INTRODUCTION 1. The 2833A Digital Line Monitor is a combined p.c.m. error detector for 2048/8448 kbit/s systems and alignment and signalling monitor for 2048 kbit/s systems. The instrument provides in-service measurement and analysis of systems using HDB3 code format and has five measurement modes which are as follows:- (i) HDB3 line-code error detector at 2048 kbit/s, (2M CODE). (ii) HDB3 line-code error detector at 8448 kbit/s, (8M CODE). (iii.) Frame alignment monitor at 2048 kbit/s, (2M ALIGN). (iv) Signalling code monitor at 2048 kbit/s, (2M SIG) . (v) Counter for an external input, (EXT. COUNT). Line-code error measurement (2M CODE and 8M CODE modes) 2. The p.c.m. bit stream is monitored and code violations of the HDB3 format are detected. The errors are either totalized and displayed on a six digit counter or displayed as an error ratio in the form of a mantissa and exponent from 0 in 10^ to 1 in 10^. 3. The totalization of errors continues when the mode is switched to error ratio, enabling error ratio to be measured without interfering with a longterm monitoring sequence. The totalizing count can be stopped then continued or reset. Count overflow is indicated by an LED. 4. A self-test facility, which provides an internally generated signal with a known error ratio, enables you to check for correct operation of the instrument. A lamp-test facility when selected will illuminate all LED's and displays. Frame alignment error measurement (2M ALIGN mode) 5. The frame alignment signal in the 2048 kbit/s bit stream is monitored and errors are determined. The display of errors is similar to the line-code error measurement mode in that totalization and error ratio facilities can be switch selected and also totalization continues when error ratio is selected. Unused bits of the frame and multiframe alignment words can be accessed via a socket on the rear panel. Channel signalling monitor (2M SIG mode) 6. It is possible to monitor Time-Slot 16 of a selected channel whereby the 4-bit signalling code, either present or previous, may be displayed as a decimal equivalent. In this configuration the last number dialled may also be displayed. Alarm state indication (2M ALIGN and 2M SIG modes) 7. LED lamps illuminate corresponding with the following error conditions listed in order of priority : Line, AIS, Frame, Errors (Frame word), Multiframe, Distant Frame, Distant Multiframe. If more than one error condition occurs simultaneously, only the highest priority lamp is lit. The lamps can be self extinguishing following clearance of the error, or can be set to remain lit by selecting a lamp lock facility. External count input (EXT. COUNT mode) 8. An external count input connector on the rear panel may be used to totalize error pulses from a multiplex alarm monitor point. Error Free Seconds mode (EFS) 9. When the 2833A is switched on it automatically monitors the input signal for code or frame word errors. When EFS is selected - via a front panel key sequence - the number of complete seconds in which no errors occurred in the period since the 2833A was switched on, mode changed or RESET, is presented as a percentage of the total number of seconds in that period. EFS is displayed if ERRORS-RATIO is selected and printed if a printer is connected.