e4d47c27__DataSheet_for_product_XE2422H__description
[pin] 9 [name] RXD [description] 
RXD provides the path for received data and modem responses to be sent from the modem to the host controller. A logic high on RXD represents a “Mark” condition and a logic low represents a “Space” condition.
Cermetek DataSheet for product XE2422H
description