RS-232 full voltage range

26th Thursday, 2012  |  Engineering News, Life at LAVA  |  no comments

When it comes to RS-232 electrical characteristics, there is one important thing to note: the voltage range for signalling in RS-232 is very wide, from 3 to 15 volts, plus or minus. The implication is that designers of serial port boards, such as LAVA, have wide latitude for electrical design, according to the specification. In reality, if a board designer takes the minimal approach, their users might run into disappointment. Let me explain.

All LAVA PCIe serial ports are engineered to provide a near-full voltage signalling range on their serial port pins. Specifically, LAVA serial ports comply fully to serial port requirements by generating a voltage range of at least +12V to -12V.

This is important when your serial data rate is high, your cable runs are long, or there is a greater than ideal amount of electrical “noise” in the area. It also matters when the serial peripheral attached makes a draw on the serial port. In those situations the added voltage differential can make the difference between a connection that works and one that doesn’t — just as with a car, you need a little more horsepower when you start to pull a trailer.

PCI and PCIe serial port add-in cards draw their power from the bus on which they are attached. In the case of PCI, there is enough voltage range on the bus to allow a serial port adapter card to operate within the specified voltage range; in the case of PCIe, there is not.

induction coil

Induction coil

You can see, on all LAVA PCIe boards with serial ports, the little copper induction coil that contributes to this capability. Check out other companies’ products on the web and you’ll see that they have cut corners here: they are in many cases not generating the -12V signal — is the risk to the reliability of your connection really worth it?