The WM232-UHF, an Australian designed and manufactured radio modem, is a low-cost, plug-'n'-play connectivity solution for devices that communicate via serial links. It supports on-air data rates of up to 14.4kbps and features data encryption, error checking and acknowledged data transfer.

The basics

The modem is housed in a small ABS plastic case, measuring about 103mm (L) x 62mm (W) x 24mm (D). Antenna hookup is via an SMA connector that protrudes through one end of the case. A tiny (60mm) helical stubby antenna can be mounted directly onto the SMA connector, making a very tidy little package.

Features Easy configuration via PC, laptop or PDA. Point to point or multipoint network capability. Repeater support for increased range. Serial port speeds of 600 to 115200 baud. On-air data speeds of 600 to 14400 baud. On-air data encryption and error checking. Diagnostic "Radar" and "Ping" functions. DTR-controlled "sleep" mode Internal 9V battery or external DC plugpack. Range of up to 500m with coax-fed half-wave antenna. CE & Australian C-Tick approval for use on licence-free UHF Band.

Power for the unit can be provided by either an internal 9V PP3 battery or an external 7-15V DC source. A 9V DC plugpack was supplied with our evaluation unit. The average current drawn during transmit/receive is quoted at 40mA, falling to 15mA when idle.

This means that an external power source will be necessary for most applications. In power-down mode, consumption drops to a more respectable 400μA.

Connection to the modem is made via a 9-pin female 'D' connector. Pin assignments conform to the PC modem (DCE) standard, so for connection to a PC (DTE) serial port all that's required is a 9-way pin to pin cable (supplied).

The unit operates in the 433.920MHz LIPD (license-free) band, with transmit power limited to 10mW (ERP) to satisfy the regulatory requirements.

Receiver sensitivity is quoted at -100dBm for a 1ppm BER (Bit Error Rate), giving a range of about 300m line-of-sight and 50m indoors with the helical stubby fitted.

An optional coax-fed, ground-independent half- wave dipole extends the range to around 500m line-of-sight. Of course, you could also fit a more elaborate "gain" antenna for much greater range, but that wouldn't be legal, now would it?

Wireless simplicity

In their simplest configuration, two WM232-UHF modems can function as a reliable two-way wireless link.

Apart from a certain amount of latency in the data exchange, the modems appear transparent to the connected devices, functioning just as they would if they were hard-wired together. This mode of operation is called "Point-to-Point".

For more demanding applications, multiple pairs of modems can operate within the same air space.

This is achieved with a two-tier addressing system, composed of "unit" and "site" addresses.

Up to 16 pairs of modems can operate within a single site address, with up to eight possible site addresses. This corresponds to a maximum of 127 possible point-to-point links.

Site addressing effectively partitions groups of modems for repeating purposes. Any WM232-UHF can operate as a dedicated repeater unit.
Up to three repeaters can be used within a network to extend the operating range. A repeater retransmits any data it receives with the same site address, allowing very flexible networks to be constructed.

It is important to note that effective data throughput for any pair in a network will decrease as the network is expanded, as the available airtime always remains constant.

An LBT (Listen Before Transmit) scheme is employed to minimise the interference that occurs when multiple modems attempt to transmit simultaneously

More about networking

A common requirement within a wireless network is to be able to monitor and control multiple nodes from a central location. This is the purpose of the "Acknowledged-Point-to- Multipoint" mode, where a single modem is configured as the master and all other modems as slaves.

Each slave is assigned a unique address, using the same addressing scheme employed in Point-to-Point mode.

In order to communicate with a particular slave modem, the host must first program the desired slave address into the master modem. In Acknowledged-Point-to-Multipoint, the master modem must initiate a link with a slave before data transfer to or from that slave can occur.

By contrast, "MultiMaster" mode allows up to 15 slave modems to initiate connection with a single master unit. In this mode, data transfers must be kept as short as possible to prevent "hogging" of available airtime.

Design your own network

If none of the above modes suit your requirements, then it is possible to design your own network from the ground up using "Broadcast Multi-drop" mode.

In this mode, all aspects of network management, including routing, node addressing and error recovery, are assumed to be host-controlled.
Data sent to a modem over its RS232 link is transmitted (broadcast) to all modems within range.

Receiving modems simply decode the data and pass it on to their hosts.

Configuration

Modem configuration is accessible, strangely enough, via configuration mode. A PC, laptop, or PDA with an RS232 port and suitable terminal software (such as HyperTerminal) are required for the job.

Normally, modems automatically enter data transfer mode at power up. To exit data transfer mode and enter configuration mode, it's just a matter of sending an "escape" sequence from the terminal program. The factory default sequence is three consecutive "+" characters.

Typing "help" at the configurator prompt displays all possible commands, whereas "list" displays current settings (see Figs. 4 & 5). A variety of useful commands allows mode and address selection and fine-tuning of network parameters.

It's also possible to modify most of these settings remotely, from a second modem via a wireless link. Of course, this feature can be disabled for security purposes if so desired.

Link testing

Two diagnostic commands have also been included to aid network setup and testing.

The "ping" command looks for a modem within range with the specified address and returns its status. This is a useful "go-no go" test and it allows verification of which unit addresses are in use.

The "radar" command allows link range and reliability to be quickly determined. This mandatory feature will help to eliminate most of the guesswork from a simple network installation.

Impressions

Setting up a simple, reliable wireless network with these modems should be a painless exercise.

And if you have something more elaborate in mind, the WM232-UHF can probably handle that too.

Getting more information

The WM232-UHF Wireless Modem is manufactured and sold in Australia by Telelink Communications, phone (07) 4934 0413 or email sales@telelink.com.au.

You can reach them on the 'net at www.radiotelemetry.net

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