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I am developing one software for managing parking lot. System is designed to use four HID RFID devices for authorization clients on gates. Each device should be connected to server by Serial port RS232. Now I wondering is there some device or method to join all that devices on one device and after that to I have only one RS232 cable connected to server. My problem is in that RFID card readers are simple (stupid) devices and there I am not able to change anything, but Ill like to know from which specific device is coming input. What Ill want to do with that new device which need to join four readers is to add suffix or prefix to string that sent from RFID card reader. So I can identify device that is sending card number to my server. I am developing my system on Win platforms.

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6 Answers

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RS-232 is point to point, so you cannot do anything else other than bring all the devices into your server on separate lines. You could use some sort of concentrator (either pure RS-232, or RS-232/USB converter), however the physical link problem you are dealing with was solved a long time ago in industrial processes and is RS-485. This allows point to point and multi-drop network topologies for the physical layer and has a lot longer range than RS232. In order to communicate with the devices you would have to run some sort of intelligent protocol that addresses each node. (Typically something like Modbus would be used). Thus your terminal equipment would then have to become intelligent enough to respond to the network addressing. This would also fulfil your requirement of adding a prefix to the data stream - it would end up being the network address. But the downside is that your software implementation on the server side also has to become more complex in order to speak the same protocol and allow for things like network configuration.

IMHO if you only have a small number of devices to connect to, you are probably better off using a multi-port serial card in your server and running all the devices to a central location. However this is predicated on how much cabling you will have to run in parallel. It could be that the overall cabling cost could push you into the realms of an intelligently designed network system (pun intended).

Edit Note that given your application is a parking garage, I imagine that your cabling lengths are going to easily exceed RS-232 maximum distances (50 feet). In which case you will also have to be implementing some sort of media converter on each end of the cable so you end up with something like:

RFID Device ==> RS232/RS485 ====(long cable)====> RS485/RS232 ===> Serial port ===> Server

(Note that you could also replace RS485 with your fav physical layer - including fiber)

Edit Not that I have used them, but you can get RS232/Ethernet extenders that could also work in place of RS485. Cat5 is a well known cabling system (so should be cheaper to manage) and Ethernet has a longer range that RS232. But once you exceed the Cat5 range (100m, about 328 feet) you will need to have some sort of media converter as well.

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Not with bare RS232. It may be possible to create a "hub" that would be able to collect all the communications from each device and send them to the PC preceded with a header, but the easiest thing to do is just to run USB and use converters at the end.

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Thanks on point me on that, Problem with USB is that they act like Keyboard mostly time. Than I have more headache to recognize which device is sending data, Can You please recommended any USB RFID card reader which can fit to my needs. –  adopilot Jun 1 '10 at 14:30
    
Converters. As in USB-to-RS232 converters. –  Ignacio Vazquez-Abrams Jun 1 '10 at 14:44
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You could get a USB Hub and enough USB Serial Adapters for the RFID Devices, but there are distance limitations to RS232.

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4-port serial cards are available. The other suggestions of just using multiple USB-Serial adaptors is a fair idea too.

Early in computing history, the most common need for more serial devices than you had ports was for connecting all your serial terminals (VT100's or whatnot). The class of devices to solve this problem became known as "terminal servers". The job of a terminal server is to provide a lot of physical serial ports for terminals or other serial devices to connect to, and tunnel their connections over a network connection to pseudo-terminal devices on your host. Even after serial terminals went away, terminal servers lived on as a way to connect a set of modems for PPP dial-in, and also as a way to connect to all the serial console ports of all your networking equipment and servers in a rack. So you might want to look at Terminal Servers. Enough different people have needed to use terminal servers in enough different ways that there are probably already solutions to your problem.

Also, beware that both RS-232 and USB have length limitations, so you'll probably already need to use some special long-distance RS-232 repeater of some sort. You'd probably be best served by talking to your RFID reader company and asking them what equipment they've used to solve these kinds of problems in the past.

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Look for a RS 485 port. RS 485 also known as EIA 485. It supports multiple connections.

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You may want to look into this site. Look for Serial Data Switches. They offer the multi-RS232 serial ports (up to 17) controlled by ONE RS232 serial port.

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