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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
   
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)

LittleProf_MTA5077_Back.jpg (140033 Byte)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.

LittleProf_LTA4276_PCB.jpg (131577 Byte)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.

 

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If you have additions to the above article please email: joerg@datamath.org.

Joerg Woerner, November 18, 2023. No reprints without written permission.