Behavior Research Methods & Instrumentation
1981, Vol. 13 (2),221-226
Low-cost microcomputer networking: How to
get high throughput on a low budget
ADAM V. REED
Graduate Faculty ofSocial andPolitical Science
NewSchoolforSocial Research. New York. New York 10011
DANIEL LEWART
HolmdelHigh School. Holmdel. New Jersey 07733
and
LINDA H. SCHNEIDER
New York University. New York. New York 10003
A very low-cost, high-throughput laboratory data acquisition and experiment control.ysteDl
may be configured by using a 8tar network architecture with a IOW-C08t microcomputer u
network controller and one or more microprocessor-based single-board controllen a. satellites.
A network of this type, using Apple II microcomputer (fl,600 with 48K RAM and one
minidiskette drive) a8 main node and up to seven KIM-I microcontrollers (fl69 each) a8
satellites is described, and its development is discussed in detail.
This paper describes the local network approach to
the problem of increasing the throughput of laboratory
computer systems. The main advantage of this approach
is low cost. A rough rule of thumb used by system
planners is that with the traditional single-processor
approach, every doubling of throughput triples the cost
of the system. With the network approach, each satellite
processor adds about 10% to the cost of hardware for
the system.
With this kind of cost advantage, it is rather surprising
that the network approach is not more widely used in
experimental laboratories. One likely explanation for
this is the mystique acquired by networking in the early
days of computing. According to this mystique, network
development requires sophisticated development systems
and large amounts of professional effort. One of our
goals for this paper is to show how outdated this mystique has become in the last couple of years.
The microcomputer network described in the present
paper was developed with the network controller, a
standard integer-version Apple II microcomputer, as its
own resident development system. The software development effort took a total of 10 h of the first author's
time, as well as about 16 h of spare time contributed by
the second author while he was a student at Holmdel
High School. The hardware interface was built by the
third author in slightly under 5 h. While a psychologist
who is not also a computer engineer may have to add
between $500 and $1,000 in consultants' fees to the
cost of the network described in this paper, a microcomDaniel Lewart is now at the Massachusetts Institute of
Technology.
Copyright 1981 Psychonomic Society, Inc.
puter network could still constitute the most costeffective approach to adequate system throughput.
A computer network is a system of two or more
independently programmable processors, at least one of
which is capable of exerting complete control over all
the others under the direction of its own program. Our
design objectives were to provide adequate throughput
for current experiments, to provide future expansion
capability up to twice the maximum requirements of
any single foreseeable experiment in cognitive psychology, and to minimize development time for the present
system as well as for future experiments. Note that a
capability for simultaneous execution of several experiments was not one of our design objectives. In general,
the most cost-effective way of running several experiments is to run them on separate systems.
HARDWARE
The Central (Controller) Node
The Apple II was originally chosen as a laboratory
computer primarily because its all-socketed construction
facilitated the hardware modifications (Reed, 1979)
necessary for tachistoscopic stimulus presentation and
accurate timing of stimulus-response latencies. Socketed
construction also minimizes down time, since most
malfunctions may be repaired on site by removing the
malfunctioning integrated circuit (Ie) from its socket
and plugging in a replacement. The Apple also meets
both requirements for a central node in a local computer
network. The first of these requirements is that the
central node be equipped with digital input and output
lines capable of communicating with satellite processors.
221
0005 -7878/81/020221-06$00.85/0
222
REED, LEWART, AND SCHNEIDER
The other requirement is that the central node be usable
as a development system for real-time routines being
developed for implementation on the satellite processor.
If real-time program development is being done in
assembly language, this means that central node software
must include a symbolic assembler capable of assembling
code to an object (OBJ) location in random-access
memory (RAM) other than the actual logical origin
(ORG) the program is to have on the system on which it
will be eventually executed. In general, any assembler
having separate OBJ and ORG pseudooperations or
assembler directives will be capable of doing the job. The
assembler included in the Microproducts, Inc., development system package for the Apple II is equipped with
these features, and it is this package that is currently
used on the network to develop real-time software for
the KIM-I. Unfortunately, assemblers seldom have the
above features in the same package with interactive
assembly capabilities necessary for network development.
The networking software for the Apple II was actually developed without a symbolic assembler. We used
the interactive miniassembler included in the firmware
of the standard integer-version Apple II. The absence of
symbolic labeling capability in this firmware was more
than made up for by its interactive debugging and
structuring features. The integer-version Apple II is
equipped with a debugging monitor that has go (run),
list (disassemble), trace, single step, register examine,
and register fill commands, as well as multiple command
sequence capability. Any debugging monitor command
sequence may be executed without leaving the miniassembler. Moreover, the go command emulates a JSR
subroutine call, a feature that encourages FORTH-like
structured programming. An interactive assembler of
this type facilitates the segmenting of code into short
externally callable subroutines, each of which is fully
debugged and tested at the time it is written, and before
the programmer goes on to write the next segment.
The Satellite Node(s)
The microcomputer/microcontroller selected for the
function of a satellite in a laboratory microcomputer
network must satisfy four criteria. First, the instruction
set of its processor should be the same as that of the
central node's, a condition essential to uncomplicated
cross-development of real-time software for experiments.
Second, an external device, capable of being simulated
by the central node processor, should be able to load
desired information into the satellite's memory, to
obtain a dump of the satellite's memory on demand, and
to start program execution at any desired location in the
satellite's memory. Our requirement of reducing development effort to the minimum added an extension of
this condition: The software for all of the above functions had to be already present as firmware in the
satellite's read-only memory (ROM). This would free us
from having to write any real-time networking software
for any microcomputer other than the central node, on
which adequate development software was known to be
available. The third and fourth conditions were the
obvious ones: The satellite had to be able to perform the
functions necessary to carry out its role in psychological
experiments, and it had to fit our budget.
The first of the above conditions narrowed consideration to microcomputers using the 6502 processor used
in the Apple II. The second condition narrowed consideration to the several industrial microcomputer/
microcontroller boards designed for use with ASR serial
terminals, and for using the terminal's local storage facility (originally, the paper-tape facility of the ASR 33
Teletype) for memory loading and dumping and the keyboard of the terminal for control commands, including
the command to begin execution at a specified location
in memory. The boards in this category included the
JOLT and the SUPERJOLT, the KIM-I, the SYM, the
SUPERKIM, and the AIM-65. We selected the KIM-I,
at $159 the second cheapest of the above boards,
primarily because the second author was already thoroughly familiar with this unit, having used it as the
central component of his personal computer system.
With 30 digital input/output (I/O) lines and two precision timers, it clearly had the capability for taking over
the task of monitoring and recording subject responses
while the Apple dealt with precise presentation of experimental stimuli.
The Hardware Interface
While the serial interface of the KIM-I is designed for
20-rnA current-loop operation, serial information is
actually sent and received for processing at standard
5-V transistor-to-transistor logic (TTL) levels. Hence, we
decided to attach the interface cable to the KIM with
clip-on connectors bypassing the 20-rnA current-loop
circuits. The resulting interface is shown in Figure 1. In
keeping with our decision to minimize design time, we
did not bother to balance the transmission line. The
interface shown in Figure 1 works fine to 3 kbaud.
KIM-I firmware limits transmission rates to 9.6 kbaud in
any case, and the relative improvement obtainable by
designing a balanced-line interface would have been
limited to that rate. Apart from connectors, the interface shown in Figure 1 consists of three wires and one
resistor.
On the Apple II side of the interface, a single KIM-l
is attached to Input SWO and to Output ANO. Three
KIM-Is could be connected by using SWO-SW2 and
ANO-AN2. A multiplexer routing SWO and ANO according to ANI-AN3 may be used with existing software to
run a network comprising up to seven KIM-Is, with one
of the eight possible multiplexer codes reserved for loading and starting all the satellites simultaneously.
SOFTWARE
Teletype Emulation
In developing network control software for the
Apple II, we minimized development work by making
maximum use of published software. Sender software for
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KIM-I network interface connection.
Teletype emulation was adapted from the TTYDRNER
program (see Apple Computer, Inc., Note I, p. 119) by
R. Wigginton and S. Wozniak. This program was revised
to run at 30 times its original baud rate. The KIM-I
TrY input routine was adapted to the hardware available on the Apple II by replacing its timing subroutines
by corresponding subroutines from TTYDRNER
(Apple Computer, Inc., Note 1), again modified to run
at 3 kbaud.
NetworkControl
A IK area of the Apple's memory was set aside for an
input map of the KIM's memory. We then wrote a program for the Apple that uses the KIM's built-in TTY
dump routine to transfer an exact map of all of KIM's
RAM (except for the stack area in the top half of
Page 1) to the map area in the Apple. A similar program
transfers the content of a similar IK-byte "output map"
area from the Apple to the KIM (taking care not to overwrite RAM locations used by the KIM TrY load routines the program invokes). A third Apple program
starts the KIM at a location in KIM-I RAM specified by
4 bytes of RAM in the Apple II.
A fourth program for the Apple sends out a synchronization pulse that can be read by the sequence
BIT 1740
other. Finally, we insure the transfer of control back to
the Apple by having the Apple wait for its input to go
high. The KIM is programmed to return to "TrY"
control by ending its program with an unconditional
JMP IDBC.
Since all Apple II programs are also usable as subroutines, it was a simple matter to combine the above programs into two programs used in actual applications of
the network. The first of these maps from the Apple to
the KIM and then starts the latter at the desired location. The second waits until the KIM returns to Apple
control and then maps the KIM to the Apple. Each of
these programs takes approximately 15 sec. This is not
an unreasonable amount of time, especially since the
KIM has an unbroken block of 512 locations usable for
data storage. This is enough to store 4 bytes of data
from each of 128 trials, a reasonable size for a session in
most psychological experiments. If the amount of storage or I/O in the KIM proves inadequate, it may be
readily expanded, or additional nodes may be added to
the network, and so on.
REFERENCE NOTE
I. Apple Computer Inc. Apple 11 R(/ennce Mtlnlltll. Apple
Part No. 03().()()()4-(J(). Cupertino, Calif: Author. (Available from
Computer, Inc., 10260 Bandley Drive, Cupertino, California
9SOl4).
BPL#FB
when this sequence is executed by the KIM-I. This
simple protocol synchronizes the satellite node(s) to
within 6 microsec with the central node and with each
REFERENCE
A. V. Microcomputer display timil1l: Problems and solutions. Belulvior RU«lrch Methods tl Instrumentation, 1979,
11, S72-S76.
REED,
224
REED, LEWART, AND SCHNEIDER
Appendix A
Network Software Memory Map (Apple D)
9200-95FF
Map of KIM for next invocation of
BAOO or 8C40
8EOO-9lFF
Map of KIM from last invocation of
8BOO or 8C40
8AOC
8A82
8BOO
8COO
8000
Accept one character from Kim into A.
OUtput character in A to KIM.
Map KIM to Apple (16 sec):
0000-017F to 8EOO-8F7F
l780-l7FF to 8F80-8FFF
0200-03FF to 9000-91FF·
Map Apple to KIM (12 sec);
9200-92EF to OOOO-OOEF
9300-937F to 0100-017F
9380-93EF to l780-l7EF
9400-95FF to 0200-03FF.
Start KIM at the indicated IDea t i on . The command
format is *HHHH SOOOG, where HHHH is the starting
address for the KIM.
8063
Transfer control back to Apple after KIM does
8033
JMP lDBC.
Send a synchronization pulse to be read by the
KIM loop BIT 1740; BPL loop.
8D2A
Synchronize (8D33), then wait until KIM returns
8040
Map Apple to KIM; start KIM at desired location;
control (8063).
wai t until KIM returns control; then map KIM to
Apple. Command format analogous to 8DOO. Takes
30 sec plUS time to execute KIM program.
Appendix B
Network Software Listing
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Appendix C
Network Initialization Procedure for Satellites
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1. Set KIM-l KBD-TTY switch to KBD.
2. Press the following sequence of keys on the KIM keyboard:
(RS) (AD) 17F2 (DA) 18 (+) 00 (AD)
This sequence sets the baud rate to 3,000 baud.
3. Set the KBD-TTY switch to TTY.
4. Run appropriate network software from the Apple II.