Chapter 3 Connecting loggers to computers

3.1 Serial port

Serial protocol is the simplest way to communicate between computers and hardware. Physical serial ports have been removed from new computers, but the protocol is still used by numerous devices with USB-adapters.

• Has drivers for any operating system and can be programmed wit almost any programming language.
• Most microcontrollers have a serial port.
• The user needs to set the baudrate (=speed) and sometimes other communication protocols. Maximum speed 115.2 kbits/s.
• Only two devices can be communicate using the same port.
• Several devices interface with microcontrollers with serial port e.g. GPS, LCDs, GSM and GPRS.

Serial ports were originally physical ports in the computer, but hardware serial ports are now longer used in similar form because they have been replaced by USB-ports. However the communication protocol is so simple and convenient to program that e.g microcontrollers use the same protocol and USB-adapters are used on the computer to from a serial port. You can find the serial ports connected to your computer from “Device manager”(You can type devmgmt.msc ) in windows run dialog (press win+R) to open Device Manager.

The easiest way to see traffic from a serial port is to use a terminal program such as free Bray’s Terminal ¹

There are different voltage levels that the serial port protocol can use: microcontrollers typically use TTL level (3.3V or 5V) and computers use 15V, therefore you need to use correct adapter for each device. The communication protocol with different voltage levels is the same so you can use a level shifter circuit to if you need to connect two devices using a different voltage level.

Some instruments still use the +15V serial ports (e.g. Agilent 34970A panel meter that is used in the exercises.) The advantage of the higher voltage level is that it can be used with longer cables.

Serial ports have several different settings for baudrate and other traffic options. Also the wiring may differ between devices. So if you experience problems always check the traffic settings and make sure you use the correct cable. Open the cable to check if you can or make a new one, if you can’t open a cable without breaking it you can use a multimeter to check which wires are connected.

3.2 USB (universal serial bus)

USB-port has become the most popular connection for dedicated DAQ boards. USB has generic drivers for keyboards, mice, printers etc. which makes it possible for devices from diffent vendors to work without separate drivers.

unfortunately there is no device class in the USB standard for for DAQ cards. Therefore you Vendor specific drivers, that need to be programmed separately for each manufacturer and you may not have drivers for all operating systems.

• Programming USB devices is usually more complex than serial port devices.
• Generally used USB 2.0 is quite fast: data transmission at 480Mbit/s
• Newer USB 3.0 standard is specified up to 5 bBit/s, however it is not yet widely used in data acquisition hardware.
• USB port can be used to power devices: 5V, 500 mA.
• Only one device can be connected per port, but 127/controller are allowed using a hub.

3.3 Ethernet

Can be connected directly directly to computer or local network, or to Internet.

• Speed 10, 100 or 1000 Mbit/s.
• Devices get an IP address and multiple devices can be connected via internet or a local network.
• Traffic using TCP/IP or UDP traffic.
• Fairly simple to program. Works on any operating system and can be interfaced with microcontrollers.
• Popular in industrial devices.

3.4 Wireless connections

There are several possibilities for wireless connections that vary in speed , power consumption and signal transmission range . It pays off to spend some time choosing the right one for the application.

The range of the radio signal depends on:

• Frequency. Higher frequencies can transfer data faster, but have shorter range.
• Obstacles. Structures attenuate signals considerably.
• Transmission power
• Antennas. Transmitting and receiving antennas.
• Receiver sensitivity and SNR.

3.5 Exercises

1. Compare serialport, USB and Ethernet connection. What is the advantage of each type?
2. What factors influence the range of a radio link?