. I need the relay to remain off until my
program is running. I have read a suggestion that said to setup a
hardware tri-state condition and use a second data line to act as
control valve? Essentially, setup a unique state that is not normally
produced by the computer or OS during the boot process. Can somebody
suggest a way to modify the circuit in the above link. I am not a
electronics expert, so providing a simple schematic would be helpful.
Thanks!

 

[scada]

I am trying to control a relay from a computer's parallel port using
the circuit labeled Fig B from this webpage
http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm. The
circuit works fine and I am able to control it with my software. But, I
have no control while the computer is booting, where the the data lines
can be set high or low. I need the relay to remain off until my
program is running. I have read a suggestion that said to setup a
hardware tri-state condition and use a second data line to act as
control valve? Essentially, setup a unique state that is not normally
produced by the computer or OS during the boot process. Can somebody
suggest a way to modify the circuit in the above link. I am not a
electronics expert, so providing a simple schematic would be helpful.
Thanks!

The relay is being energized because you choose a circuit that will trigger
on a logic "1", or a +5V (high). The computer parallel port pin you are
using will boot to a "high" before it sees your programmed information. You
could simply use a control "Low" (0 volts) and use either circuit "A" (with
some modification) or circuit "C" as is. Please note that if your circuit is
critical, do not rely on this alone! There are safer, better ways to ensure
the proper decoding is received from the port! I would also like to add the
fact that direct connecting to an external circuit, such as circuits B & C,
are not good practice. The circuit "A" diagram offers isolation from the
computer port, that way if something goes wrong it cannot back into &
destroy the computer! If you post your requirements in detail, the talent on
this NG can come up with a solution.

 

[scada]

The relay is being energized because you choose a circuit that will trigger
on a logic "1", or a +5V (high). The computer parallel port pin you are
using will boot to a "high" before it sees your programmed information. You
could simply use a control "Low" (0 volts) and use either circuit "A" (with
some modification) or circuit "C" as is. Please note that if your circuit is
critical, do not rely on this alone! There are safer, better ways to ensure
the proper decoding is received from the port! I would also like to add the
fact that direct connecting to an external circuit, such as circuits B & C,
are not good practice. The circuit "A" diagram offers isolation from the
computer port, that way if something goes wrong it cannot back into &
destroy the computer! If you post your requirements in detail, the talent on
this NG can come up with a solution.

A quick answer to this would be to add some gates to your circuit that would
decode a series of port pins. For instance, if you used all eight data port
pins (D0 - D7), that would give you a 1 of 256 decoder. That output (high)
would then input to your existing relay circuit. The way you wire the gates,
would reflect the code number, which would be from 0-255. I suck at ASCII
art, so I won't attempt to draw a circuit here. Perhaps someone else will do
that, or I can email you a scanned pdf if needed.

 

[[email protected]]

Thanks for the reply. The unknown for this problem is what parallel
port state is set by BIOS and OS during the boot process. Its not
consistent from among computers, so some kind of unique coded control
is needed to control the relay. I just saw the second reply about
using a gate and that appears to be the answer to my problem. About
the isolation.. I am hoping the diode across the coil is good enough to
stop any back voltage from blowing up the port.

 

[[email protected]]

I never thought about using gates and that would solve my problem. Do
you have any suggestions about which chips I should be using? If you
can send that scanned circuit, I would appreciate it. Thanks!

 

[[email protected]]

Hi,
i understood your problem. you can do one simple thing . use nomally
open switch for off state, instead of normally closed. Now logic 1 will
appear at the printer port while computer is booting.

Pravin Falcao

 

[ehsjr]

Thanks for the reply. The unknown for this problem is what parallel
port state is set by BIOS and OS during the boot process. Its not
consistent from among computers, so some kind of unique coded control
is needed to control the relay.

If you do not know what appears on the port during boot, it
is impossible to design a circuit using that port that is
*guaranteed* to avoid operation during boot. Therefore, go
to a manual solution. Use a toggle switch to switch the relay
out of circuit during boot. After boot, flip the switch to on.

To do it without needing manual intervention, you will
have to discover some condition that occurs *only*
after boot is completed - which implies knowing what
happens during boot.

Ed


I just saw the second reply about

 

[budgie]

If you do not know what appears on the port during boot, it
is impossible to design a circuit using that port that is
*guaranteed* to avoid operation during boot. Therefore, go
to a manual solution. Use a toggle switch to switch the relay
out of circuit during boot. After boot, flip the switch to on.

To do it without needing manual intervention, you will
have to discover some condition that occurs *only*
after boot is completed - which implies knowing what
happens during boot.

or institute some form of hardware time delay on the relay board, assuming the
board has another way of sensing that the PC is turned on.

(I cheat - all my PC's have the internal +12V and gnd brought out to the rear of
the case, for use on add-ons. Perfect for driving a time delay).

 

[[email protected]]

[email protected] wrote:

If you do not know what appears on the port during boot, it
is impossible to design a circuit using that port that is
*guaranteed* to avoid operation during boot. Therefore, go
to a manual solution. Use a toggle switch to switch the relay
out of circuit during boot. After boot, flip the switch to on.

To do it without needing manual intervention, you will
have to discover some condition that occurs *only*
after boot is completed - which implies knowing what
happens during boot.

Ed

This is quite true if the OP needs a guaranteed error free situation.
OTOH if all s/he wants is something with 99.6% chance of doing what its
told, which is enough for some apps, the OP could use the code 10101010
to turn that relay on.

This can be done with discrete gates. Put your 4x 0 lines thru a hex
invertor. Now all you need is to AND your 8 lines. Not rocket science.

If you're working with critical apps, eg life support, military, safety
critical etc, dont even think about using this approach.

Also I dont know whether you could put something in bootup files to set
the port before the PC gets into windows/etc. Doable with DOS but with
NT I really woudnt know.

With a little more complication, one could add a timer triggered by any
line going high. Once a line goes high, the timer waits for long enough
for bootup, then starts listening for that 10101010. Now you've got a
very low error rate with cold boots, warm reboots will give you that
99.6% again.


NT

 

[Fred Bloggs]

I am trying to control a relay from a computer's parallel port using

the circuit labeled Fig B from this webpage
http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm. The
circuit works fine and I am able to control it with my software. But, I
have no control while the computer is booting, where the the data lines
can be set high or low. I need the relay to remain off until my
program is running. I have read a suggestion that said to setup a
hardware tri-state condition and use a second data line to act as
control valve? Essentially, setup a unique state that is not normally
produced by the computer or OS during the boot process. Can somebody
suggest a way to modify the circuit in the above link. I am not a
electronics expert, so providing a simple schematic would be helpful.
Thanks!

That cannot be done reliably. The only reliable way would be to add a
PIC or Basic Stamp that holds the relay driver off until it is unlocked
by a 32-bit code shifted out a data line by your program.

 

[Sjouke Burry]

That cannot be done reliably. The only reliable way would be to add a
PIC or Basic Stamp that holds the relay driver off until it is unlocked
by a 32-bit code shifted out a data line by your program.

What I did is just check port pins during bootup, and find one
which stayed in a stable condition, Then i used that one,as
power reset on my electronics. After you start your program,
invert that pin, to start the fun.

 

[Bob Monsen]

I am trying to control a relay from a computer's parallel port using
the circuit labeled Fig B from this webpage
http://ourworld.compuserve.com/homepages/Bill_Bowden/page6.htm. The
circuit works fine and I am able to control it with my software. But, I
have no control while the computer is booting, where the the data lines
can be set high or low. I need the relay to remain off until my
program is running. I have read a suggestion that said to setup a
hardware tri-state condition and use a second data line to act as
control valve? Essentially, setup a unique state that is not normally
produced by the computer or OS during the boot process. Can somebody
suggest a way to modify the circuit in the above link. I am not a
electronics expert, so providing a simple schematic would be helpful.
Thanks!

I'm assuming you have an external supply which powers the relay.

Use a counter with an active low reset. The port will try to pull down to
gnd when the computer is off (you probably want to have a pull down
resistor of like 10k in case it doesn't). If the counter comes out of
reset because the pin connecting it is high, it'll still be reset to its
initial state.

So, the process of turning on the relay is to bring the counter out of
reset, and then to clock it using another data pin which is connected to
the clock input. To add to the fun, you can have a feedback pin at the
output, connected to a status pin, that tells you when the count is
complete. Any binary 4 bit counter should work. You could use the Q3
output, and then it would take 8 clocks to make it turn on. When you want
it off, just reset the counter. A '163 should work.

This will be safe unless some other program tries to flip bits on the port
before you get to it.

--
Regards,
Bob Monsen

"Nothing before had ever made me thoroughly realise, though I had read
various scientific books, that science consists in grouping facts so
that general laws or conclusions may be drawn from them."
-- Charles Darwin

 

[scada]

I'm assuming you have an external supply which powers the relay.

Use a counter with an active low reset. The port will try to pull down to
gnd when the computer is off (you probably want to have a pull down
resistor of like 10k in case it doesn't). If the counter comes out of
reset because the pin connecting it is high, it'll still be reset to its
initial state.

So, the process of turning on the relay is to bring the counter out of
reset, and then to clock it using another data pin which is connected to
the clock input. To add to the fun, you can have a feedback pin at the
output, connected to a status pin, that tells you when the count is
complete. Any binary 4 bit counter should work. You could use the Q3
output, and then it would take 8 clocks to make it turn on. When you want
it off, just reset the counter. A '163 should work.

This will be safe unless some other program tries to flip bits on the port
before you get to it.

--
Regards,
Bob Monsen

"Nothing before had ever made me thoroughly realise, though I had read
various scientific books, that science consists in grouping facts so
that general laws or conclusions may be drawn from them."
-- Charles Darwin

I posted a schematic to Alt.binaries.schematics.electronics, for your
review.

Good Luck!