Teardown: Digi International Edgeport/8 RS-232 to USB Converter

In the things that have happened to me last week, getting a Digi International Edgeport/8 donated to me was one of the more exciting events.

A Little Bit of History

It wasn’t that long ago that if you wanted to add something to a computer externally, chances are you would have used the serial port. This could be used to connect a ball mouse, a modem, a null-modem cable for transferring data between computers, a light-pen or digitizing tablet, amongst other devices. Most home users didn’t have needs for many serial ports – most computers came with two by default as part of a multi-I/O card, with either the 8250, 16440 or 16550 UARTs, of which the later ones were preferred for high speed modem usage (19,200bps and above). The serial port transmitted data asynchronously with start and stop bit framing, and operated commonly at rates of 300, 2400, 4800, 9600, 19200, 38400, 57600 and 115200bps. Other rates were available where the right frequency oscillator, prescaler/divider values were programmed into the UART and used mainly for special applications.

Two ports weren’t enough for more advanced users. Of course, you could add another serial card and get another two ports, but after getting to that stage, you would often find issues with IRQ sharing leading to strange behaviour (e.g. my modem on COM3 only transmits data when I move my mouse connected to COM1) and silly workarounds (e.g. polling for data consuming cast amounts of CPU time). The configurability of many of these add-in boards was limited for I/O base address and IRQs, leading to more headaches if you wished to have more ports.

However, more ports was exactly what you needed if you wanted to run a set of dumb terminals in a mainframe style set-up, or if you wanted to run a BBS (the precursor to the internet forums) which often required having a bank of modems to ensure your visitors could get on reasonably easily. Similar set-ups with banks of modems could be needed if you wanted to set up a fax gateway for an organization. Otherwise, you might be running a point-of-sale style system where you have a custom keyboard, docket printer, cash drawer, barcode scanner, and swipe-card reader all of the serial variety.

This is how Digi International came into being in 1985. Their product portfolio initially consisted of their DigiBoard, an ISA to multi-port Serial card, often recommended in product manuals for BBS and fax gateway softwares. I never owned one of them, but I’ve always heard about them.

However, despite the Edgeport being branded a Digi product, its heritage is actually rather different. Once serial and parallel ports started to be displaced thanks to USB and the drive for more compact computers, ways of connecting these devices to more modern machines were sought and the Edgeport/8 sought to fill this gap. This product was actually designed by Inside Out Networks, formerly taking the URL ionetworks.com. This company specialised in high performance USB adaptation solutions, with the Edgeport already showing up in their catalogues as of 1999. This particular solution became quite popular amongst industrial users and garnered the support of certain companies as the approved method of migrating their serial based products to USB.

How Digi came to be involved was that they acquired Inside Out Networks in September 2000, citing that they are a “natural partner for many of the solutions we offer.” By 2006, the URL redirected to digi.com.

Digi International is still alive and well today, and still sells and provides support for these products with drivers supporting Windows 8 (officially). Rather surprisingly to me, they are the company behind the rather popular Xbee modular radios used very often with Arduinos and other microcontrollers.

The Goodies

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Rather surprisingly, the unit was in its original box, with the tape unopened except by the donor to check the contents. It’s probably a case of “bought as a spare” or “thought we needed it, but we didn’t,” but that’s all good for me.

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From the side, it looks to have been supplied by a scientific equipment company called MDS SCIEX. The unit seems to have been labelled in Week 5 of 2008, making it eight years old.

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Inside, the unit was still sitting in its plastic-wrap on cardboard cradle.

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The unit itself was very much a plastic box, with the front still adorned by the Inside Out Networks branding, maybe using up the old stock of front-labels. One status LED window is provided on the front.

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A distinctive feature is the corner which is slightly recessed and made smaller, with ribbing. Otherwise, it feels slightly hollow and slightly like a jiffy box you used to pick up from your local electronics shop.

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The rear features 8 DB-9 ports in close proximity, with just enough space for regular plugs to clear. A single USB B connector provides data and power, with no external power required. This seems to be quite an efficient design.

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The underside has it branded Digi with dating to Week 47 of 2007 (a few weeks earlier).

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The unit also comes with a USB cable, a drivers and manuals CD and a support leaflet.

If you wanted to purchase this today, it is still available. Digikey has it listed for US$455, whereas Express Systems & Peripherals claims a price of US$344.43 and Bressner Limited has it for GBP300. It is also available from Synnex.

In an attempt to try and find out the actual cost of the unit, I was digging around and I found a tender spreadsheet where Applied Biosystems were sourcing the unit from the same supplier with a claimed list price of US$1000 as of 2009, with a discount to US$950. Pricey!

edgeport-price

Teardown

I suppose now that I have the unit, it’s worth trying to find out how it works and what it’s made of. To open the unit, three feet have to be removed and four screws have to be undone. The case parts although the front label holds the three segments together, and the board can be slid out taking care not to lose the standoffs.

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The converter is mainly comprised of four Texas Instruments TUSB5052PZ USB to 2-port serial with configurable hub, each with its own ST 24C02WP serial EEPROM for ID and configuration. The hub is used to daisy chain a total of four chips together to form the 8 port model. The outputs of these go to Texas Instruments MAX211C level converters which feature a charge pump circuitry to produce RS-232 levels from TTL signals and vice-versa. The board itself is copyright Inside Out Networks in 2004, and the date of the PCB is Week 42 of 2007.

Many of the footprints on the PCB are unoccupied and are likely used to build other models of converter, such as the galvanically isolated enhanced safety models and possibly even the RS-422 models.

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The underside of the board does not house any particular components. For the price, it does seem somewhat devoid of components.

In Use

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The first step was to install the drivers. I decided to forego the drivers offered on the CD, as they were certain to be out of date, and went straight to the ones offered online. It was a hassle free install, and off we went. I did note that plugging in the device without drivers led to no drivers being installed as they could not be sourced from Windows Update, but apparently there is kernel level support for Edgeport devices in Linux.

edgeport-config-utilityOf course, while USB to serial adapters are fairly common products today, their driver quality and support do vary depending on the chip. The Edgeport drivers come with a configuration utility for you to tweak some settings which can ease compatibility issues.

This can be accessed from the Port Flags page, which can allow you to toggle low-latency mode, baud rate remapping, ignore flush, fast write/reads, ignoring transmit purge and delaying setting the port configuration.

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It also offers a self-testing page which allows you to test the ports by digital loopback and by external loopback (requiring the loopback plug wired to their specifications).

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There is the more boring pages where it shows you the basic information of the device and the ability to rename or reassign the COM port numbers.

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Aside from that, there are also more information and advanced configuration pages.

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This is a more comprehensive offering of options than any other USB to serial device I have used so far. However, the one thing I am interested in is seeing it push data back and forth – for that I got out my two trusty null modem cables and hooked them up between Port 1 and 2; and Port 3 and 4.

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Starting up four sessions of Hyperterminal, I checked the maximum baud rate it would respond to. Surprisingly, rates up to 460800bps were working, whereas 921600bps would respond with an error opening the port. Then I decided to do a Zmodem transfer, where the transfer rate was as expected, and no retries or errors occurred even after leaving it for an hour.

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This shows me that the flow control is pretty good, with sufficient buffering and good drivers to make sure nothing is lost even at high bitrates beyond their claimed 230400bps supported.

For a full-speed USB device on a 12Mbit/s maximum bus, I wondered just how many serial connections could realistically be hosted. Often 10% of the bus is reserved for management, and assuming no other devices or overheads, that leaves 10.8Mbit/s available. If each port operated at 230400bps full duplex, that means a maximum of about 23 ports, so thus it seems their Edgeport/16 model being the largest they offer is a reflection of this fact. In reality as most devices operate with a lower baud rate, or half-duplex, this is probably unlikely to be a real constraint.

Maybe I’ll find myself a set of dumb terminals, or maybe start up a bank of modems … but what a shame it’s not easy or cheap to get eight phone lines at home … and the software … *thought fades into the distance*.

Conclusion

This unit isn’t as old as I first thought, but it definitely still works under a modern OS and has also Linux support (although I didn’t try it with Linux yet). The fact that it’s still available on the market today, essentially unchanged for the better part of a decade and a half really is a rare sight in technology. It definitely works as expected, and seems to have a slight bonus when it comes to faster baud rates. Null-modem based transfers saw no overflows/errors indicating a very well implemented hardware flow control, sufficient buffering and good drivers overall. Compared to other USB to serial products on the market, the Edgeport drivers do offer more configurability and even VXD compatibility on older OSes to allow for emulation of hardware serial ports in terms of IRQ/Base Address style access of more than 4 COM ports. These may be the options that can save your bacon when it comes to finicky industrial systems with very fixed design expectations. I suppose that would be what you expect when you pay this amount for the hardware, from a brand as long-standing and reputed as Digi.

Note: For my Australian visitors who may be suffering inconsistent and slow performance as of late today, it seems that Cloudflare is having some difficulties which they haven’t noticed or seems to only affect this site. Accesses outside Australia seem to be unaffected, as is direct access to the origin server (which is what I’m using to actually write the article because it’s too annoying otherwise). Unfortunately, this is something that even Developer Mode in CloudFlare doesn’t get around. My apologies for this, and hopefully you can be a little patient as this hopefully clears up on its own.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
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4 Responses to Teardown: Digi International Edgeport/8 RS-232 to USB Converter

  1. sparcie says:

    Wow that certainly is an interesting device, think of the possibilities. I only ever saw 4 port PCI cards because no-one really needed any more ports than that. I also wasn’t aware of any devices that could go faster than 115200bps, very cool indeed.

    Oh there is so much you could do with a device like that… Dumb Terminals and modems of course, but something that I thought of is using all 8 ports to make a sort of serial based network for older PCs (that don’t do ethernet). You could use pppd and some clever routing setup on a linux (or *BSD if they support it) machine to make a LAN! It probably wouldn’t be terribly useful or fast, but definitely a bit of a challenge.

    • lui_gough says:

      Great ideas … reminds me of the old Windows 95/98 style “direct cable connection” networking that was so awfully slow.

      That being said, peripherals that go faster than 115200bps are generally rare, but I’ve had a few Swann SpeedDemon modems which claim 230400bps support, mainly as V.44 compression (under V.92 connections) can achieve > 5 to 1 on certain text-based transmissions, and with a line rate of 52000bps achievable on good lines, you could easily see serial being the bottleneck even with a higher than normal serial speed, although this is rarely the case.

      Serial adapters on computers generally didn’t do faster than 115200bps mainly due to the reference clock and prescaler values – some ISA boards I’ve come across allow you to change the prescaler value to allow for faster bauds but at the cost of losing the lower baud options.

      USB to serial adapters by Prolific and FTDI seem to have no trouble with 921600bps, although at such speeds with such voltage swings, errors seem to crop in occasionally.

      Thanks again for your comments on all of my recent posts – definitely inspiring and enlightening.

      – Gough

  2. Jimmy says:

    When you did your testing, what kind of flow control did you use? I am having issues with this same converter in which hardware flow control seems to have some hickups in OpenVMS.

    Thanks,
    Jimmy

    • lui_gough says:

      All testing was done with hardware RTS/CTS flow control, the null modem cable used was wired with the hardware flow control lines correctly assigned. I did not test XON/XOFF software flow control as it’s very rarely used in my experience.

      – Gough

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