Triggering and Suppresion

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[edit] Trigger Electronics

Trigger Logic
Trigger Logic
Master triggers are generated through programmable logic modules. The wiring, as it looked as of 2005, is shown here. This may need to be updated.

[edit] Suppression

The germanium pre-trigger can be programmed to accept all germaniums (unsuppressed), or to only accept those that do not have an accompanying signal in the suppression shield (suppressed).

Hardware suppression is performed in Lecroy 4516 Trigger Logic units. It has three sets of 16 input banks Ai, Bi, and Ci, i = 0...15. These inputs and the output are ECL. The output logic is (A\ c_1\ B)\ c_2\ C, where c1 and c2 are chosen as either boolean AND or OR operators. These operators are selected either by back-panel switches or by CNAFs. In the rest of this page and in the Lecroy documentation, c1,2 are denoted C1 and C2 (not to be confused with input C).

BGO Suppression Cabling. Note that in this drawing, pulses are drawn as if they were NIM pulses -- i.e., a negative-voltage signal is true.
BGO Suppression Cabling. Note that in this drawing, pulses are drawn as if they were NIM pulses -- i.e., a negative-voltage signal is true.
The inputs for BGO suppression of Ge triggers are:
  • A: NOT-Bgo (True when BGO not present, False when BGO CFD fires)
  • B: always True
  • C Ge (True when Ge CFD fires)

Suppression is achieved by (A\ \cdot\ B)\ \cdot\ C, i.e. with C1 set to AND and C2 set to AND. With the inputs just described, this reduces just to (NOT-Bgo) AND Ge, as it should be. Suppression can be turned off by setting C1 to OR, in which case the logic reduces just to the value of Ge.

The multiplicity condition for Ge pre-triggers is derived from the number of time-coincident True signals in the output of the 4516 units after passing through the suppression logic. TDC start and ADC gate signals are also derived following suppression logic.

[edit] Back-Panel Switches

The back-panel switches for C1 must be set in "Remote", and C2 must be set in "AND". In this way, the suppression mode (on or off) is selected by CNAF commands issued at the beginning of a run. The mode is selected by an ODB variable. This operation feature may be (accidentally or intentionally) overriden by setting the back-panel switches by hand or by removing the True inputs to A. If the suppression is not working as expected, or if the germanium detectors are not triggering at all, verify that the back-panel switches are set correctly.

[edit] Inverting an ECL Signal in a Ribbon Cable Header

The ECL used on the Lecroy modules in the suppression logic is differential logic. As such the NOT operation can be realized by simply reversing the two wires carrying the signal -- i.e., changing the polarity. In the hardware suppression logic, this is how the Bgo and False signals are inverted to form NOT-Bgo and True signals for inputs A and B of the 4516s. For individual signals carried on twisted-pairs with two-hole plugs, this is as simple as making sure that (e.g.) the white wire connects to + on the output and - on the input, and vice versa for the black.


Figure 1. Regular header, ribbon-cable side, before modification
Figure 1. Regular header, ribbon-cable side, before modification
A regular stores-item dual-row female ribbon cable plug, such as the 34-pin ribbon-cable headers, can be made into an inverting header with some work. Figure 1 shows the ribbon-cable side of a common plug. Position 1 of the header is the rightmost contact on the top row (farthest back); you can just see the notch indicating "position 1" behind it. Positions 3, 5, 7, and part of 9 are also visible in that row, counting to the left. Position 2 is the contact on the bottom row (closest forward), with 4, 6 and part of 8 visible to the left. Note that the little jaws for the even positions (front row) are in between the odds. In this way, when the ribbon cable is pressed onto the contacts, the back row cuts into and contacts alternating wires 1,3,5 ... and the front row cuts into and contacts wires 2,4,6 ...


Figure 2. Contact for Position 2, Wire 2 removed.
Figure 2. Contact for Position 2, Wire 2 removed.
Each of these contacts can be carefully removed with a pair of pliers. In Figure 2, the contact for Position 2 has been removed. If you look carefully you can see the gold coloring on the part of the contact that mates with the pins of a receptacle.


Figure 3. Contact for Position 2 has been re-installed to intercept Wire 1.
Figure 3. Contact for Position 2 has been re-installed to intercept Wire 1.
To turn a regular header into an ECL-signal inverting header, simply remove each contact and re-install it turned around by 180 degrees, so that the jaws are pointing to the opposite side. Figure 3 shows this for Position 2. As installed, this contact will cut through the insulation on Wire 1 of a ribbon cable. It will make contact with Wire 1. If you repeat this operation for all the contacts, you will end up with a connector where the Even Positions 2,4,6 ... contact Wires 1,3,5 ... , and the Odd Positions 1,3,5 ... contact Even Positions 2,4,6 ... . When such a connector is used on a ribbon cable with, for example, normal Lecroy ECL-Line electronics, where the ECL signals are carried on lines 1-2, 3-4, 5-6 ..., this will have the effect of inverting all the ECL logic signals between the ribbon cable and the receptacle.


This method has in fact been used to make certain cables for the 8pi suppression logic. It is strongly recommended that such inverting connectors, once fabricated, are labelled in an unambiguous way. Use a hot soldering iron to burn the letters "INV" into the side of the connector. Paint the ribbon-cable cover and strain relief with liquid paper, then write "Inverting" on them with a fine-tipped red permanent marker.

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