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Working with an unknown serial port device

Posted on 2003-03-14
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Last Modified: 2010-04-26
I have a telephone switch that is plugged into a serial port. This switch is very simple: it has 3 telephone plugs, one for the phone line, one for phone 1 and one for phone 2. It also has a serial port and a power cable. By sending info on this serial port, the switch changes from phone 1 to phone 2. It only allows one phone to use the line at a time.

The problem is that I dont know what info I must send into the serial port to make it switch the phone lines. There are only 2 wires connected to the serial cable:

x  x  o  o  o
 o  o  o  o

The x's are the connected wires (pins 4 and 5 I guess).

Any ideas on how to make this device work? I work with delphi on windows xp.
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Question by:acerola
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Expert Comment

by:kiranghag
ID: 8141232
which telephone..???
provide info regarding model/make of the device and we can try to find info about it...
no points in shooting in dark...
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by:kiranghag
ID: 8141242
give whatever information u can find on the switch...any numbers, name, fine prints...
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Author Comment

by:acerola
ID: 8141412
As stated in the title, it is an UNKNOWN serial port device. No model, brand or anything. I really must guess how it works.
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Expert Comment

by:rid
ID: 8141639
If this is indeed a port for serial communication, you seem to have only the TX and RX lines in it. Common lead will be in the cable shield, probably.

Any communication with this device will have to be using XOn/XOff (control character flow control).

It is also possible, that the device will only accept incoming info, and doesn't send any, one of the pins will be signal ground. Or the other way around... The pins you mention to be in use are not what would be standard signal pins for either a DTE or a DCE device.

Sometimes, DB9 connectors were used for other purposes than serial communication; this may just be a rather clumsy headset connector or something.

Can you open it and find any kind of info on whatever circui board is inside?

Regards
/RID
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Expert Comment

by:aLk0
ID: 8141701
If this device is Plug'n'Play try to identify it by Vendor / product numbers. But your device is external(not P'n'P), so try to search by chip names used on the board of device.
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Expert Comment

by:iclarius
ID: 8142603
Acerola,

If the pins you identified are truly pins 4 and 5, then all it is doing is reacting to a logic level. The Data Terminal Ready line is a discrete line that is either logic hi (5 vdc) or logic lo (<1.8 vdc). All it takes to command the device is to raise pin 4 to +5 Vdc and leave pin 5 grounded (the return) to switch it to phone 2. Apply 5 vdc to pin 4 and the phone device will switch to line 2. This has to be a simple device. I presume phone 1 is the default line and thus if you did not connect anything up to the phone line mux device, it would default to line 1.

George
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by:acerola
ID: 8143035
iclarios/George,

You got it right. It is a logical device and I also guess it is responding to DTR state. The device is supposed to be connected to the serial port of a computer, so how do I send the +5 and 1.8 vdc through the serial port, to make it work? What I need is a piece of software to work the serial port in that way.

I forgot to tell how it is inside: it has a very small chip, with only 3 connectors on each side, a capacitor, a resistor and something that looks like a transistor. It also has a big thing that I guess it is a relay where the phone lines are connected. The device looks home-made. It was assembled by my client's former software company, so I must make my software compatible with this device, or loose the client :(
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Accepted Solution

by:
iclarius earned 2000 total points
ID: 8143060
acerola,

I presume you are a programmer and are programming in Delphi. When you want to send a command to access line 2, you would issue the command from you application to send a Data Ready command for the RS-232 port the device is connected to.

Are you familiar with RS-232 ports and how to command them? I am sure that there are applications/code for you to glean that information from on any of the Delphi support sites.

George
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Author Comment

by:acerola
ID: 8143157
Oops, my bad. After all the problem was a defective power cable. I tried making a simple program just to open the port and set the DTR state. It didn't work yesterday, when I posted this question, but it worked today after I replaced the power cable. ;)

Points for you George, sice you guessed how the device works. Thanks all.
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