References
- DCP is a proprietary protocol developed by Bell Laboratories to allow communications of voice, data, and control information between the new digital telephones and the System 85. Voice digitizing uses the standard North American PCM scheme of 8,000 samples coded to 8 bits per sample. Therefore, DCP uses an 8,000-frame-per-second transmission rate, with 20 bits transferred per frame-3 bits for a frame delimiter (flag), 1 bit for a signal, and 16 bits for other information (8 bits for data and 8 bits for voice). The DCP rate is therefore 160K bits per second and allows both voice and data to be sent between the terminal (e.g., 7403 complete with DTDM) and its interface card in the System 85 PBX .
- SSI is a proprietary protocol developed by Teletype and used for communications between the BCT 500 and 515 CRT terminals and applications processors. The SSI interface uses two-pair wiring, one for transmitted data and one for received data, at a bipolar signal rate of 56K bits per second. Control character information and data are sent in 18-bit frames-bit 1 is a start bit; bit 2, an indicator bit (i.e., whether the next 16 bits are data or control); and bits 3 through 18, two 8-bit data or control characters .
- The PDM connects data terminals to the System 85 when a digital telephone is not required or cannot be cost-justified. Speeds up to 19.2K bits per second asynchronous or synchronous are supported. The TDM connects the System 85 to modems for connection to analog circuits. Speeds are limited to those of standard voicegrade modems, whether synchronous or asynchronous. Both the PDM and the TDM communicate with the System 85 interface card through the DCP protocol. (The second 8-bit field in the information field is bit stuffed because no voice is supported with these two units.) .