Serial cable length
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The stamp programming manual outlines the use of a 22K resistor when using the
serin command (and direct connection when using the serout command). I've used
this setup successfully, but was wondering how long of a cable I can use,
without using a line driver? About 2' of cable seems to work well now.
This isn't for a production product, and I don't have to worry about connecting
to unknown serial devices.
Thanks,
Kevin
serin command (and direct connection when using the serout command). I've used
this setup successfully, but was wondering how long of a cable I can use,
without using a line driver? About 2' of cable seems to work well now.
This isn't for a production product, and I don't have to worry about connecting
to unknown serial devices.
Thanks,
Kevin
Comments
a regular Stamp pin, look into a MAX232 chip. One chip and 4 capacitors
gives you two channels of true RS232.
The resistor thing is a workaround to drop the voltage to Stamp levels.
Original Message
> The stamp programming manual outlines the use of a 22K resistor when using
the
> serin command (and direct connection when using the serout command). I've
used
> this setup successfully, but was wondering how long of a cable I can use,
> without using a line driver? About 2' of cable seems to work well now.
>
> This isn't for a production product, and I don't have to worry about
connecting
> to unknown serial devices.
But does anyone have any experience with the current limiting resistors approach
and cable length? I'd guess this is highly dependent on the individual serial
implementation on the other side of the line, just looking for others' data
points.
Thanks,
Kevin
Rodent wrote:
>
> If you are going to talk to a true serial device (PC, printer, etc...) with
> a regular Stamp pin, look into a MAX232 chip. One chip and 4 capacitors
> gives you two channels of true RS232.
>
> The resistor thing is a workaround to drop the voltage to Stamp levels.
>
>
Original Message
>
> > The stamp programming manual outlines the use of a 22K resistor when using
> the
> > serin command (and direct connection when using the serout command). I've
> used
> > this setup successfully, but was wondering how long of a cable I can use,
> > without using a line driver? About 2' of cable seems to work well now.
> >
> > This isn't for a production product, and I don't have to worry about
> connecting
> > to unknown serial devices.
>
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This is what limits your distance. I suspect 3-5 feet with a good 20 gauge
or larger shielded cable may be pushing it.
Receiving the RS232 signal with the Stamp should not be a problem -- the
resistors drop the voltage to Stamp levels. Its the maximum of 5 volts the
Stamp can put out transmitting the RS232 signal that can be an issue.
Depending on the baud rate and the type of cable you have, you can get over
500 feet with the MAX232's
Original Message
> Yes, thanks, understood.
>
> But does anyone have any experience with the current limiting resistors
approach
> and cable length? I'd guess this is highly dependent on the individual
serial
> implementation on the other side of the line, just looking for others'
data
> points.
> > If you are going to talk to a true serial device (PC, printer, etc...)
with
> > a regular Stamp pin, look into a MAX232 chip. One chip and 4 capacitors
> > gives you two channels of true RS232.
> >
> > The resistor thing is a workaround to drop the voltage to Stamp levels.
> >
> >
Original Message
> >
> > > The stamp programming manual outlines the use of a 22K resistor when
using
> > the
> > > serin command (and direct connection when using the serout command).
I've
> > used
> > > this setup successfully, but was wondering how long of a cable I can
use,
> > > without using a line driver? About 2' of cable seems to work well
now.
> > >
> > > This isn't for a production product, and I don't have to worry about
> > connecting
> > > to unknown serial devices.
>
> RS232 is spec'd for + / - 12 volts -- the Stamp can put out 0 to +5 volts.
> This is what limits your distance. I suspect 3-5 feet with a good 20 gauge
> or larger shielded cable may be pushing it.
Yep. From what I'm seeing there a number of interpretations of the 'spec'. I
suspected the same thing, just thought I'd ask to see if anyone had actually
fooled around with this.
> Depending on the baud rate and the type of cable you have, you can get over
> 500 feet with the MAX232's
Yes yes, maxim is good, got it (I'd probably use an LTC part though), just
thought I see what sort of experience folks had without using a line driver.
Thanks,
Kevin
>serin command (and direct connection when using the serout command).
>I've used
>this setup successfully, but was wondering how long of a cable I can use,
>without using a line driver? About 2' of cable seems to work well now.
>
>This isn't for a production product, and I don't have to worry about
>connecting
>to unknown serial devices.
>
>Thanks,
>Kevin
Hi Kevin,
There are 3 issues I think:
1) capacitance of the cable degrades the signal.
2) noise induced from outside sources fakes a signal.
3) ESD from lightning or accidents, oops.
#1 can be improved by operating at a lower baud rate. I have run 200
feet at 300 baud at TTL levels for a kluged setup, through Belden
8443 unshielded cable. Not a scientific test. Depends on the cable
and on the resistance (RC time) driving the cable. If you are using
a stamp, use no more than 220 ohms in series to drive the line.
#2 Using shielded twisted pair cable can cut down the noise. However,
shielded cables will have a higher capacitance. Noise can be awful
on an unshielded cable. Of course, depends on where you are. The
stamp input does not have any hysteresis, and the asymmetric
threshold is not ideal if you are receiving TTL levels.
Funny things can affect the results. Say your cable runs on the
ground outdoors. It can work at first but stop working when it
rains, because of changes in capacitance or noise. Or somebody can
turn on a motor nearby. Okay for a kluge, but don't depend on it for
the long term.
#3 Well, ESD is separate issue.
-- Tracy
BTW, what's the 220ohm resistor for?
I'll be driving this with a BS2 (maybe a PIC too, but I guess I can't
admit to that on this list ;-)
Kevin
--- In basicstamps@y..., Tracy Allen <tracy@e...> wrote:
> >The stamp programming manual outlines the use of a 22K resistor
when using the
> >serin command (and direct connection when using the serout
command).
> >I've used
> >this setup successfully, but was wondering how long of a cable I
can use,
> >without using a line driver? About 2' of cable seems to work well
now.
> >
> >This isn't for a production product, and I don't have to worry
about
> >connecting
> >to unknown serial devices.
> >
> >Thanks,
> >Kevin
>
>
> Hi Kevin,
> There are 3 issues I think:
> 1) capacitance of the cable degrades the signal.
> 2) noise induced from outside sources fakes a signal.
> 3) ESD from lightning or accidents, oops.
>
> #1 can be improved by operating at a lower baud rate. I have run
200
> feet at 300 baud at TTL levels for a kluged setup, through Belden
> 8443 unshielded cable. Not a scientific test. Depends on the cable
> and on the resistance (RC time) driving the cable. If you are using
> a stamp, use no more than 220 ohms in series to drive the line.
>
> #2 Using shielded twisted pair cable can cut down the noise.
However,
> shielded cables will have a higher capacitance. Noise can be awful
> on an unshielded cable. Of course, depends on where you are. The
> stamp input does not have any hysteresis, and the asymmetric
> threshold is not ideal if you are receiving TTL levels.
>
> Funny things can affect the results. Say your cable runs on the
> ground outdoors. It can work at first but stop working when it
> rains, because of changes in capacitance or noise. Or somebody can
> turn on a motor nearby. Okay for a kluge, but don't depend on it
for
> the long term.
>
>
> #3 Well, ESD is separate issue.
>
> -- Tracy
>Thanks for the reply.
>
>BTW, what's the 220ohm resistor for?
>I'll be driving this with a BS2 (maybe a PIC too, but I guess I can't
>admit to that on this list ;-)
>
>Kevin
> > and on the resistance (RC time) driving the cable. If you are using
> > a stamp, use no more than 220 ohms in series to drive the line.
Hi Kevin,
The 220 ohm resistor the output of the stamp is there simply to
protect it from accidents. Anytime an IC pin (Stamp, PIC, HC11,
'555, CD4011, whatever) is connected to the "outside world", it is
prudent to put at least resistor in series just in case... The
resistor limits fault currents. There are some chips specifically
designed as "line drivers" where this is not necessary. You should
not go overboard with the protection, because more resistance will
slow down the signal.
-- Tracy
protection and lost signal. 5/220 = about 23mA. Any smaller and I'll go over
the 25mA current sink limit, any larger and I needlessly loose signal strength
if I'm just trying to protect against 5V, yes?
Thanks again,
Kevin
Tracy Allen wrote:
> The 220 ohm resistor the output of the stamp is there simply to
> protect it from accidents. Anytime an IC pin (Stamp, PIC, HC11,
> '555, CD4011, whatever) is connected to the "outside world", it is
> prudent to put at least resistor in series just in case... The
> resistor limits fault currents. There are some chips specifically
> designed as "line drivers" where this is not necessary. You should
> not go overboard with the protection, because more resistance will
> slow down the signal.
>protection and lost signal. 5/220 = about 23mA. Any smaller and I'll go over
>the 25mA current sink limit, any larger and I needlessly loose signal strength
>if I'm just trying to protect against 5V, yes?
Exactly!
I have an application that requires me to periodically access the
Stamp via the Serial port. It is located in an awkward position and I
would like to bring a serial cable from the Stamp to a more
convenient location. The board has decoupling capacitors to the DB9
and to ground on the ATN line (as done on the BOE). I have a
shorting plug that would hold ATN to ground at the remote serial
port, when not in use.
Thus my questions for the collective wisdom:
1) How long a serial cable can I have between the computer and the
DB9 on the Stamp board?
2) Is the shorting plug sufficient to prevent false ATN interrupts to
the Stamp? (Or is it really necessary?)
Thanks for you input,
Phil
>All,
>
> I have an application that requires me to periodically access the
>Stamp via the Serial port. It is located in an awkward position and I
>would like to bring a serial cable from the Stamp to a more
>convenient location. The board has decoupling capacitors to the DB9
>and to ground on the ATN line (as done on the BOE). I have a
>shorting plug that would hold ATN to ground at the remote serial
>port, when not in use.
>
> Thus my questions for the collective wisdom:
>1) How long a serial cable can I have between the computer and the
>DB9 on the Stamp board?
>
>2) Is the shorting plug sufficient to prevent false ATN interrupts to
>the Stamp? (Or is it really necessary?)
>
> Thanks for you input,
>Phil
I have made 200ft runs with plain-jane 4-conductor telephone wire up
through ceiling tile and over an area with heavy machinery without any
problems. I did have a 1K resistor tied between the ATN and GND at
the stamp location however.
Beau Schwabe IC Mask Designer
National Semiconductor Wired Communications Division
500 Pinnacle Court, Suite 525 Mail Stop GA1 Norcross, GA 30071