# How to set frequency and distance in ARINC 429?

How can I set frequency & read distance in ARINC 429?

For example can you give an example to tune a 1105.5 MHz and read 25786 ft.

LABEL | INFORMATION CONTAINED IN LABEL | FORMAT | SPEED
------+--------------------------------+--------+----------
035   | DME Frequency                  | BCD    | Low Speed
202   | DME Distance                   | BNR    | Low Speed

• Your formatting was pretty confusing. I took a guess at fixing it up. – abelenky Feb 14 '18 at 20:53
• In what context? ARINC 429 is just a protocol, and its content is very equipment-dependent. If you have two units on the same bus, usually the SDI bits select one; otherwise some of the data bits may select a channel. You need to read documentation on your particular unit. – Zeus Feb 15 '18 at 2:30

The first thing you need to understand is that DMEs are not tuned by their actual frequency but by the frequency of the paired VOR/VHF Com, ILS, or MLS frequency. For the vast majority of operations, this means a VOR or ILS frequency. This frequency as well as mode settings are coded in the Label 035 as shown below:

And the text from reference to Section 4.1.2:

So you wouldn't attempt to tune 1105.5 MHz. First, it's not a valid DME frequency, as DME channels are assigned to whole MHz. 1105 MHz is valid, but would be tuned by the paired frequency of 113.40 or 113.45 MHz (VOR) or 5067.6 or 5067.9 MHz (MLS). You don't need to know the actual DME frequency as the DME knows the pairing and the DME ground stations are charted using the paired frequency. The snapshot above includes an example of coding to different frequencies.

The unit you mention in the comments is a two channel DME. DMEs are available with up to 5 channels. Tuning of a specific channel is done using bits 11-13 (DIR FREQ 1-5). Labels are sent sequentially. The DME expects a minimum of 5 labels per second and any one channel label must be received at least once per second.

The DME distance is output on label 202 as an unsigned twos complement fraction as shown in the following from ARINC 709:

There are 16 significant bits. Bit 28 is equal to 256 NM and each successive bit is half the value the previous. (e.g., bit 27 = 128 NM) The LSB is Bit 14 and is approximate 0.008 NM.

The distance and frequency labels are output from the unit in blocks so you can correlate distances to stations.