serial to PSTN modem

[overmind]

hi everybody, i have a device that sends data serially. however this device would be installed 3 km from the PC that receives the data. because of the distance between the two, is it possible to convert the serial data to classic telephone signals and let it pass through a single twisted pair and at the end convert it back again to serial data and connect it to a certain COM port in the PC? are their such devices that translates the data in these two formats? please help.. thanks.

 

[Nigel Goodwin]

hi everybody, i have a device that sends data serially. however this device would be installed 3 km from the PC that receives the data. because of the distance between the two, is it possible to convert the serial data to classic telephone signals and let it pass through a single twisted pair and at the end convert it back again to serial data and connect it to a certain COM port in the PC? are their such devices that translates the data in these two formats? please help.. thanks.

Just a standard serial port modem, presumably you'd have to simulate the exchange voltages on the wires?.

How fast does it need to be, and does it need to be bidirectional? - consider how old telegraph systems worked (Indians cutting the wires etc.)

 

[overmind]

hi thanks for the swift reply.. the baud rate of the device is at 19200 and sad to say "yes" it needs to be bidirectional.. the pc sends certain commands and the device will reply based on these commands. sorry for this ignorant question but what do you mean by "simulate the exchange of voltages on the wires?" is there a device that would do all those stuff for me? that i would just plug at both ends and it would operate like a normal serial line?

 

[Nigel Goodwin]

hi thanks for the swift reply.. the baud rate of the device is at 19200 and sad to say "yes" it needs to be bidirectional.. the pc sends certain commands and the device will reply based on these commands. sorry for this ignorant question but what do you mean by "simulate the exchange of voltages on the wires?" is there a device that would do all those stuff for me? that i would just plug at both ends and it would operate like a normal serial line?

As you're wanting a pretty fast data rate, it makes things more difficult - but you can't just connect two modems together, they expect to connect via an exchange, which provides voltage they expect to see.

What are you actually doing?, and why does it need to be that fast?.

EDIT:

I just did a quick goggle for "connecting two modems together", here's one result:

https://www.jagshouse.com/modem.html

 

[overmind]

hi sir.. you see, i have a weather station that sends data serially.. in its datasheet, it specifies that its baud rate is at 19200. since it is stationed 3 kms away from the server, the only way i can think of to successfully send data with this distance is to convert it to classic telephone signal and reverse it on the other end... will a standard serial port modem suffice the problem?

 

[Nigel Goodwin]

hi sir.. you see, i have a weather station that sends data serially.. in its datasheet, it specifies that its baud rate is at 19200. since it is stationed 3 kms away from the server, the only way i can think of to successfully send data with this distance is to convert it to classic telephone signal and reverse it on the other end... will a standard serial port modem suffice the problem?

I don't know, I've never tried it - but I doubt the weather station sends out continuous streams of 19200 baud anyway, I would imagine it only sends small quantities of data occasionally. It would be easy to buffer it and send it out slower, but I would suggest connecting it directly to a PC first and monitoring how much data is sent, and how often. Do you have mains at the station site?.

 

[KeepItSimpleStupid]

Why not go Serial to fiber? **broken link removed** 40 km max

 

[overmind]

I don't know, I've never tried it - but I doubt the weather station sends out continuous streams of 19200 baud anyway, I would imagine it only sends small quantities of data occasionally. It would be easy to buffer it and send it out slower, but I would suggest connecting it directly to a PC first and monitoring how much data is sent, and how often. Do you have mains at the station site?.

yes, it sends small quantities of data on a polled basis. when it receives a particular command coming from the PC, it sends a small stream of data.. around 40 bytes and the power source is not a problem since it is available at the station site..

Why not go Serial to fiber? **broken link removed** 40 km max

this would really solve the problem, however the cost of fiber cable at 3 km would create monetary issues for me and since i have in my disposal a twisted pair of this length so my first option would be to utilize this in anyway possible..

 

[KeepItSimpleStupid]

I lost what I was putting together. Ideas to explore are ISDN, DSL and wireless.

DSL is probably too complicated and expensive, but you could end up with an Ethernet link. You would have to put a DSLAM at one end. DSL can run over dry pairs.

ISDN may be a reasonable choice. See: http://www.arcelect.com/isdn.htm

Remember with the ISDN and DSL solutions, your being the phone company and the subscriber.

Not sure if you can put a telephone line simulator at one end and use a conventional modem. e.g. http://www.microseveninc.com/telephonelinesimulators.htm

and a multi-port DSL DSLAM: **broken link removed**

These are only ideas. I'm not endorsing any of them.

The Telco line simulator could be problematic because it's not always on. ISDN is just plain wierd.

http://www.buytelco.net/NetworkApplications.asp?ID=601

 

[Nigel Goodwin]

yes, it sends small quantities of data on a polled basis. when it receives a particular command coming from the PC, it sends a small stream of data.. around 40 bytes and the power source is not a problem since it is available at the station site..

So speed isn't an issue, particularly as it only sends data on request.

A simple PIC could easily receive the request at (say) 300 baud, and send it on to the station at 19200 - it could then receive the data, buffer it, and send it back out at 300 baud again.

As you have mains power at the station, you could also do the same thing using a PC - but it's a bit of overkill

I would suggest, for a start, getting two old serial port modems, and a couple of PC's, and see what you can do via the wires.