What is a Serial Device Server?

The Problem

One of the major and real limitations of RS-232, and to a lesser degree other serial modes like RS-422 and RS-485, is cable length. For RS-232, cables getting beyond 50 feet are pushing it. While this distance is three times farther than USB's cable limits, in many applications 50 feet is still not very far. Run a serial cable from a computer up into a drop ceiling, along for 20 feet, and back down to a printer and you're just about there.

A second limitation of RS-232 is that peripherals on a serial port are only available to a user on the computer that has that serial port.

The Solution

These two limitations are both addressed by a device called a "serial device server." As the name suggests, a serial device server is networked device, almost always an Ethernet device, that provides network access to a serial port. It is typically a small enclosure with a serial port (and sometimes with several serial ports) on one side and a network port on the other. At a minimum, its function is to receive data on its serial port and then to prepare it for sending across its network port. The converse is also true: a serial device server will receive network packets with serial data, and prepare them for delivery as serial data on the serial port .


Ether-Serial Link one-port

The function of a serial device server is simple therefore, but the details are complicated. First, serial device servers need to be able to operate on a network — so they need a MAC address; they need to be configurable as network devices  — with an IP address,  netmask, and gateway address; they need to be able to process network traffic effectively; they need an internal IP stack; and so forth. Second, on the serial port side, they need to be configurable too — the standard serial port settings for baud rate, parity, stop bits, and flow control all need to be available. That's for starters.

In addition, a serial device server will also have some sort of security interface to prevent unintended changes or tampering; software for configuration (in the form of an application, an internal web server, a Telnet console, or some combination of these options); a variety of serial port modes of operation (more on these in later articles); and possibly a device driver that will pernit the serial port in the serial device server to appear as a virtual serial port in a computer that is accessing it.

With all these things, you can see that a serial device server needs, on the hardware side, a power supply, physical connectors, memory, a processor, a network interface, a serial port Universal Asynchronous Receiver-Transmitter chip (UART), and the supporting componentry to make this all work.

Another way to think about a serial device server is as a specialized single-purpose computer — consider it a PC with a network interface, a serial port, memory, and a processor — but without a keyboard, mouse, hard drive, video display, fans, or operating system. It will however have embedded software to manage the tasks it is meant to do. Indeed, a computer could be configured to do the job of a serial device server, but at greater cost and with less reliablility.

In fact, LAVA's Ether-Serial Link device servers have half a megabyte of flash memory and an AMD 8086 processor — in these regards they are IBM PCs, stripped down to a single-purpose hardware and firmware.

So back to the "solution" part of this article — first, as network devices, serial device servers can make their serial ports available anywhere — on a LAN, a WAN, or over the Internet. Cable distance limitations — gone.

Second, serial device servers can make (or disallow) access to their serial ports by any number of clients — the single-user model is finished, with the proviso that just one user can access the serial port at a time. This actually makes sense, when you think about it.

The Conclusion

Serial device servers solve the two major limitations of serial ports, and especially those of RS-232, economically and efficiently. Further articles will fill in this picture more fully.

All About Serial

Welcome to an informational web site sponsored by LAVA Computer MFG. Inc. Detailed information on LAVA and its products can be found at www.lavalink.com

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