By using a selfmade hardware based on an 8051 compatible microcontroller and a 433 MHz radio transceiver IC it is possible to have several clients communicate with a server. The client and server software currently exist for the TI-82 as well as the TI-86 and a Windows PC respectively. Due to the specific approach the server controls the transmission whilst the clients (calculators) act merely as dumb terminals. As the server software supports some Internet services, these can be used on the clients. If there's no Internet connection available local data can still be sent to the clients.
Radio transmission is based upon the nordic nRF401, which is a simple 433 MHz transceiver. Theoretically this part could easily be exchanged by another transceiver solution, but this would of course result in loss of compatibility to the current nRF401-based hardware version. On the PC side a level converter between RS232 and TTL voltage levels is necessary. The client hardware, which means the interface between transceiver and calculator, is built around the 8051 derivate Atmel AT89C51/52 and could of course be substituted by another derivate or a completely different microcontroller, as long as the radio and TI<->µC protocols are supported. We developped and produced a PCB with transceiver, microcontroller, supplemental parts and a loop antenna on it.
Client user interface
As of now the client software on the calculators allows for three basic content types: Plain text, fullscreen menus and edit fields. Out of this it is possible to make a simple menu structure with the opportunity to have the user input text. For our plans on further additions to this take a look at the section "Future".
After logging into the system each client user has to authenticate by entering username and password. If the password cannot be verified by the server, further use of the system is prohibited. In case such mechanism isn't considered necessary, password verification can be disabled in the server configuration.
Supported Internet services
As of now the server software supports only sending and reception of e-mail.
Supported local "demo" services
For demonstration purpose we included some local (i.e. not Internet-based) services into the server software, for example a message system between TI-Net users, a timetable, a dictionary and several additional static content. Any more local services can be added without modifying the server software itself. Modifications can even be performed while the server is running.
The radio transceiver IC is able to send binary data at a maximum speed of 20kbit/s, as of technical reasons we transfer data at 19.2kbit/s with a payload of 15.4kbit/s which means that theoretically 1920 data bytes can be transferred each second. However this rate is decreased a little due to switching between transmission and reception. Further cutting of bandwidth may result from bad transmission quality. Altogether it may seem as if the system is quite slow and we actually have to admit that we're not really content with transmission speed, but as there is no huge amount of data transferred, overall speed is ok for the user.