How to add RS-232 serial ports to a computer

It's all well and good to talk about connecting serial-based POS peripherals such as sensors, printers, barcode scanners, and card readers to a computer — or any other type of serial peripheral for that matter — but not every computer has serial ports built in to make this possible. At other times, there are just not enough serial ports in a system to meet the need. So what's to be done?

Some of the many RS-232 adapter options

It's actually simple: there are a wide variety of methods of adding serial ports, depending on the interfaces a computer has available. The table below shows options for the PC in its various forms (desktop, server, workstation), as well as for portable computers — laptops and netbooks.

Serial port expansion options

Interface

Computer Type

PC: desktop, server, workstation

Portable: laptop, netbook

PCIe slot

yes, newer motherboards

no

Low Profile PCIe slot

yes, newer motherboards

no

PCI slot

yes

no

Low Profile PCI slot

yes

no

ISA slot

yes, legacy motherboards

no

USB port

yes, external device

yes, external device

Ethernet port

yes, external device

yes, external device

WiFi

yes, external device

yes, external device

IrDA (infrared)

yes, depending on motherboard

yes, depending on computer

ExpressCard slot

no

yes, depending on computer

CardBus slot

no

yes, legacy portables

No surprise: LAVA makes many of these interface adapters. You can see the full lineup at www.lavalink.com.

Which adapter method you choose will depend on serveral factors. The obvious one is the system you want to add the port to: you're not going to add a PCI card to a laptop, or an ExpressCard serial adapter to a normal desktop system. That's all clear in the table above.

But there are other considerations.

SPEED

Actually, I'm listing this as a consideration just to get it out of the way, because it's not really a factor at all, on the interface level. All of the buses supporting serial port expansion (with the possible rare exception of IrDA) are plenty fast enough to carry the data of an RS-232 serial connection, as well as any associated protocol and conversion overhead.

COST

This can be a consideration, as a WiFi serial adapter will cost more than an adapter installed directly into a computer, for instance. So between one option and another, the cost of adding a serial port can vary. On the other hand, none of them are terribly expensive.

CONVENIENCE

Here's where personal preference comes into play, but generally speaking it's more convenient in the longer term to add a serial port directly into slots in the system's hardware then to have an external device. So the "convenience prize" goes to PCI, PCIe, ISA, and ExpressCard expansion over external options such as USB or Ethernet.

SOFTWARE COMPATIBILITY

All these options will require some degree of software interfacing, but this is generally well-travelled ground. You will need to ensure that software drivers exist for the interface and OS you are matching up. In this area PCI and PCIe are good options, especially in Windows, where many interfaces have driver support native to the operating system.

HARDWARE COMPATIBILITY

While this is generally not much of an issue, there can be things that will trip up a user using a serial adapter. "But I thought RS-232 was RS-232?" I can hear people saying. Well, it's almost that easy, but not quite. For one thing, we at LAVA have had more than a few people come to us after trying off-brand USB to serial adapters without success. Generally the problem seems to be timing issues in signalling.

Another area of concern relates to the voltages output on the serial pins. Unfortunately, the RS-232 specification gives more leeway here is ideal. Given that some serial peripherals expect more voltage than some serial adapters deliver (especially those on USB and PCIe), problems can result. LAVA is very careful in this regard: a look at all our PCIe serial interface cards for example shows an induction coil that makes sure the voltages output from the card will properly supply RS-232 peripherals.

USE OF PHYSICAL SPACE

Do you have slots available? and if so, which should you use? A little planning here is a good idea: if, for example, you have both a PCI and a PCIe slot available, it usually makes sense to use the PCI slot for serial expansion and to keep the PCIe slot for higher-bandwidth expansion needs. On a laptop, I'd probably want to keep expansion as internal as possible and keep USB slots free, so I'd probably look at ExpressCard options if I wanted a more or less permanent serial port installed. But if my laptop serial port needs were intermittent, I'd probably go the other way, and expand on USB.

SERIAL PORT DISTANCE

Here's where the WiFi and particularly the Ethernet options shine. These port expansion options can locate a serial port where it will be used, regardless of where the "host" computer is located. Serial device servers, as Ethernet-to-serial expansion deivces are often called, do this job beautifully.

 

So you can see you have lots of options for serial port expansion; which one works best will depend primarily on your individual needs and context. You can however look to LAVA for many serial expansion options for PCIe, PCI, USB, Ethernet, and for the retro-minded among you, even ISA.