DACS App Note

RIU limitations on a TDM ring (#30)


The purpose of this application note is to provide the user with information regarding the number of RIUs that can be put on a TDM ring.


At the time of this writing, the latest RIU hardware version is 4.1. In the 4.xx series of Model Builder, 4.04e and RIU firmware version 2.3.1 are the most recent. In the 3.xx series of Model Builder, 3.12i and RIU firmware version 1.1.1 are the most recent.

There are a total of 30 data slots available on a TDM ring. Each separate voice stream consumes one data slot each. The two serial port devices on an RIU occupy a single data slot together. For example, a four channel RIU with four input voice streams and two serial port devices connected will consume 5 data slots.

The user must also take into account the number of audio output streams available. There are initially 30 data slots available. Each RIU consumes a single data slot plus audio. As the number of RIUs on a ring increases the number of data slots, and hence audio output streams, decreases.

A total of 14 RIUs can be put on a TDM ring. Refer to the table below for a breakdown of available input and output channels versus the number of RIUs on the ring.

#RIUs on ring #Audio streams possible from each RIU #Audio output streams available
1 4 24
2 4 24
3 4 24
4 4 24
5 4 24
6 4 24
7 3 22
8 2 22
9 2 20
10 2 20
11 1 18
12 1 18
13 1 16
14 1 16

Model Configuration Notes

The default value for the number of RIUs on a TDM ring is 6. This value can be viewed/modified from the Model Configuration Window within Model Builder. The field "No RIUs on TDM" can be set on a per model/TDM basis. There is no detrimental effect to MB operation if you are using fewer RIUs on a ring then the default value specified.

If it is necessary to have more than 6 RIUs, increase the number in even (8, 10, 12) increments until the desired value has been reached. Ensure that you save the model before exiting MB in order for the change to take effect.

This document will be updated as later revisions of Model Builder and RIU firmware are released.