Introduction to the Wireless Modem

A wireless modem is a type of radio converter that is able to convert information that exists on a wired network, such as one connected using an RS-232 or RS-485 interface, etc., to a wireless network. The wireless modem is designed for data acquisition and control applications between a host and any remote sensors that are connected to the network. It is also useful for those applications where it is inconvenient or impractical to install physical cable wire. Not only can a wireless modem be used in peer-to-peer mode, it can also be used in a multi-point structure.

Normally, transmissions at a lower frequency or at a lower speed data rate are able to be transmitted over a longer distance. However, the effective communication distance for a wireless module is very much dependent on environment where it is installed. Obstacles, such as walls or metal, etc., will all influence the communication distance. The best approach when installing a wireless module is to select a suitable location that takes these factors into consideration.

The 429 MHz Wireless Band
Although the 429 MHz wireless spectrum is low speed, it can provide wireless communication over a wide area. Generally, the RF Baud Rate is similar to that of an RS-485 device, i.e., from 650 bps to 115200 bps. However, the maximum line of sight (LOS) transmission distance that can be achieved at the lowest Baud Rate of 650 bps is around 2 km.

Consequently, this frequency is only suitable when the application requires less data to be transmitted, or when there is a long time interval between transmissions, but the transmission is over a long distance, such as when monitoring the temperature or rainfall, or when monitoring and controlling solar panels, etc.

Note that 433 MHz is the ISM band for use in ITU Region 1, which is generally Europe, Africa, Russia, etc., and 429 MHz is the ISM band for Japan.

Application Case Study - Collecting data from solar panels at a data center

As a result of increased environmental awareness, the installation of solar panels on the roof of a building to save energy has become common practice. However, when these solar panels need to be monitored at a central control room, installing physical wiring between the various buildings is generally impractical. As frequent monitoring of these solar panels is not explicitly required, implementing a network of RFU-400 modules operating at low speed and over a long distance can operate as viable alternative to physical wiring.

The 915 MHz Wireless Band
The 915 MHz wireless spectrum ranges between 429 MHz and 2.4 GHz. The communication distance over this range is shorter than that of the 429 MHz frequency, but is longer than for the 2.4 GHz frequency. Additionally, the communication speed at this frequency is slower than 2.4 GHz, but is faster than for the 429 MHz frequency. Consequently, the 915 MHz frequency is the medium of choice for communication speed and distance. However, in some countries, such as in America, the 429 MHz band can't be used to transmit data as it is dedicated to Amateur Radio and Radiolocation services, meaning that the 915 MHz frequency is a more suitable choice than 2.4 GHz when considering the communication distance.

Note that 915 MHz is an ISM band only for use in ITU Region 2, which is mainly North, Central and South America (plus a few others like Australia and India).

Application Case Study - Crane motion control

In a traditional crane system, there is typically a cable connection between the remote control site and the crane. However, there is a high potential that this cable can become damaged either by vehicular traffic or through the operation of the crane itself, causing inconvenience and downtime of it needs to be repaired or replaced. Using an SST-900B module in conjunction with either control panel or a joystick rather than the cable not only removes the possibility of damage to the cable, but also extends the range of movement of the crane that was previously limited by the cable.

The 2.4 GHz Wireless Band
The 2.4 GHz wireless spectrum operates in the ISM band using a transmission frequency range of between 2405 and 2480 MHz. Unlike the 429 MHz and 915 MHz bands that can only be used in certain countries, the 2.4 GHz ISM band can be used worldwide, and is based on Direct Sequence Spread Spectrum (DSSS) RF technology that is able to resist noise and other interference. The communication distance for the 2.4 GHz band is up to 700 m.

Application Case Study - Multi-point Communication

In this example, a number of RFU-2400 modules are acting as wireless converters responsible for broadcasting data to multiple remote devices in Broadcast (half-duplex) mode. A variety of PLCs are wirelessly connected to one or more RFU-2400 devices and the remote I/O modules are each connected to additional RFU-2400 devices via the serial RS-485 port. The RFU-2400 devices are then able to communicate wirelessly to enable data transfer between the PLCs and the I/O modules.