Radio Modems (RM96XX) FAQs

The maximum transmission distance depends on a number of factors, such as type of antenna used, height of antenna, line-of-sight etc.

Firstly, the output power of the RM96XX series is 500mW, designed for use in the low power license free bands. This relatively low output power will limit the maximum distance, but because the receiver has good sensitivity and the data speed is relatively low, this still allows a reasonable distance.

The 1/2 Wave whip antennas have no gain (in fact probably about –2dB) so are suitable for short range, up to about 1km.

The Dipole antennas are omni-directional and have 0dB gain. These are normally mounted outside on poles, giving additional height and can therefore cover about 5km.

If a high gain Yagi antenna is fitted to the receiver, distances of 10km and above can be achieved.

These figures are reasonably conservative and many customers achieve far more, in some cases over 20km. It depends very much on the local terrain, the height of the antenna and having a clear line-of-sight. For a full description see the line-of-sight page.

If you are trying to operate non-line-of-sight, then the performance is difficult to predict. It depends entirely on the nature of the obstruction. Metal clad buildings or reinforced walls will totally block the signal. You may actually receive a signal, but this is probably being reflected, rather than taking a direct path.

Other buildings will seriously attenuate the signal, possibly by about 50% for each wall. This is just an estimate because it depends on the walls structure and the angle that the signal hits the wall.

Trees and other foliage will also affect the signal and may be variable in different weather conditions i.e. wet trees with dense leaves will present a greater barrier than a dry tree in winter.

Yes, a modem can be set-up using the on-board DIP switches to work in a number of different modes. There are three different repeater functions, depending on the application. The different operating modes and setting instructions are detailed in the user manual supplied with the units.

The LEDs are very useful for diagnostics and have two main functions.

During programming using the DIP switches, the LEDs indicate the position of the DIP switches. For example, if the rotary switch is turned to position 3, then the LEDs will indicate which of the 8 switches are ON. (The 8 LEDs correspond to the 8 DIP switches). This makes it easy to check the programming in each rotary switch position.

When in normal ‘run mode’ (rotary switch position ‘0’), the LEDs indicate the current status.

For example, when transmitting data, LED labelled D15 will flash. D10, the carrier detect is very useful for indicating interference. When the modem is in the normal receive mode and it is not receiving any data from another valid modem (during this test other modems could be powered down), this LED should not be active. If it is flashing or permanently on, this indicates interference. You may need to change channels.

The LEDs can also be used as a signal strength indicator in the special test mode. For a full description see the user manual.

Changing channels is very simple using the on-board DIP switches. In normal run mode, the DIP switches 3 to 8 set the current channel. With all the switches OFF, the lowest channel is selected and with all switches ON the highest channel is selected.

For a full description see the user manual.

Yes. There is a special test mode that allows the signal strength to be checked in both directions. The LEDs act as a signal strength indicator. The test mode is set using the DIP switches.

On the transmitter, set the rotary switch to position 1 and DIP switch 1 to ON. The modem will permanently transmit. On the receiver, set the rotary switch to position 1 and DIP switch 1 to OFF. The LEDs will act as a signal strength meter. See the user manual for more information.

The modem is supplied with a set of default values that we find are the most commonly used. These are shown in bold in the user manual. If you need to change any of the settings, this can be achieved using the on-board DIP switches.

As an example, the radio modem may be supplied configured to use the RS232 port (normal default). If you wish to use the RS485 port, for controlling a video camera for example, set the rotary switch to position 3 and DIP switch 8 to ON. Press the store button to save this setting then return to run mode (rotary switch position 0).

Note: When changing a setting, pressing the store button will save the current position of all 8 DIP switches, not just switch 8 that you have changed. Make sure all 8 are in the correct position before pressing store.

See the user manual for the full list of settings available.

The modem can be set for baud rates, bits and parity, so it can be set up to work with most controllers. We have tested it with many of the common control protocols and have produced a series of application notes.

Go to the downloads section and check for your particular protocol. If it is not there, check the controllers manual and set the modem accordingly.

All the settings are made using the on-board DIP switches. See the user manual for full details.

Yes there are a number of fairly simple checks to identify the problem and confirm whether or not the unit is faulty.

Firstly, check the status of the LEDs.  The LEDs are labelled D10 to D17, with D10 towards the edge of the board and D17 near the centre.

Make sure you have antennas connected to the modems before powering the units or they may be damaged.  With the unit powered, D17 and D16 should both light red, indicating power on. If not check the power supply.
D14 should light yellow. This is the receive LED indicating that the modem is in receive or standby mode, awaiting data input.

No other LEDs should be lit or flashing.

If you have a pair of modems, it is useful to have them both on the bench for this next test. It can be carried out on-site, but it will require someone at each end, in contact by mobile phone.

Set modem ‘A’ into test transmit mode. Set the rotary switch to position 1 and DIP switch 1 to ON. This will transmit permanently and D15, the yellow LED should light.

Set modem ‘B’ into test mode receive. Set the rotary switch to position 1 and DIP switch 1 to OFF. This will set the modem to receive mode and the LEDs should indicate the signal strength. If the units are side-by-side on the bench, this should show maximum signal strength. If the test is carried out in the field, you should see at least 3 LEDs lit (2 x red and 1 yellow). This is about the minimum acceptable signal strength.

You can repeat the test with modem ‘B’ in transmit mode and ‘A’ in receive mode. This test proves that the radio is working and the 2 units can communicate.

One final test is to check for interference. Power up just one of the modems set to test receive mode. With the other modem turned off, you should see no activity on the LEDs (signal strength indicator). If there is any activity, this indicates another user on the channel. If the interference is significant, high signal strength, try changing channels and try again until you find a clear channel. Remember to change channels on the other modem(s).

Once you have connected your equipment, if there are still problems, check all the settings. These are detailed on pages 14 and 15 of the user manual. It may be necessary to change settings to suit your equipment.

The RM96XX series of Radio Modems is compatible with most of the commonly used protocols and is therefore capable of controlling most PTZ cameras.

The modems however can be set for various baud rates, number of bits and parity, so they must be set correctly for your system. See page 14 of the user manual for details of the settings. Most PTZ controllers use RS485, so make sure the modem is set to RS485, rather than RS232.

We have produced a set of application notes covering the main PTZ protocols – see the downloads section.

If you have no communication between the controller and camera, check the modems as per the previous answer above. If the modems are working, the problem may be the RS485 connection. Make sure you have the correct polarity. Try changing the twisted pair over at both ends – in other words, try all combinations. On rare occasions, there may be a problem with RS485 levels and you may need to fit a terminating resistor or bias resistors. If in doubt, contact RDT technical support.

This is probably caused by the camera missing a command from the controller. When the joystick is moved to the left, most controllers send a number of commands, but once released, they send just one stop command, that could be missed.

Check that the modems are working correctly and that you have a good signal strength, as described above. If they appear to be working, check carefully for interference. You may need to set a unit in test mode receive and watch the LEDs for some time. The interference may be intermittent, but just enough to cause the occasional missed command.

Check also that all the modem settings are correct for your protocol. See the downloads section for the application notes.