e4d47c27__DataSheet_for_product_XE2422H__description
[pin] 8 [name] txd [description] 
TXD provides the path for transmitted data and modem commands to be passed from the host controller to the modem. A logic high on TXD represents a “Mark” condition and a logic low represents a “Space” condition.
Cermetek DataSheet for product XE2422H
description