 ### Network # How drive LEDs for RS232 input 3v to 30v + and -

G

#### [email protected]

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic 1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference? If I can I guess the impedance
of any signal would be very high and then the LED driver would need to
be very high input. I guess an op-amp would do it but not practical as
I need eight. Is there a better way? Much appreciate any help.

M

#### [email protected]

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic 1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference? If I can I guess the impedance
of any signal would be very high and then the LED driver would need to
be very high input. I guess an op-amp would do it but not practical as
I need eight. Is there a better way? Much appreciate any help.

Do you need accurate voltage detection, or are you just trying to
determine what is on the line? [High, low, pulled to ground via input
resistor]

Maxim's first generation auto-shutdown just detected if input was
within a VTH of ground to classify the signal as ground.

If the situation has a line at hard ground and lines at ground due to
the input resistors on the 232 device, it will take some sophistication
to find the real (hard) ground.

R

#### Rich Grise

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic 1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference? If I can I guess the impedance
of any signal would be very high and then the LED driver would need to
be very high input. I guess an op-amp would do it but not practical as
I need eight. Is there a better way? Much appreciate any help.

Sounds kind of pointless, if you don't even know which pin is ground.
How are you going to know what any of the rest are anyway?

Thanks,
Rich

J

#### jasen

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic 1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference?

try this:

+--->|----(in1)---->|----+
| |
+--->|----(in2)---->|----+
| |
+--->|----(in3)---->|----+
| |
+--->|----(in4)---->|----+
| |
+--->|----(in5)---->|----+
| |
+--->|----(in6)---->|----+
| |
+--->|----(in7)---->|----+
| |
+--->|----(in8)---->|----+
| |
+----[10K]--+--[10K]-----+
|
+-- ref

diodes are 1N914 or 1N4148 (or similar in an 8-pack, etc)

that'll be less succeptible to signal changes, as long as one signal is low
ans one is high it'll give an output that's close to ground (as long as it's
not passing much current).

you'll need battery power or a well-isolated supply.
If I can I guess the impedance
of any signal would be very high and then the LED driver would need to
be very high input. I guess an op-amp would do it but not practical as
I need eight. Is there a better way? Much appreciate any help.

some op-amps come 4 per chip...

B

#### [email protected]

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic 1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference? If I can I guess the impedance
of any signal would be very high and then the LED driver would need to
be very high input. I guess an op-amp would do it but not practical as
I need eight. Is there a better way? Much appreciate any help.

It sounds perfectly practicable to me. Practical is another question.
There is nothing impractical about using eight op amps - you can get

Farnell lists more than a page of bipolar input quads, half a page of
FET-input quads and nearly a page of CNMOS-input quads. Some of them
are quite cheap.

J

#### James Harris

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V
and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though
would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic
1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the
ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference? If I can I guess the
impedance
of any signal would be very high and then the LED driver would need
to
be very high input. I guess an op-amp would do it but not practical
as
I need eight. Is there a better way? Much appreciate any help.

Do you need accurate voltage detection, or are you just trying to
determine what is on the line? [High, low, pulled to ground via input
resistor]

Maxim's first generation auto-shutdown just detected if input was
within a VTH of ground to classify the signal as ground.

If the situation has a line at hard ground and lines at ground due to
the input resistors on the 232 device, it will take some
sophistication
to find the real (hard) ground.

No, the voltage doesn't need to be too accurate, though +/-3V would be
ideal. Yes, one line coming in from the rs232 port would be expected to
be an a signal ground, though it would not be obvious before testing
which one. That is one point of the test. The signals will be presented
on a (possibly incorrectly wired) RJ45 and this is to be a breakout box
for the eight lines.

J

#### James Harris

Jan 1, 1970
0
jasen said:
On 2006-09-16, [email protected] <[email protected]>
wrote: snip
Now the fun part: I won't know which of the eight inputs is the
ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference?

try this:

+--->|----(in1)---->|----+
| |
+--->|----(in2)---->|----+
| |
+--->|----(in3)---->|----+
| |
+--->|----(in4)---->|----+
| |
+--->|----(in5)---->|----+
| |
+--->|----(in6)---->|----+
| |
+--->|----(in7)---->|----+
| |
+--->|----(in8)---->|----+
| |
+----[10K]--+--[10K]-----+
|
+-- ref

diodes are 1N914 or 1N4148 (or similar in an 8-pack, etc)

that'll be less succeptible to signal changes, as long as one signal
is low
ans one is high it'll give an output that's close to ground (as long
as it's
not passing much current).

That's the kind of thing I was thinking of but using, say, 10k resistors
fed to a common point. I don't think I can see what the diodes would do.
Say, for example, only in1, in2 and in3 were connected and that they
were at +12v, 0v, and -12v. Would the +12v raise the RHS to +11.3, and
the -12v drop the LHS to -11.3?

J

#### James Harris

Jan 1, 1970
0
It sounds perfectly practicable to me. Practical is another question.
There is nothing impractical about using eight op amps - you can get

Farnell lists more than a page of bipolar input quads, half a page of
FET-input quads and nearly a page of CNMOS-input quads. Some of them
are quite cheap.

Great! I'm just looking at op-amps and they look to be really useful for
this application. It looks like any one op-amp will drive its output
positive or negative depending on the greater of the non-inverting and
inverting inputs. I should they be able to use the single output to
drive two LEDs, one for +ve and one for -ve. The problem is, I need a
gap of about 6v in between when both LEDs should be off. I'll read up
some more.

G

#### Gerard Bok

Jan 1, 1970
0
Is it practical to put together a circuit that would show either a
green or a red LEDs for each of eight inputs which are between -30V and
+30V. I only know the basics of a bipolar transistor-based driver and
am faced with the problem of the wide input range that the RS232
standard permits. In practice 3V to 20V should be enough, though would
prefer it to be safe to 30V. The intention is:

1) an input between -30V and -3V to light a red LED for RS232 logic 1,
a -ve voltage
2) an input between -3V and +3V to light no LED
3) an input between +3V and +30V to light a green LED for RS232 logic
0, a +ve voltage

and this will be needed eight times. Perhaps a driver IC?

Now the fun part: I won't know which of the eight inputs is the ground
line. Can I connect all via separate high value resistors to a common
point and use that as ground reference? If I can I guess the impedance
of any signal would be very high and then the LED driver would need to
be very high input. I guess an op-amp would do it but not practical as
I need eight. Is there a better way? Much appreciate any help.

If you are looking at actual RS232 circuits, why not use an out-

6 or 8 bicolor leds to show you the status of each line.
(They just use a resistor and a bicolor led per line.)

Ready made, must set you back at least 2 pound J

#### James Harris

Jan 1, 1970
0
Gerard said:
On 16 Sep 2006 10:33:20 -0700, "[email protected]" snip
If you are looking at actual RS232 circuits, why not use an out-

6 or 8 bicolor leds to show you the status of each line.
(They just use a resistor and a bicolor led per line.)

Ready made, must set you back at least 2 pound I have a couple of these. The problem is they require you to know which
line is ground. This is OK for a 9- or 25-pin standard serial cable but
the 8-pin RJ45-type serial connectors seem to vary. Many are
Yost-standard pinouts with the two central pins as ground, but not all.
Hence this idea.

G

#### Gerard Bok

Jan 1, 1970
0
I have a couple of these. The problem is they require you to know which
line is ground. This is OK for a 9- or 25-pin standard serial cable but
the 8-pin RJ45-type serial connectors seem to vary. Many are
Yost-standard pinouts with the two central pins as ground, but not all.
Hence this idea.

Wouldn't it be possible to use an external ground and just 'test'
all 8 leads for possible RS232 signal ?
Those pins that don't light a LED could well be ground G

#### [email protected]

Jan 1, 1970
0
Gerard said:
On 18 Sep 2006 05:50:18 -0700, "James Harris" snip
Wouldn't it be possible to use an external ground and just 'test'
all 8 leads for possible RS232 signal ?
Those pins that don't light a LED could well be ground You mean test them in isolation? I don't think that would work. Each of
the signals is only what it is relative to its own ground. Just as the
"voltage" of a line is only relative to another line and birds can
perch on power lines etc......

The idea is to take all eight lines to a common point with high value
resistors. Then measure the difference between that point and each
line. Because the potential difference between the reference point and
each line will be high impedance I will need an amplifier to drive LEDs.

J

#### jasen

Jan 1, 1970
0
. .
. .
. .
+--->|----(in8)---->|----+
| |
+----[10K]--+--[10K]-----+
|
+-- ref

diodes are 1N914 or 1N4148 (or similar in an 8-pack, etc)

that'll be less succeptible to signal changes, as long as one signal
is low
ans one is high it'll give an output that's close to ground (as long
as it's
not passing much current).

That's the kind of thing I was thinking of but using, say, 10k resistors
fed to a common point. I don't think I can see what the diodes would do.
Say, for example, only in1, in2 and in3 were connected and that they
were at +12v, 0v, and -12v. Would the +12v raise the RHS to +11.3, and
the -12v drop the LHS to -11.3?

yeah,

now suppose in4-8 were all +12V

with the diodes ref (the output) would still be about 0V

but with resistors you'd get the average of all the inputs

1x0 + 1x-12 + 6x12
------------------ = 60/8 = 7.5V
8

the diodes and two resistors above get you the voltage half-way between the
highest and lowest voltages.

Bye.
Jasen

G

#### Gerard Bok

Jan 1, 1970
0
You mean test them in isolation?

Well, I don't know your actual situation.
If your source is 'true RS232' one of the wires should be pretty
close to ground level.
Your problem would be, to find out which one is.

My suggestion is, to connect all 8 wires to a detector that is
connected to 'real ground'.
That may not be a reliable situation for datacommunications but
it may well be an easy way to detect signal levels on each wire.
And to detect, which of them is RS232-ground.

would it take to probe the idea? What would it save you if it
just works ?
I don't think that would work. Each of
the signals is only what it is relative to its own ground.

It is not uncommon for RS232 ground to shift several volts from
'real ground'. But it is not common to float.
If you are in doubt, fuse the wires.

But you are quite right if you already know that your RS232
source will be floating.

J

#### James Harris

Jan 1, 1970
0
jasen wrote:
snip
the diodes and two resistors above get you the voltage half-way between the
highest and lowest voltages.

Cool!

Replies
14
Views
2K
Replies
2
Views
1K
P
Replies
40
Views
2K
Baron
B
J
Replies
2
Views
933
Rich Grise
R
V
Replies
11
Views
979
valentin tihomirov
V