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AM64/128A User Manual System Overview
8
The interface circuit performs a number of functions : it converts between the user data from the
appropriate interface and the 64kbps or 128kbps, which is transmitted down the line; it links the control
circuit to the control channel; it interfaces the control circuit to the control lines of the data ports. Again,
these functions are performed by a custom IC supported by a number of standard components. Looking
at each function in turn, user data at 64kbps and 128kbps is sent without processing; byte-timing is
maintained by the transmission system and is available if required. Data at 56kbps is sent in a 7+1 format,
i.e. data plus status information. At all lower rates the data is transmitted as octets comprising 6 data bits,
1 framing bit and 1 status bit. At rates below 48kbps the octets are iterated as appropriate to fill the
transmission channel.
The control channel, uses 4 bits from each transmission frame to transmit control information to the unit
at the other end of the line, e.g. user data rate selection, and to monitor the overall performance of the
link.
With regard to the remaining function of the interface circuit, the control lines of the data ports mainly
require buffering; however, in the case of X.21 the various control patterns have to be generated or
detected. The control circuit is based on a microcontroller and determines the state of the unit according
to the state of the transmission system, the state of the data interface and the configuration information
received from the front panel and from the control channel. It also provides the front panel display with
information on the current state of the unit which can be reviewed via a simple menu structure.