DATAMATH CALCULATOR MUSEUM
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DATAMATH CALCULATOR MUSEUM |
Texas Instruments Little Professor (1976 Version XC)
| Date of introduction: | June 13, 1976 | Display technology: | LED-stick |
| New price: | $19.95, £11.95 | Display size: | 8 |
| Size: | 5.0"
x 3.5" x 1.1" 127 x 89 x 29 mm3 |
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| Weight: | 4.2 ounces, 119 grams | Serial No: | n.a. |
| Batteries: | 9V | Date of manufacture: | wk 38 year 1978 |
| AC-Adapter: | Origin of manufacture: | USA (MTA) | |
| Precision: | Integrated circuits: | TMS0975/ZA0356 KCS or GCS | |
| Memories: | Displays: | DIS713 | |
| Program steps: | Courtesy of: | Joerg Woerner | |
| Download manual: | 
(US: 7.3M Bytes) |
The
final version of the original Little
Professor introduced in June 1976 before it was replaced in December 1978
with the even more cost optimized Little Professor
(1978) based on the TI-1000.
From the technology this
First Generation Little Professor introduced in Summer 1976 is related
to the TI-1200 calculator of the same
year. You'll notice a similar evolution during the production of the Little
Professor to optimize
manufacturing costs but a minor mishap added one interims step not observed with
the TI-1200 and its sibling TI-1250
and all their variations like the fancy
T-1225. The pictures on the right compares two Little Professors, an
early one and a later one. Here at the Datamath Calculator Museum we named them
based on the Revision of the used single-chip calculator circuits accordingly
Version A, Version B,
Version C, and Version XC.
Dismantling the featured Little Professor (Version
XC) with Date code 3878 MTA and manufactured in
September 1978 in Midland, Texas reveals an internal construction very similar to the
TI-1200/TI-1250. The single-sided printed circuit board (PCB) is centered around
a TMS0975/ZA0356 single-chip calculator circuit and powered by a 9V alkaline
battery. The keyboard with 5 rows of keys is borrowed from the TI-1200 although
the Little Professor is using only 4 rows of keys.
The
internal design of this final version of the Little Professor (Version XC) looks
identical to the Version Little Professor
(Version C) and they share the layout of their PCBs.
The
TMS0975/ZA0356 chip is a member of the
TMS0970 Product Family introduced in March
1976 with the TI-1200 and based on
the TMS1000.
The TMS0970 integrated both segment and digit drivers to the TMS1000 feature set
allowing for highly cost-optimized designs and paving the way of four-banger
calculators with 4-key memory selling below the magic $10 threshold. While the
original TMS0970 chips were housed in a standard 0.6” wide 28-pin DIP (Plastic
Dual In-line Package with a 0.1” / 2.54 mm lead pitch), started Texas
Instruments in 1977 to use a smaller 0.4” wide 28-pin SPDIP (Shrink Plastic Dual
In-line Package with a 0.07” / 1.778 mm lead pitch) design.
We repeated our little experiment to calculate the frequency of the internal clock oscillator for this TMS0975NL/ZA0356 KCS, a TMS0975/ZA0356 GCS from a second Little Professor (Version XC) manufactured in September 1978, too and an early TMS0975NL/ZA0356 AP and summarized the results:
| DUT | Display Scan Rate with CLK PCB |
Measured CK (Ext) |
Display Scan rate w/o CLK PCB |
Calculated CK (Int) |
| 3776 LTA AP 7636 |
93 Scans/sec | 150 kHz | 198 Scans/sec | 320 kHz |
| 5076 LTA BP 7640 |
60 Scans/sec | 150 kHz | 130 Scans/sec | 325 kHz |
| 1878 MTA CSP 7818 |
58 Scans/sec | 150 kHz | 162 Scans/sec | 420 kHz |
| 3878 MTA KCS 7832 |
58 Scans/sec | 150 kHz | 160 Scans/sec | 415 kHz |
| 3678 MTA GCS 7827 |
58 Scans/sec | 150 kHz | 164 Scans/sec | 425 kHz |
The "Shrink" in the two measured devices at the bottom of the above list and marked with CSP and GCS suggests that they were manufactured in an improved 6.0 um metal gate PMOS process while the two devices at the top of the list still used the original 8.0 um metal gate PMOS process". Main reason to shrink the transistors, metal connection traces and other features on silicon chips is reducing manufacturing costs (less silicon, higher yield) but an obvious side effect is the improved speed, too. While the original TMS0970 chips operates in the Little Professor at about 320 kHz, oscillate the revised TMS0970S chips at much higher pace of around 420 kHz.
Preparing our
DCM-50A Platform
to allow the Characterization of Single-Chip Calculator Circuits
of the TMS0970/TMC0900 Family, we studied a TI-1270 calculator manufactured in
July 1976, a Little Professor (Version A)
manufactured in September 1976, a TI-1200
manufactured in October 1976, a Little Professor (Version B) manufactured in December 1976, a
TI-1250 calculator manufactured in
August 1977, a Little Professor (Version C) manufactured in May 1978, a WIZ-A-TRON
educational toy assembled in June 1978 and this Little Professor (Version XC) manufactured in September 1978.
Learn more about the different Versions of the Little Professor (1976) and its Product Labels on the backside of the calculators.
While the "look and feel" of the four versions of the Little Professor (1976) is identical, did we encountered significant differences in the software implementation of the TMS0975NL chips despite their identical "ZA0356" marking. Follow this link to deep dive into the TMS0975NL/ZA0356.
We couldn't identify any differences in the chips with the CSP and GCS or KCS markings and assume that Texas Instruments changed in 1978 just the definition of their package codes. The main differences between the remaining three iterations of the TMS0975NL/ZA0356 can be summarizes in the following table:
| Version | Package | Level Switch Scanning |
Segment Scanning Style |
"EEE" Output Approach |
Display Scan Cycle @ 150 kHz |
| TMS0975NL ZA0356 AP |
DIP28 | Segment F | O6 → O0, Blank Fixed Timing |
PLA E Normal Scanning |
12.4 ms |
| TMS0975NL ZA0356 BP |
DIP28 | Segment F | O6 → O0, Blank Variable Timing |
PLA 6 Segment C Skipping |
16.8 ms |
| TMS0975NL ZA0356 CSP |
SPDIP28 | Segment DP | O7 → O0, Blank Variable Timing |
PLA 6 Segment C Skipping |
17.1 ms |
Looking into the much slower Display Scan Cycle of the "BP" chip we assume that it was already prepared for a faster internal clock oscillator but didn't meet some other specifications, causing Texas Instruments to "repair" Little Professor's in November and December 1976 before having a more cost-effective fix available that included switching to a smaller SPDIP28 package of the "CSP" chips.
This first version of the Little Professor could easily recognized by the two sliding switches to the left and right of its face, in December 1978 they were replaced by three additional keys [OFF], [SET] and [LEVEL] slightly disturbing the design of the product. Meet the Little Professor (1978.
Another education toy of this time are the rare MATH MAGIC and its sibling WIZ-A-TRON.
If you have additions to the above article please email: joerg@datamath.org.
© Joerg Woerner, November 18, 2023. No reprints without written permission.