Simple PC Serial Port Help

[[email protected]]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,
generally speaking, but I am definitely a "software guy." Once
connected, I'd like to be able to open the serial port and query (or
receive) the current state of the switch. The rest of my project can
be handled in software (for now) so I'd like to keep everything on the
hardware side as simple as possible.

Googling, I find a good number of circuits involving using the PC
serial port to control the state of LEDs/relays/etc. but not much in
the way of simple receiving of data. I'm not opposed to using the
parallel port, I'd just prefer to use serial for this project, unless
there is a significant advantage (lower cost, simpler design). I'm
also not against using a USB port, but I am assuming that involves more
complexity on the hardware side as well as the software side.

Anyway, thanks for any links or information you can provide. I'd be
glad to provide the final design and example C code for my project when
I get it going, if that is useful to anyone.

oz

 

[Chris]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,
generally speaking, but I am definitely a "software guy." Once
connected, I'd like to be able to open the serial port and query (or
receive) the current state of the switch. The rest of my project can
be handled in software (for now) so I'd like to keep everything on the
hardware side as simple as possible.

Googling, I find a good number of circuits involving using the PC
serial port to control the state of LEDs/relays/etc. but not much in
the way of simple receiving of data. I'm not opposed to using the
parallel port, I'd just prefer to use serial for this project, unless
there is a significant advantage (lower cost, simpler design). I'm
also not against using a USB port, but I am assuming that involves more
complexity on the hardware side as well as the software side.

Anyway, thanks for any links or information you can provide. I'd be
glad to provide the final design and example C code for my project when
I get it going, if that is useful to anyone.

oz

Hi, Oz. The serial port is the easiest place to accomplish your task,
and s.e.b. is the right place to ask (There are no questions too simple
or basic for s.e.b. -- just no homework problems unless you've at least
made a good effort, please ;-)

Having said that, your best bet would be to use a SPDT switch, with the
common going to one of the inputs (like RxD), and the N.O. and N.C.
contacts going to two outputs (say, TxD and CTS). Make one output high
and the other low. Then, when you switch, the logic state at the input
will change. Try to use a switch rated for dry contact (<5mA), also
called a logic level switch. View your switch hookup here in fixed
font or M$ Notepad):
|
| Serial Port
|
| .-----------< TxD
| |
| | SW1
| '--o
| __--o----> RxD
| .--o
| |
| |
| '-----------< CTS
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

This is trivial in DOS -- just input and output ports in BASIC, C, or
any other language that can access hardware directly. The Lakeview
Research site has helpful information on Windows .dlls and drivers you
might require if you're using a Windows programming language like
Visual C++.

Good luck
Chris

 

[Randy Day]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,

Build a loopback tester with the switch in the loop.
If you transmit some data, and you receive the same
data back, your switch is closed.

http://zone.ni.com/devzone/cda/tut/p/id/3450


HTH

 

[jasen]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,
generally speaking, but I am definitely a "software guy." Once
connected, I'd like to be able to open the serial port and query (or
receive) the current state of the switch. The rest of my project can
be handled in software (for now) so I'd like to keep everything on the
hardware side as simple as possible.

put the switch between the DTR and DSR pins... you should be able to tell
the state of the switch by turning DTR on and reading DSR.

in linux you open the port and use an IOCTL, to get the DSR state, in DOS
it's a BIOS call, or you can just read the io port, dunno about windows.

 

[Bob Masta]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,
generally speaking, but I am definitely a "software guy." Once
connected, I'd like to be able to open the serial port and query (or
receive) the current state of the switch. The rest of my project can
be handled in software (for now) so I'd like to keep everything on the
hardware side as simple as possible.

Googling, I find a good number of circuits involving using the PC
serial port to control the state of LEDs/relays/etc. but not much in
the way of simple receiving of data. I'm not opposed to using the
parallel port, I'd just prefer to use serial for this project, unless
there is a significant advantage (lower cost, simpler design). I'm
also not against using a USB port, but I am assuming that involves more
complexity on the hardware side as well as the software side.

Anyway, thanks for any links or information you can provide. I'd be
glad to provide the final design and example C code for my project when
I get it going, if that is useful to anyone.

In addition to all the good info from the other responses, one general
source of port info is Lakeview Research at www.lvr.com.

I personally have always preferred the parallel port, but that is
no longer easy to use on any system later than Win9x, without
a special Ring 0 driver. I expect that even the serial port's days
are numbered as well. Ahh, progress!!!!

Best regards,


Bob Masta
dqatechATdaqartaDOTcom

D A Q A R T A
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Home of DaqGen, the FREEWARE signal generator

 

[Chris]

Chris wrote:

... The Lakeview
Research site has helpful information on Windows .dlls and drivers you
might require if you're using a Windows programming language like
Visual C++.

Good luck
Chris

Forgot to provide the link:

http://www.lvr.com/serport.htm

"Serial Port Complete" by Jan Axelson of LVR is a great resource on
hacking the serial port. It has a CD which includes numerous
programming examples in many languages, as well as the Windows files
you'll need. You can purchase the book from the website or many
hobbyist resources such as Jameco. It's also available at many
libraries.

Good luck
Chris

 

[Rich Grise]

[email protected] wrote: .... ....
|
| Serial Port
|
| .-----------< TxD
| |
| | SW1
| '--o
| __--o----> RxD
| .--o
| |
| |
| '-----------< CTS
|

I'd do almost this, except I wouldn't use TxD and RxD - they go through
the UART and are expected to be a serial data stream, with start bits
and all that.

Just use, lessee...
http://www.aggsoft.com/rs232-pinout-cable/pinout-and-signal.htm

So, you'd use your control register to output, say, a high on DTR, and
a low on RTS, and come in on your choice of DSR or CTS:

| Serial Port
|
| .-----------< DTR
| |
| | SW1
| '--o
| __--o----> DSR/CTS (just use the appropriate bit).
| .--o
| |
| |
| '-----------< RTS
|

Then, you just poll the status byte when you want to check the
switch.

(on that webpage, the computer's on the left, so you really should
look at a mirror image of the ASCII art above.)

Have Fun!
Rich

 

[Chris]

I'd do almost this, except I wouldn't use TxD and RxD - they go through
the UART and are expected to be a serial data stream, with start bits
and all that.

Just use, lessee...
http://www.aggsoft.com/rs232-pinout-cable/pinout-and-signal.htm

So, you'd use your control register to output, say, a high on DTR, and
a low on RTS, and come in on your choice of DSR or CTS:


Then, you just poll the status byte when you want to check the
switch.

(on that webpage, the computer's on the left, so you really should
look at a mirror image of the ASCII art above.)

Have Fun!
Rich

Yup. Sorry, Rich -- I first did this more than a quarter century ago
with a footswitch on a tank Kaypro II with Z80 assembler -- I forgot
the details. Ah, the joys of that monster 9" green screen!

http://oldcomputers.net/kayproii.html

You're right, the OP should use control signals, they're immediately
addressable from most UARTs including the PC. As I remember, I'd also
have occasional problems with resistive switch contacts. Much better
to just assert a "1" on one output control bit, and a "0" on the other,
and use a SPDT switch to signal.

Cheers
Chris

 

[petrus bitbyter]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,
generally speaking, but I am definitely a "software guy." Once
connected, I'd like to be able to open the serial port and query (or
receive) the current state of the switch. The rest of my project can
be handled in software (for now) so I'd like to keep everything on the
hardware side as simple as possible.

Googling, I find a good number of circuits involving using the PC
serial port to control the state of LEDs/relays/etc. but not much in
the way of simple receiving of data. I'm not opposed to using the
parallel port, I'd just prefer to use serial for this project, unless
there is a significant advantage (lower cost, simpler design). I'm
also not against using a USB port, but I am assuming that involves more
complexity on the hardware side as well as the software side.

Anyway, thanks for any links or information you can provide. I'd be
glad to provide the final design and example C code for my project when
I get it going, if that is useful to anyone.

oz

You need some basic understanding of the serial port. All you need for that
you can find in
www.beyondlogic.org
chapters on RS-232 interfacing. You need to know which (two) lines are meant
to transmit and receive data and what modem controll- and sense lines you
have available. The original use of the latter lines is not important, but
you must distingguish inputs from outputs.

Next, you did not tell what OS you're using. In plain old DOS, you can
access the UART directly. In W95, W98 and WME you can do so as well. For
WNT, W2000 and WXP you'd better find a driver or use a language that
supports serial data communication. For Linux, you need to handle the TTY
controls. Some others told you already how you can connect the switch.
Beware! An open input line has no defined logical value so make sure it is
connected to either a high- or a low output line.

Now you need to decide what you really want to achive. So do you want to
look at the switch after asking the user a question or do you decide for
yourself when you want to read out the switch or do you want to catch every
change the switch makes? The first two cases are simple. You poll whenever
you need to. I'd go for the single pole, single throw switch that can
(dis)connect TxD and RxD. Send a byte, for instance 0x55, and check whether
the byte has been received or not. If you want to catch every change, you
need to poll continuously or use an interrupt. I'd go for the interrupt, but
being a programmer, you will know what to do.

petrus bitbyter

 

[Si Ballenger]

Please point me toward any applicable FAQs or other groups if I am
breaking protocol or in the wrong place, thanks.

That said, I am trying to find some information on building a very
simple circuit that combines a momentary contact switch and my PC's
serial port. I have a very basic understanding of electronics,
generally speaking, but I am definitely a "software guy." Once
connected, I'd like to be able to open the serial port and query (or
receive) the current state of the switch. The rest of my project can
be handled in software (for now) so I'd like to keep everything on the
hardware side as simple as possible.

Googling, I find a good number of circuits involving using the PC
serial port to control the state of LEDs/relays/etc. but not much in
the way of simple receiving of data. I'm not opposed to using the
parallel port, I'd just prefer to use serial for this project, unless
there is a significant advantage (lower cost, simpler design). I'm
also not against using a USB port, but I am assuming that involves more
complexity on the hardware side as well as the software side.

Anyway, thanks for any links or information you can provide. I'd be
glad to provide the final design and example C code for my project when
I get it going, if that is useful to anyone.

oz

For the serial port, connect the tx to the rx thru the switch.
Then send some bytes and if the switch is closed, the bytes will
be received. For the parallel port, below is a simple setup.

http://www.geocities.com/zoomkat/status.htm