Polar BCAM Head (A2051) Manual

© 2004-2021 Kevan Hashemi, Brandeis University


Radiation Tolerance
Laser Isolation
Image Geometry
Image Contrast
Power Consumption


The Polar BCAM Head (A2051) is a Long-Wire Data Acquisition (LWDAQ) Device that reads out two TC255P image sensors and drives four light sources. When the A2051 is connected to a pair of Polar BCAM Side Heads (A2040), as shown below, these light sources are laser diodes (LDP65001E or equivalent). A BCAM is an optical surveying instrument (Brandeis CCD Angle Monitor).

Figure: Black Polar BCAM (P-BCAM-L). The top label is the serial number. The front label gives the lower two digits of the serial number, and provides a way to resolve ambiguities about BCAM serial numbers when our database of BCAMs and mount locations is corrupted.

The Black and Blue Polar BCAMs each use one Polar BCAM Head. The Black BCAM is part number P-BCAM-L, where the "L" is for "left-handed". The Blue BCAM is P-BCAM-R, where the "R" is for "right-handed".

Figure: Blue Polar BCAM (P-BCAM-R). The RJ-45 socket is at the back, out of view in this picture. The brass insert is the lens holder. The two holes on either side of the lens contain laser diodes that act as point sources. The central laser is device element number three (ON3) and the outer one is number four (ON4). The same is true for the Black version, which we show in transparent view below.

By default, the device is equipped with two cameras and four lasers, so it is "double-ended". We have made versions that have only a single camera, facing either forward or backward. These are the P-BCAM-LSF, for "left-handed single front-side camera", P-BCAM-RSR, for "right-handed single rear-side camera" and so on.

Figure: Black Polar BCAM, Transparent View. Inside the enclosure are a Black Polar BCAM Head (A2051L) and two Black Polar BCAM Side Heads (A2040L).

In a double-ended BCAM, the A2051 works with two BCAM Side Heads A2040s to drive two TC255P image sensors and four laser diode light sources. In a single-ended BCAM, the A2051 needs two A2040s, but only one image sensor and two lasers are present on the A2040s.

CCamera Head (A2051L modified for TC237 readout, no lasers)
FFiber Illuminator (A2051L with laser circuits only, two-pin headers)
LBlack Polar BCAM Head (mirror image of A2051R)
QQuad LED Head (A2051S with pigtails and LEDs)
RBlue Polar BCAM Head (mirror image of A2051L)
SAzimuthal Source Head (A2051L with laser circuits only)
Table: Versions of the Polar BCAM Head (A2051).

The A2051L is designed to operate with the Black Polar BCAM Side Head (A2040L). The A2051R is designed to operate with the Blue Polar BCAM Side Head (A2040R).

Figure: Black Polar BCAM Head (A2051L). Here we see the bare circuit, top and bottom sides.

The A2051S has six-way zero-insertion force flex connectors in place of twelve-way low-insertin force flex connectors, and operates either with the Black Azimuthal BCAM Side Head (A2049L) or Blue Azimuthal BCAM Side Head (A2049R). It also omits all parts in the A2051 circuit that are necessary only for CCD readout. The footprints for these parts on the circuit board are present but empty.

Figure: Azimuthal Source Head (A2051S). Here we see the bare circuit, top and bottom sides. The two six-way connectors have pin one on the outside, and over-hang the edge of the circuit board.

The A2051S operates with two Azimuthal Source Heads (A2049), either a pair of A2049M mounted on black plates, or two A2049N mounted on blue plates, to make an Azimuthal Source Plate, also known as a Saloon Door Source, as used in the ATLAS end-cap alignment system.

Figure: Black Azimuthal Source Plate, consisting of one A2051S and two A2049M.

The A2051Q is an A2051S without the flex connectors. Instead, we solder four twisted pairs of wire into the flex connector footprints. Each twisted pair provides a single high-power LED for connection to an optical fiber. There is a resistor in series with the LED acting to control the LED current. The twisted pairs receive 15 V running through two 47-Ω, 0.5-W resistors on the A2051Q board. Thus connecting a diode without a resistor would allow 150 mA to flow through the diode. Additional resistance reduces the current.

Figure: Fiber Illuminator (A2051F). Four two-pin connectors provide power for LEDs or lasers. Each connector is identified by the LWDAQ element nuber we use to flash its LED. The negative terminal of each connector is pin one, which is on the top side in the photograph. The serial number of the assembly is on the bottom side.

The A2051F is an A2051S in which the flex connectors for lasers have been replaced by four short twisted-pair cables running to four two-pin 0.1" headers. We can connect fiber-coupled LEDs or lasers with laser drivers directly to these two-pin headers and switch them on and off using the A2051F. Thus the A2051F is the same as an A2051Q with the wires cut near the circuit board and terminated with two-pin headers. For mechanical strength, we mount the two-pin headers on a circuit board of their own, and enclose this and the main circuit board in a plastic box.

The A2051W has eight-way flex connectors occupying pins 1-8 of J2 and J3. These connect to two TC255P Minimal Heads (A2016P) via two 8-way flex cables. In the place of pins 11 and 12 of J3, we solder a pair of wires with a two-way 0.1" socket on the end.We modify the A2051 circuit by placing R23 and R24 in parallel instead of in series. We solder the two resistors on top of one another and short the empty footprint with a piece of wire. Now we can connect the two-way socket to a Nine-LED Array (A2041W). The 22 Ω parallel resistance of R23 and R24 allows roughly 80 mA to flow into the LED array when we connect 15 V across the combination. For a photograph of the A2051W and A2041W installed in a Wire Position Sensor, see here.

Figure: Camera Head (A2051C) used in a Double-Ended Black Polar BCAM. Note the two TC237B image sensors connected to the A2051C with two eight-way flex connectors.

The A2051C is similar to the A2051W. It is designed to work with one or two TC237B Minimal Heads (A2070). The board provides two eight-way flex connectors for the connection to the minimal heads. We change R29 and R35 to 3.3 kΩ in order to increase the gain of the output amplifier to account for the lower output of the TC237B sensor. Lasers or LEDs can be powered using pins 10 and 12 of both the J2 and J3 footprints.

All versions of the A2051 connect to a LWDAQ driver (such as the A2037) or multiplexer (such as the A2046) with a LWDAQ cable. The A2051 is LWDAQ device type 2. The DAQ software selects a camera or laser diode in the BCAM by specifying a device element number to the driver. On flash jobs, elements one to four select lasers one to four. On image retrieval jobs, elements one and two select cameras one and two.


The A2051 complies with the LWDAQ Specification. It's LWDAQ Device type is 2 (TC255P). Its laser drivers are elements 1 through 4, and its cameras are elements 1 and 2.

Table 1:Command Bit Allocation on the A2051. An "X" means the command bit serves no function.

The signals acronyms are DCEN for direct clock enable, SRGD for serial register gate digital, SAGD for storage area gate digital, IAGD for image area gate digital, ABGD for anti-blooming gate digital, ABEN for anti-blooming enable, and LB for loop back.

The CCD1 bit, when asserted, selects the image sensor connected to J3, which is the rear-facing camera on a polar BCAM. We select the rear-facing camera by specifying device element 1 during image readout. When we specify device element 2 or 0, the CCD1 bit will be unasserted, and the image sensor connected to J2 will be selected. This sensor is the one used by the front-facing camera. By default, the device element number used by LWDAQ Drivers is 0, so by default, the rear-facing camera will be the one that gets read out.

Here is a diagram showing the names of the lasers and cameras in a Black Polar BCAM. The image sensorss and lasers share channel numbers. The driver knows that flash jobs are directed towards the lasers, and image retrieval jobs are directed towards the image sensors.

Figure: Channel numbers of lasers and cameras on a Black Polar BCAM.

The Blue Polar BCAM is a mirror image of the Black Polar BCAM. Lasers 1 and 3 are to the right of lasers 2 and 4 respectively. This exchange of positions allows us to distinguish between black and blue polar BCAMs in our field of view.


The A2051, when combined with two Polar BCAM Side Heads (A2040), provides two TC255P image sensors and four laser diodes. Let us assume you have the A2051 in a polar BCAM enclosure, along with its A2040s. If the BCAM has lasers and lenses on both sides, you will be able to capture images from both TC255Ps, and flash all four of the lasers. If the BCAM has a lens only on the front side, which is the side opposite the connector, then you will be able to flash only the two lasers at the front, and capture images only from the image sensor at the back.

Once you are capturing images, you will need to adjust the exposure time until you get the image you want. If the exposure time is too long, the image will be white. If it is too short, it will be black. The black image from a working A2051 is, however, different from the black image from a broken A2051, or one that is not connected. Each driver introduces its own noise, but this will be less than the noise present in the black image from a working head.

If your A2051 is not mounted in a Polar BCAM enclosure, you will have to install it in such an enclosure and connect it to its two BCAM Side Heads (A2040). For these connections you need twelve-way 1-mm flex cables.

The A2051 provides exposure of the TC255P with and without anti-blooming, and drives four light sources. When connected to a pair of A2040s the A2051 drives four lasers and two TC255P image sensors.

When an image sensor pixel fills up with light-induced charge, the charge tends to flow into neighboring pixels and cause them to saturate as well. A bright spot in an image can turn into a large, bright smear, in a phenomenon called blooming. The TC255P image sensor provides an anti-blooming clock input that you can pulse during an exposure, and which gets rid of excess charge before it can spread into neighboring pixels. The anti-blooming process does, however, degrade the linearity of the pixel response. We do not use anti-blooming when we take BCAM images, and the LWDAQ Software's BCAM Instrument does not support anti-blooming. Nevertheless, when we take diagnostic images of the field of view of the BCAM, with exposure times of a few hundred milliseconds, anti-blooming makes sure that bright reflective objects, or overhead lights, do not flood the image. In such cases, we can use the Camera Instrument to obtain images from a BCAM. The Camera Instrument supports anti-blooming. We say more about anti-blooming, and provide some example images, in our Camera Head (A2056) Manual.

To expose the image sensor without anti-blooming, send the following instructions to the LWDAQ driver:

  1. write 2 to the device type register, specifies TC255P device
  2. write 1 or 2 to the device element register, selects image sensor.
  3. move, clears the image sensor)
  4. wake, leaves the image sensor to gather light
  5. delay or flash to expose the sensor, requires writing to delay timer
  6. alt_move, moves the image into the image sensor's storage area
  7. read, moves the image into the driver memory

An anti-blooming exposure requires the following LWDAQ driver instructions:

  1. write 2 to the device type register, specifies TC255P device
  2. write 1 or 2 to the device element register, selects image sensor.
  3. move, clears the image sensor)
  4. wake, leaves the image sensor to gather light
  5. write 4 to delay timer, sets anti-blooming clock to 1000 ns HI and 625 ns LO.
  6. write value to repeat counter, sets exposure time to 1.625 ns × value.
  7. toggle, clocks the anti-blooming gate for a specified time
  8. alt_move, moves the image into the image sensor's storage area
  9. read, moves the image into the driver memory

To flash one of the A2051 lasers, send the following instructions to the LWDAQ driver:

  1. write 2 to the device type register, specifies TC255P device
  2. write 1, 2, 3, or 4 to the device element register, selects laser.
  3. write a value delay timer, sets flash time in multiples of 125 ns
  4. flash, flashes the laser

To measure the propagation delay of signals traveling from the driver to the A2051 and back again, execute a loop job and read the loop time out of the driver. The A2051 is asleep when it powers up, and goes to sleep when you execute a sleep job.

You will find the above read-out sequences layed out in BCAM.tcl, the TclTk script that defines the BCAM Instrument in our LWDAQ Software.

Radiation Tolerance

See here for a discussion of the radiation tolerance of the A2047, which is a camera circuit very similar to the A2051.

Laser Isolation

The A2051 can turn on its lasers only if their metal cases are isolated from the BCAM chassis, as we describe in the A2049 Manual.

Image Geometry

For a discussion of image geometry, and how to translate between points in the image sensor and points in the image on our computer screen, see the Image Geometry section of the TC255P Minimal Head (A2016) Manual. For instructions on finding Pin One on a TC255P, see the Pin One section of the same manual.

Image Contrast

We discuss the image quality provided by our various TC255P readout circuits in the Image Contrast of the A2036 Manual.

Power Consumption

We picked an A2051L at random and measured its power consumption in three states. The A2051L was connected to two fully-populated Black BCAM Side Heads (A2040L).

State+15 V −15 V +5 V
Asleep10 μA10 μA2.9 mA
Awake, no activity55 mA48 mA3.2 mA
Awake, 6 images/s55 mA48 mA3.4 mA
Awake, 1 laser on84 mA48 mA3.4 mA
Awake, 2 lasers on107 mA48 mA3.4 mA
Awake, 3 lasers on129 mA48 mA3.4 mA
Awake, 4 lasers on136 mA48 mA3.4 mA
Awake, 4 lasers on, −15V disconnected240 mA0 mA3.4 mA
Awake, any command, +15V disconnected0 mA0 mA0 mA
Table 1: Power consumption of the A2051L.


Note: All our schematics and Gerber files are distributed under the GNU General Public License.


S2051_1: LVDS Transceiver and Power Switches
S2051_2: Command Receiver
S2051_3: Level Shifters
S2051_4: CCD and Laser Connections


A205101L: For A2051L, A2051S, and A2051W.
A205101R: For A2051R.
A205101L_Panel: 2×5 Panels of A205101L
A205101R_Panel: 2×5 Panels of A205101R


A2051L: BOM, PIK, KIT, and Cost for A2051L production
A2051R: BOM, PIK, KIT, and Cost for A2051R production
A2051S: BOM, PIK, KIT, and Cost for A2051S production