Behavior Research Methods & Instrumentation1979, Vol. 11 (2),314-315

A microprocessor- (HeathET-3400) basedbackup control system for

laboratory experiments

W. LLOYD MILLIGAN and ANTHONY RICHARDSONNeuroscienceLaboratory, Veteran'sAdministration Hospital, Columbia, South Carolina 29201

This paper describes a psychology laboratory experimental control system that utilizes aPDP-UI10 minicomputer to perform both instrument control and data acquisition functions.The minicomputer is backed up by a Heath ET-3400 microprocessor trainer system whichperforms control functions only. Major components of the hardware and software comprisingthis system are described.

Psychophysiological experiments in the NeuroscienceLaboratory, Veteran's Administration Hospital,Columbia, South Carolina, are automated by a DigitalEquipment Corporation PDP-I 1/10 minicomputer withan LPS-ll lab peripheral system. The computer per­forms both control and data acquisition functions, usingthe LPS-ll digital input/output (I/O) registers, eight­channel multiplexor, and analog/digital (A/D) converter.Data sensed from laboratory instruments are stored inmemory or on scratch disk files until the end of a testsession (typically about 90 min), when they are analyzedby a background program. Upon completing the analysis,the program writes session parameters (classificationvariable values and dependent variable means) to perma­nent data files.

Development of this system has evolved continuouslysince acquisition of the PDP-ll in November 1975. Atfirst, the minicomputer was used for control functionsonly, and these were shared by external hardwired logiccircuits (BRS Digibits). The external logic also servedas a backup system whenever the PDP-II failed. Withimplementation of computer control, and later auto­matic data acquisition, the complexity of experimentsincreased. The relative ease with which parameters couldbe varied in software enticed us to make modificationsthat were not readily duplicated in the hardwired back­up system. Moreover, with increasing disuse and hybridi­zation, the reliability of the backup system suffered. Iteventually became apparent that a more dependablebackup system was needed. Renovation of procedurecontrol rooms in the spring of 1978 provided a favor­able opportunity to replace this system with amicroprocessor-based system.

Two important considerations influenced the overalldesign of the backup control system. First, it wasevident that the system should be microprocessor based,in order to preserve the flexibility of varying experi­mental parameters in software. A second considerationwas the requirement that both systems be capable ofcontrolling the same external laboratory (stimulus and

response) instruments, that is, those used in experi­mental procedures. The problem of controlling identicallab instruments by two different computer systemswas solved by designing "front ends" for each instru­ment. Thus, we designed a polygraph controller, shockstimulator controller, tone generator/amplifier controller,and set switch (Milligan, 1977) controller. The frontends each presented a single standard TTL load persignal line to the controlling device. One other devicewas needed to distribute the signal lines. This card, thedistribution module, performs minimal logic functionsin conveying computer or microprocessor outputs to theinstrument controllers.

The choice of microprocessor was influenced by athird consideration. It was felt that the Heathkit ET­3400 (Motorola M6800) trainer would serve the dualpurpose of teaching the principles of microprocessorcontrol systems and providing the central componentof the backup system. In retrospect, we are satisfiedwith this choice, as many examples and exercises in theHeath training course are directly applicable to the psy­chology laboratory situation. Subsequent experiencewith the Commodore PET 2001 microcomputer suggeststhat it also would serve well as a psychology laboratorycontrol computer. The PET has an IEEE bus interfaceand a general-purpose user port which consists of the Bside of a 6520 peripheral interface adapter (PIA). ThePET would be a more suitable choice for the user whowishes to program his application in an advanced lan­guage with a minimum of hardware and software de­velopment costs.

Figure 1 shows the arrangement of components inour laboratory. All components to the right of, andincluding, the distribution module are common to boththe PDP-II and ET-3400 control systems, with theexception of the dashed line at the top. This line repre­sents eight channels of analog data from the polygraphto the PDP·ll.

Inputs to the distribution module are conveyed viaa 25-conductor ribbon cable that plugs directly into the

314

MICROPROCESSOR BACKUP SYSTEM 315

REFERENCE

the DR-A. For example, Bit 0 of the low-order byterepresents the "trial interval," Bit 1 is CS on, Bit 2 istest-trial select, and so forth. All definitions are "lowenable"; the final output stage of the DR-A is an open­collector buffer (7416) normally pulled high. Intervaltiming is accomplished identically by both systems. Inthe PDP-ll, the system time-of-day (60-Hz) clock isinvoked by a .TWAIT macro; in the ET-3400, a 60-Hzinterrupt clock is included in the control program. Thesequence of time intervals is embedded in a set of nestedcounters: SETS C 8-TRIAL BLOCK C 8-BWCKSESSION. A minor disadvantage of the microprocessorsystem is that the control program and experimentalconstants (time intervals, counters, etc.) must be keyedin manually. Since it is primarily an emergency backupsystem, this drawback can be tolerated.

Figure 2 shows the interface circuitry that connectsthe ET·3400 system to the distribution module. All ofthe bus connections (address, data, and control lines)on the ET-3400 are fully buffered.

The distribution module and each controller isconstructed on a separate prototyping PC board, and allboards are contained within a single shielded enclosure(BUD AC1428). Control signals are conveyed from thedistribution module to the controller cards via double­ended DIP jumpers that plug into l4-pin sockets on eachboard. Outputs to the instruments leave the cards atthe foil-edge connectors.

A typical control program for executing one of theexperiments performs all the control functions of atwo-tone autonomic conditioning experiment for fouranimals simultaneously. The analogous control and dataacquisition program for the PDP-ll is approximately4K bytes long. In the ET-3400 backup system thecontrol part alone is approximately 300 bytes long.

In summary, the microprocessor-based system wehave described is an economical and reliable backup forthe control functions of the laboratory minicomputer.In addition, a level of flexibility is attained that wouldbe difficult to duplicate with hardwired logic. Thesystem, as described, retains the possibility of futuremodification and expansion to accommodate experi­ments of increasing complexity.

MILLIGAN, W. L. Electromechanical switching of preamplifierinput leads: A method of increasing the usefulness of electro­physiologicalrecording instruments. Psychophysiology, 1977, 14,507·508.

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Figure 1. Arrangement of components in NeuroscienceLaboratory, Veteran's Administration Hospital, Columbia,South Carolina.