EPICURE DESIGN NOTE 108.00
G128 - Tevatron Clock Repeater / Fanout
Allen Forni, Terry Kiper
October 2, 1991
I. General Description
The Tevatron Clock Repeater Fanout (TCRF) is a single wide board used for receiving, reshaping and repeating through multiple outputs the 10 Mhz Tev Clock. The TCRF is designed to occupy one slot in the existing Serial Link Repeater (SLR) chassis G-Bus crate.
The TCRF receiver has a 75-ohm transformer-coupled input. The TCRF input uses a Manchester encoder/decoder interface adapter to recover data and clock from the input signal. The data is in a 10-bit format that begins with a start bit followed by 8 bits of data, and ends with a parity bit (even). On completion of the received data and verification of the parity bit, the data word will be stored in a FIFO. On parity error, data received will not be stored in the FIFO, the cycle will start over. On completion of the received data a local oscillator will clock out the recovered data from the FIFO, and insert a start and parity bit into the Manchester encoder chip. Each repeater will delay the received serial data by about 1.3 usec.
II. Input and Output
The Tevatron Clock and Repeated Clock outputs will connect to the TCRF module by way of a cable harness from the G-Bus backplane. The output drivers will be TTL line drivers using transformer coupled outputs. Multiple transformer-coupled outputs will be available by way of the cable harness at the G-Bus backplane. This harness will support 4-foot lengths of RG59 coaxial cables having BNC connectors. This will connect to a relay rack mounted isolated BNC patch panel. All user connections will be made from this labeled panel.
III. Status Indicators
Front panel status indicators will show input and output Tev Clock activity, parity errors on received data, and power supply indicators. The TCRF will provide a connector for external remote monitoring and reset of the parity error latch.
Appendix LINE DRIVER TESTS
The TCRF repeater input sensitivity requires a minimum-transformer coupled peak to peak signal of 600 millivolts. To allow for reliable operation, a minimum transformer coupled signal of 1.2 volts is recommended.
Research of Ethernet type line drivers found that they require circuits for receipt of data, collision detection and line drivers. Their output drive levels tend to be around 4 volts. Thick wire Ethernet cable specifications limit the cable length to 1640 feet. These chips are in 16 pin dips, each having one channel. Cost is on the order of $45 per chip in small quantities. The devices checked were SMC82C501AD and NE8392A.
The TTL line drivers use inexpensive drivers having two drivers in each 8 pin package. The output drive voltage is determined partially by the power supply being used. Circuit protection is accomplished by using transorbs (diodes), transformer coupled outputs and RC networks.
Below is a table of line driving tests done using existing cable feeds for a total distance of approximately 2500 feet. The line driver circuit in Fig A was used for this test. The dc cable resistance measured with an ohm meter was 55 ohms. The cables are all 75 ohm impedance. A RG59 section feeds from the 12E floor to 13NW. From there the cable is RG11 to WH basement SE. Then the cable changes to a 1/2 inch hardline to SW-YARD at which point the cables are jumpered to loop-back the signal. Test equipment included the use of a VME based TEV Clock Generator. Parity errors were monitored at an output pin on the receive circuit.
Line Driver Test
Driver Vcc 4.5V 5V 10V 15V
Signal Out PP 2.0V 2.5V 7V 10V
Received Signal PP 600mV 700mV 2V 5V
Receiver Data Errors yes no no no
Figures B thou D are circuit variations that could be implemented. An output voltage in the range of 10V P/P will be needed for the worst case distance of around 3500 feet from NS1 to WH13. The worst case cable distance from repeater to repeater within the EAD beam lines is around 1500 feet (excluding Wilson Hall to beam lines).
Figure A uses a TTL line driver with transformer coupling to drive the 75 ohm cable. The transformer coupling gives protection against high voltage transients.
Figure B has the secondary windings in series to allow for more drive voltage from a smaller power supply on the primary side.
Figure C has primary windings in parallel.
Figure D, like figure B has the secondary windings series. It is driven from two drivers in parallel.