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http://physgi01.phy.bnl.gov/~mps1/dm.html .
All readout planes consist of alternating anode and field wires, with the anodes operating at ground and the field wires at about 1.8 kV. The anode-to-anode spacing is 6.35 mm, with field wires between anodes. Each readout plane is sandwiched between a pair of cathode planes, operated at about 2.3 kV; middle cathode planes are shared by two readout planes. The gas mixture consisting of (approximately) 75% Ar, 12% Isobutane and 13% Methylal. The chambers were originally constructed from 1978 to 1982, as part of MPS II.
The readout electronics were built at BNL, and consist of a 256-bit shift-register TDC for each channel. The full drift-distance spans about 16 TDC bins, each bin being 4 ns in width. Encoding is performed in FASTBUS modules (called DFBs), also built at BNL, which generate a data buffer from the chamber hits for readout by DAQ. Only 32 TDC bins are utilized, of the shift-registers' 256 bins, with the remainder serving the purpose of a delay.
This is a brief description of the signals required to encode and read out the drift modules.
This is a NIM-true signal which indicates that the AGS spill is in progress. This is used to shut down the fast write clock to the shift-registers between AGS spills, which extends their life by reducing the heat-generating duty-cycle.
This stops the shift-registers, freezing the timing information in the TDCs. This is typically derived from a lower-level trigger (high rate, small deadtime trigger). This must occur within about 150 ns of the beam crossing or else the TDC timing information is lost. Readout noise is occasionally observed in the first 5 or 6 TDC bins in some channels. The Strobe is timed so that the first real time-bin (corresponding to particles crossing at an anode, with 4ns or less drift) occurs in readout-bin 8 or 9. This permits any readout noise to be discarded by software, during analysis, while leaving ample room for the full-drift hits (16 TDC bins) in the 32 TDC bins actually utilized.
In non-T3 mode, an encoding cycle automatically follows a received Strobe, after which the fast clock is restarted. Encoding takes about 600us, and DAQ readout typically adds from 100us to 200us to that. This is the simplest way to readout the DMs, but does not permit trigger hierarchies.
The Strobe is also used by the MPS PWCs, to latch in their data.
T3 mode permits experimenters to hold off on the readout of the drift modules until a higher level trigger is made, with the option of aborting without adding nearly a millisecond to the deadtime. In addition to permitting trigger hierarchies, T3 mode also permits the experimenter to delay DM readout, which is a fairly noisy operation (electronically speaking), until other, noise-sensitive detectors have been read out. An example of this occurs when both DMs and TPCs are read out. The sensitive TPC electronics are prone to seeing the noise from the DM readout, so the DM readout is postponed until the full drift-volume of the TPC has cleared for a triggered event, typically about 50 us.
T3 mode is selected with a small toggle switch, located on the front panel of the DM clock generator, in fast trailer rack 3; the position labelled "1" selects T3 mode, and the position labelled "0" selects non-T3 mode.
In T3 mode, once a CS has been received, one of the following two signals must be received, else the fast clock will remain off until the next AGS spill, rendering the DMs useless for the remainder of a spill.
This is a NIM-true pulse which initiates encoding, which requires about 600us. The fast clock automatically restarts after encoding is complete.
Trig is also used by the MPS PWCs, to initiate the encoding process.
This restarts the fast clock, without encoding, requiring less than 1us.
Each readout plane has its own threshold adjustment; the U and V planes are split in half, with each half having its own threshold, so there are a total of 9 threshold adjustments for each 7-plane DM. The threshold controls consist of a set of modules in a special-purpose electronics bucket, in Fast Trailer rack 26. Each module has nine 14-turn dials, one for each threshold. Threshold values are expressed in terms of the values read from these dials.