Rutgers Physics Student Response System

Overview of the Hardware

This summarizes the design of a ``student response system'', which enables students to key in numerical answers on a telephone-style keypad, accumulates all the inputs and makes them available on a PC style central computer on the lecturer's desk. This should enable a wide variety of interactive pedagogical tools. It is designed for the Physics Lecture Hall, an amphitheater style room with 330 seats.

This design is hierarchical, starting with an extremely simple device at each student seat, a keypad consisting of a telephone keypad and three LED lights, red, yellow and green. These are formed into groups of at most 16, each group interfacing with a subsubstation. Each row of seats in each of the two central sections of the lecture hall constitutes such a group, served by one subsubstation. In the side sections, where it is possible to run cables from one row to another along the wall, a few rows are grouped together, making four groups in each side section. Thus we have four subsubsections for each of the two side sections, and 11 subsubstations for each of the two central sections, for a total of 30 subsubstations. Each subsubstation is mounted under one of the seats in a row which it serves.

A group of up to 8 subsubstations is served by one substation. Due to the layout of the room we are using 5 substations, although the system was designed to use 6. There is one substation for each side section of the lecture hall, one which controls the first few rows of both of the central sections, and one each for the back of the two central sections. The substations are located on the ceiling of the storage room which extends under most of the seating area of the lecture hall.

The substations have their signals brought together at a single distributor station located centrally in the storage room. It is connected to each of the substations by cables of differentially driven pairs, and is connected to the controller by a similar long cable of differential pairs.

In the lecturer's desk is the final stage, the controller, which must be connected by short ribbon cables to the interface board in the PC. This device controls the system, working through a cycle during which all keys are polled, and the state of the lights sent out to the corresponding substations. This arrangement makes it possible to transmit the data to and from the PC during an interrupt, which leaves the PC available most of the time for the higher level software which is designed to implement the instructor's needs. Each cycle lasts approximately 55 milliseconds. During this time the subsubstations go through a poll of the 48 columns (three columns each for sixteen keypads), reading the status of the four buttons in each column, and also lighting (or not) the LED light for that column. During each of these 48 steps the collecting station will gather the information from each substation and subsubstation, and reset the light controls. This is all done without help from the PC. At the end of this polling, the PC takes over control of the collecting station, quickly reading the status of all the keys and setting the state of all the lights for the next cycle. When it is done, the next cycle commenses.

The Student Station

The crucial design criterion was to keep the student station as simple as possible, both because we need 330 of these, and because they are the most vulnerable to abuse. Also the number of wires to each keypad needed to be minimized, as the connection of the keypads to the subsubstations constitutes the bulk of the wiring. The keypad below enables three lights, red, yellow and green, to be separately controlled, and uses only eight wires for each keypad. The four row lines and the LE line are in common for the sixteen keypads controlled by a given subsubstation.

Diagram of Keypad Picture of Keypad

More detail on the student station is available.

The Subsubstation

The subsubstation can control up to 16 student stations, Logically, each of these is connected to the subsubstation with three individual column wires C0, C1, and C2. All student stations are also connected in parallel to one LED driver line LE and for row wires R0 - R3. For ease of wiring and connection, however, each station is actually connected with a separate 8 conductor modular cable

The function of the subsubstation is basically to compactify the signals. Six signals control which of the 48 columns is currently active. The Light enable signal is amplified, and the row input signals are buffered. This is connected to the substations by a 15 wire cable.

Diagram of Subsubstation Picture of Subsubstation

More detail on the subsubstation is available.

The subsubstation is mounted under one seat in each row (one in every other row in the side sections), and distributes the signals to, and collects those from, the individual student keypads. Each keypad is connected by an 8 conductor cable to the subsubstation. Even though five of the 8 signals are in common for all the keypads, it was deemed easier to connect, with modular cables and RJ45 connectors, if they were seperately brought to the subsubstation.

The Substation

Each of the substations controls up to eight subsubstations. It is mainly a signal compactification device, together with line drivers so that the communication with the collecting station is impervious to noise. It also stores the light signals for the current cycle. The row inputs from the eight subsubstations enter multiplexers which are controlled by the SS lines generated by the controller.

Diagram of Substation Picture of Substation

More detail on the substation is available.

The Signal Distributor

Because the controlling station needs to be located in close proximity to the PC, in the central lectern, it is not convenient to have it directly control the substations, which are located under the seats a minimum of one hundred feet of conduit away. Thus a signal distribution station was built in the center of the room on the ceiling of which the substations are mounted. The distribution station is again a collecting device. Each of the six substation inputs goes into one of two multiplexers, controlled by the SL signals. Together the two multiplexers deliver a byte at at time to the collecting interface. The light enable signals are treated differently, with the byte for each substation sequentially delivered to all workstations, but with a latch pulse sent by the collecting station to the appropriate workstation.

Due to the large number of signal lines (23 differential pairs to each substation and 23 differential pairs to the collecting interface), it was necessary to implement the collecting station on three boards, employing a mix of printed circuitry and wire-wrap.

Picture of Distribution Station

More detail on the signal distributor is available.

The Controller

This board controls the system when it is active, and is controlled by the PC during the time for interchanging data, during which the rest of the system is inactive. Using an oscillator and a counter, it generates the control signals for the rest of the system, reads in and writes out the data to the various substations, storing the results in RAM. The controls and address lines of the RAM are multiplexed between the PC output board and a EPROM which contains the control program for the active period. This board was implemented in wire-wrap.

Diagram of the Controller clock Diagram of the rest of Controller Picture of the Controller

More detail on the controller is available. In addition, there is a description of the commercial I/O board inside the PC which communicates with the controller.

Interconnections of These Compontents

A connection diagram will clearly needed to show how the 332 student stations, the 30 subsubstations, the five substations, the signal distributor, the controller, and the PC are all connected. One in PostScript is available here.
Revised October 11, 1995
shapiro@physics.rutgers.edu