Specifications

62 Telephone Interface

4.2.2.2 Identiﬁcation

The TLI sends an identiﬁcation code to the DTC04 module. If two

modules are connected to a TLI, the TLI sends the same identiﬁcation

code to each module. This identiﬁcation is factory set to identify the TLI

as a North American analog loop start interface.

4.2.2.3 Reset

When the module asserts the TLI RESET signal to the TLI, the TLI

cannot accept any calls. This prevents the TLI from connecting to the

telephone network while the module is resetting.

4.2.3 Intel SLD Feature Control Chip

The heart of the telephone line interface is the Intel 29C50 control chip.

This chip is a user-programmable, fully integrated PCM codec with

transmit and receive ﬁlters, parallel input, and parallel output. The

chip communicates with the telephone line and DTC04 module through

the SLD protocol. The SLD data lead channel sends four separate bytes

of information between the module and TLI: voice, data, control, and

signaling.

4.2.3.1 Voice Byte

The voice byte is the main data path into the 29C50 chip. The TLI

converts data it receives from this byte into audio signals. The module

receives audio signals from the telephone line through this byte. Voice

data from the telephone line that is sent to the chip can use either of two

encoding schemes: -law or A-law. The data stream to or from the 29C50

chip moves at a rate of 8 kHz in 8-bit compressed samples.

4.2.3.2 Data Byte

The DECvoice system does not use this byte.

4.2.3.3 Control Byte

The control byte programs the 29C50 chip. The SLD protocol uses a

sequence of six feature control bytes to program the chip. The most

signiﬁcant bit of a control byte is used for framing. The ﬁrst byte in the

6-byte sequence must have a framing bit value of 0. The rest of the bytes

should have a framing bit value of 1. The second most signiﬁcant bit in a

control byte is a write-enable bit. When this bit is 0, writing is enabled.

The remaining bits in the control byte contain programming information.

For details on the use of the control byte, refer to the Intel

Microcommunications Handbook.