Scan control electronics

The scan electronics (except PMAC accessory cards) will be mounted in a rack-mount PC. The electronics layout is shown in Fig. 1.1, with the following subsystems:

Delta Tau digital servo system. This system consists of a main board (PMAC2) and several accessory boards. The system can read in digital positions as parallel words, and drive either piezo or stepper motors in a digital closed servo loop. The system will have six 24-bit parallel word inputs (provided by three ACC-14D boards powered by the ISA bus backplane for $x/y/z$ actual positions from the interferometer, and two of the three $x/y/z$ desired positions from PCI-6534), two 18 bit precision analog outputs (for piezos; provided on an external ACC-8E card that will be included in the rack mounting outside the PC), and one stepping motor encoder input and pulse and direction output (for ZSTG; provided on an external ACC-8S stepper motor output card that must also be mounted outside the PC). The main Delta Tau servo control card (PMAC2-PCI with dual ported RAM for programming access, and 80 MHz processor upgrade) is mounted on the PCI bus with internal connection to the three ACC-14D boards.

Agilent laser interferometer. This system uses a N12131A PCI bus card to deliver 0.3 nm resolution position information on three axes. One can directly read positions and velocities from this card using the supplied Windows driver. It also provides its position information on three 24 bit parallel outputs that plug into the Delta Tau ACC-14D boards.

NI PCI-6534 digital I/O board. This board will be used to provide digital position outputs from the computer to the Delta Tau PMAC2 board. The sequence positions for at least an entire scan line, and quite possibly an entire scan, will be calculated in advance and stored in computer memory; these positions will then be streamed out. However this board has a total of only 32 output bits, whereas the Delta Tau servo controller ideally would be provided with three 24-bit inputs. In fact, a straightforward solution exists:

• The 32 output bits will be configured to operate as one. However, the lower 16 bits will be routed into one Delta Tau axis input, while the other 16 bits will be routed into another Delta Tau axis input. The ACK1 bit will be routed to the ICLK inputs of each Delta Tau axis.

• For different scan types, the Delta Tau input that serves as the control input will be selected from the appropriate lower or upper 16 bits of the NI PCI-6534.

• The Delta Tau bit rollover scheme described in Sec. 4.1.5 will be used to extend the resolution of the control positions beyond 16 bits.

NI PCI-6052E multi-function board. This board has a 16 channel ADC with a maximum samlping rate of 333 kS/sec, which we will use to read the segmented Si detector which has up to 10 signal outputs. The board also has two 24 bit counters; one will be used to count the pulse stream from the proportional counter detector, while the other will count pulses from a timing clock (from which one can calculate the actual dwell time of each pixel). All these three functions can happen simultaneously and even be configured to use the same handshaking signal (provided by the master clock) as trigger. We will not make use of the board's 2 channel DAC.

NI PCI-6503 digital I/O board. This board provides 24 input/output bits. It is used to control the operation of the clock interface, as shown in Fig. 8.1. Table 8.3 shows its pin assignments.

National Instruments PCI-GPIB controller card. This card will be used for talking to the Newport MM-3000 motor controllers used to control and drive the XSTG, YSTG, XDET, YDET, ZDET, and ZOSA motors.

ACS stepping motor control system. The ZSTG stepper pulse outputs from three microscopes, along with the EXS2X_I and EXS2Y_I inboard high energy exit slit positions, will go into ACS stepping motor control systems. SPD-3M drivers will be used to drive the ZSTG motors on the three microscopes based on inputs from the ACC-8S boards. SBD-6B drivers will be used to drive the EXS2X_I and EXS2Y_I motors based on serial port position instructions to the SPC-3 interface module.

Note that the Delta Tau boards must be connected to each other by ribbon cables inside the computer case. The Agilent interferometer board also delivers its three position outputs to each of the three Delta Tau ACC-14D input boards inside the computer. Each computer requires a total of 7 PCI cards, and 3 ISA slots without computer connection. The backplane layout and connector scheme is shown in Fig. 1.2.

Figure 1.1: Layout of the STXM V scan electronics. For the $x$ position Delta Tau servo control input ACC-14D board, ``XPZT pos'' refers to the present position of the $x$ piezo stage as determined by the laser interferometer, and ``XPZT ask'' refers to the desired $x$ piezo stage position calculated for the scan and provided by the NI PCI-6534 board. The Cryo II electronics are identical except that there is no laser interferometer, so the XPZT, YPZT, and ZSTG positions are all driven in open loop mode as far as the Delta Tau digital servo system is concerned (the XPZT and YPZT piezos are controlled using capacitance micrometers in an analog feedback loop in the PI piezo controller).

Figure 1.2: STXM V/Cryo II backplane layout. Currently used connectors and board connections as also listed.