TN 0038: Outline of Mark IV Electronics 

Author: Tom Droege
Date: 980107
Revision: #0 980107
Key Words: CCD, instrumentation, ADC

Table of Contents

Introduction

This is mostly a reply to Grzegorz Pojmanski who is interested in using the Mark IV electronics. I am posting it to the group as there may be general interest. The status today is that there are rough drawings for everything. The connector details have not been worked out. All the labels of signals between boards have to be determined. We started board layout today. This is the second generation design. The first is working on the bench if I can remember all the things required to turn it on. Note that the big change has been going from a design that required a computer for each channel to single computer, four channel design.

The Mark IV electronics will consist of 5 "logical" printed circuit boards. I may package these as one or two physical printed circuit boards. If one, it would be about 10" x 20". It is usually cheaper to make a single board if my vendor does not barf at the size. In any case, it will be laid out as 5 boards and pasted together as needed.

The boards are:

These boards will all now live in the telescope housing. This greatly reduces the number of cables that have to go from the telescope to the control room. The disadvantage is that the electronics now must operate at telescope temperatures. I figure -20 C to +30 C. Someone might let me know what the 20 year low temperature is at the observatory of your choice.

Stamp

The stamp board is the control center. It receives commands over an RS-232 cable from the control room and controls everything else. It contains:

  1. A 32 channel 16 bit data system to measure all the important voltages and temperatures.
  2. 16 each 8 bit DACs. These are used for CCD clock levels, offset adjustments, RA drive speed trim, and temperature commands for the TEC.
  3. 24 Output pulses. These can be used to set registers, start and stop things, and drive model airplane servos who's position is proportional to pulse width.
  4. An 8 bit bi-directional data bus. This is set up to have 16 logic level sense channels as well as being the way to load 8 bit registers for various purposes.

Scanner

This board receives start and stop pulses from the Stamp board and generats the necessary clocks (under PROM control) to scan a CCD chip. There are 8 pulse channels for the Vertical scan and 16 for the horizontal scan. It is set up for one program of 128,000 steps or two programs of 64,000 steps. A 64k step program would allow 32 micro steps for each pixel read during the horizontal scan. Sounds like a lot, but one quickly uses them up. On can get down to 100 ns or so micro step width, though 200ns is the more practical limit. The plan is (at the moment) to use one program to read out the whole chip and a second for focus. Read out time for the whole chip is 40 seconds. We might read out the center area of the chip in 1 or 2 seconds for focus.

Note that there is no provision for binning. We have not provided the clocks that are needed. This would be a whole new design.

ADC

This board contains 4 ADC channels. The first version will use a 10 us ADC chip, the ADS7805. This chip is cheap, works well, and has a 16 bit resolution if not 16 bit accuracy. The four channels drive produce 8 bytes of data per pixel scanned from the four CCDs simultaneously. Note that there is no provision to scan one chip while exposing another. All the CCDs must be exposed and read out at the same time. If alternate exposure and read out is desired it will require a second set of electronics, and probably a second computer to receive the data. The electronics is not a big expense on the scale of things. Much lest than the cost of a CCD chip.

The ADC board drives a DB-25 cable that connects to the Memory board.

Motor

The motor board contains a bunch of misc. stuff that does not conveniently go anywhere else. The stepper motor drives are bi-polar drives with current control. This means they have a built in current sense and turn off the current so is free wheels when it gets to some limit. This makes for a very efficient stepper motor drive with faster response than the common 2R system. The chip is rated at 45 volts and 1000 ma, but I would not get very close to this limit. Some of the things it contains:

  1. A VCO and RA drive stepper motor. The VCO is set by a pot and trimmed by a DAC under program control. A temperature sensor on the board will allow trimming the VCO to hold it constant under temperature. There is no good way to get a fast measure of the VCO frequency. I will bring a cable back to the control room so it can be monitored and trimmed using a frequency meter. One can also do it slowly using the PC clock. The problem here is that The only way to get time information back to the PC is through the RS-232 line. This means something not so good time wise in both the Stamp and the PC. So I can't promise how well this will work. One can always just take a long time to make the measurement.

  2. Five additional stepper motor drives. Run under program control from the Stamp.

  3. Eight Model Airplane Servo drives. Four are used for the shutters. The rest are available for things that can be done with a shaft that makes roughly a 180 degree rotation. The rotation is proportional to pulse width, about 1 degree steps can be made. Motors are available with response times of 150ms for 60 degrees, and torques up to 200 Oz In.

  4. 16 logic level sense inputs. We will provide 4 pin connectors that convently drive the common IR LED/Light Sensitive Transistor devices. One just needs to stick some sort of vane between the poles of the device, and it will sens it. On can reliably detect a limit position to 0.001"

  5. An 8 bit control register. I plan to use four of these lines to provide 4 switched AC outlets on the side of the telescope housing. One to turn on the TEC high current power supply, one to turn on the cooling water pump. This leaves a couple to power AC motors to do things like open the dome. There is no problem swithing a killowatt these days with available SCR switch modules.

  6. I will probably double up the connector to this board from the stamp. This will allow building the interface of your choice with the 8 bit bus and available control pulses.

TEC

The TEC board contains 4 power amplifiers and comparator circuits to drive the TECs in the camera head. It receives DAC temperature commands and feedback signals from the thermisters in the camera head and drives the TEC appropriately.

Memory

The memory board is 32 Mbyte board which can be expanded to 64 Mbytes. 32 is probably enough memory for three channel systems where we will start. This board lives in the PC. It is read out over the I/O bus. It can interrupt the PC, but otherwise it is a pretty simple board. It can have its memory address set to zero, and can accept bytes over the cable. It automatically advances each time it gets a byte. If it gets out of sync with the data, that is just tough.

Cables

With this design, there are only two cables between the control room PC and the camera. One also needs AC power and a cooling water pond at the camera site. I expect the system to work fine with 100' cables. There is no real limit. I think you can go 1000' feet or so with a properly designed RS-232 signal. One might have to slow down the read out for very long cables. But this is possible to do.

I have worried about sensitivity to lightning. We will see if I have worried about the right things.

Grzegorz is planning to run a telescope of his own design. So he has asked questions about how he might operate.

I will probably package the Stamp, the Scanner, and the ADC on one printed circuit board, and the Motor and TEC on another. Running a camera head separately would then only require the Stamp combination board since the controls would be elsewhere. The TEC could just be driven manually. In actually fact, the boards will be quite cheap once in production on the scale of things. Grzegorz will probably want the full compliment, just because it might be useful to have one of the features on the motor board. One can always leave most of the parts out for the small saving in parts but larger savings in stuffing labor and testing.

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