A digital data communication system enables host computer to communicate with a great number of data terminals having specified functions. Communication from the host computer proceeds through a medium or long distance, high power transceiver and through a multiplex system to one of a number of portable communication devices. The communication devices preferably contain internal power supplies, such as rechargeable NiCad battery packs. The devices may be carried on a person's belt, or may be mounted within or on a service vehicle, for example, a forklift truck for use in warehouse operations or a merchandise delivery route vehicle. The communication devices are adapted to function as communication relay devices between the host computer and selected ones of the data terminals. Each communication device includes a first type, low power transceiver to communicate within a first channel and at a low power transmission level with a selected number of the data terminals which also feature such first type transceivers. Each communication device further includes a high power transceiver to communicate within a second channel with the transceiver of the host computer. A microprocessor operated address controller allows the communication device to control low power communication among the data terminals by receiving and re-transmitting locally directed communications. The controller and the controlling protocol further relays data messages between the low power communication level and host computer. In operation the device receives data messages on one of the transceivers and transmits them on the other. The communication device further may contain sufficient memory capacity to permit periodic batch type data transmission between the host computer and the communication device. The ability to switch from a high power, high precision and high cost transmission system to a low power, low precision and low cost transmission system at the level at which a great number of data terminal devices are used provides system flexibility at a reasonable cost.