There are three factors required when choosing an Ai 429 product: Function/Class Model and Channel count. What the product’s does is its function (or class) and there are three: Transceiver Tester and Mixer. The product comes in two different models: Serial model for a RS 232 422 and USB model for a USB to UART. The product’s channel count is the number of available ARINC 429 channels: 4, 8, 12, 16 denoting (Tx & Rx) channel pairs.