These cards are used to buffer and convert the GAC output signals from the TTL to ECL differential signal. The inputs to these cards come from 8-INPUT cards and the CLOCK card. The outputs go to the ECL DISTRIBUTION bin.
There are 3 BUFFER cards per GAC cage, corresponding to one side-band. The first card from the left is used for buffering all the IA output signals. The second card buffers all the signals corresponding to all PA Sub-Group. The signals enter the Buffer cards from the backplane. In the backplane there is a special kind of cabling arrangement to bring the signal from the 8-Inout Cards section to the Buffer Card section.
Each of the BUFFER Cards have 3 stages. The First stage being the TTL buffers. The second ia teh DELAY module section and the third is the TTL to ECL conversion section. All the signals that enter the Buffer Card pass through the TTL buffers stage. This Buffer stage can work as buffer-cum-latch mode. This is done by suitable jumper settings.
1.Data Lines :
Data lines APx, BPx,, CPx and DPx are meant to carry te IA or PA signals. As soon as they enter the Buffer Card, there is a provision to terminate these signals ( if required ) using the dual termination network ( value 220 / 330-ohms, 330-ohms to VCC, 220-ohms to GND; it is a 10-pin SIL; RS part number is 140-855 year 1998 ) at the SIL sockets RP1 to RP6. All these Data sets have 11 data lines. The most significant 8 lines are used for carrying the data. In the case of DPx, the 11th line DP10, is labeled as LCK, and is brought to the jumper JP2 pin 3, instead of the buffer stage. This has been done to use this signal line to bring in an latch clock for the buffer stage.
2.Clock and Control Signals:
The Clock and Control signals required for the back-end systems are sent through the BUFFER card. These signals provide proper timing information to latch teh data. The back-ends can make use of these signals to synchronise their timing these signals. There are three signals, which are
a.CLOCK - this clock positive edge can be used to latch the data.
b.INIT - a LOW pulse which repeats every FFT cycle. Indicates the start of a new frame.
c.GAC_ON- Going High indicates that the GAC is Ready to output data. This signal will stay high, during the normal operating time.
Currently it is not enebled, and is kept always high. It is supposed to operate with a control signal from the Control PC.