Texas Instruments SR-56 (Pre-Launch)

According to Texas Instruments the SR-56 was introduced May 21^st, 1976 which corresponds to week 21 of the year. French collector Francis Martin was first reporting to the Datamath Calculator Museum that he discovered two SR-56 calculators with manufacturing Date codes of LTA 1176 and LTA 1276, a whooping 9 resp. 10 weeks before the date of the "official" introduction. Dismantling the calculators revealed Date codes of the two Scanning SCOM (Scanning Read-Only Memory) Chips of week 11, too for proof that the SR-56 were produced already in March and we assumed that Francis is the lucky owner of rare "pre-production run". We consequently started a Database with Serial Numbers and Date codes for SR-56 calculators and invited our fellow calculator aficionados to report their findings accordingly.

Next in line was Ken Kaplan reporting his SR-56 with Serial# 046479 and a production Date code LTA 0876 with SCOM Chips from week 5 before Lynn Shelley reported in March 2010 his SR-56 with Serial# 003594 and a production Date code 0476 DTA sporting SCOM Chips from weeks 02 and 03.

The discrepancy between SR-56 calculators manufactured as early as January 1976 and its "formal" introduction in May 1976 wasn't understood till we studied some notes from a former TI Quality Controls Engineer revealing that while the first member of the new TMC0530 SCOM Chip, the TMC0531 was up and running in March 1975 for a timely introduction of the SR-50A (Version 2), did the TMC0530 design not work properly and the rework of the Base Mask Design took more than 6 months. Texas Instruments consequently introduced an interims design for the SR-50A (Version 1) and SR-51A (Version 1) with the original TMC0520 SCOM Chips known from SR-50 and SR-51 and postponed the introduction of the SR-51P, later renamed to SR-56. Here at the Datamath Calculator Museum we label SR-56 calculators manufactured significantly before May 1976 as "SR-56 (Pre-Launch)" to study their differences (if any) to later SR-56 calculators.

The featured SR-56 with Serial# 004403 falls squarely within the reported Serial Numbers 003594 and 004496 of the earliest production Date codes 0476 DTA.

Dismantling the featured SR-56 (Pre-Launch) calculator manufactured in January 1976 in Dallas, Texas and comparing with an SR-56 calculator manufactured in December 1976 in Rieti, Italy reveals no major changes and incorporates the unified printed circuit board (PCB) used with SR-50A (Version 2) and SR-51A (Version 2). While the previous designs of the SR-50A and SR-51A uses a SN97227 Clock Generator Chip in a small 8-pin DIP (Dual In-line Package with a 0.1” / 2.54 mm lead pitch) housing together with a TP4011A Clock Buffer Chip in a 14-pin DIP housing to reduce both real estate on the PCB and manufacturing costs compared to the original SR-50 introduced in January 1974, are these two Integrated Circuits (ICs) gone. And no, they are not hiding under the small power supply module on the right side of the PCB boosting the output of the rechargeable BP1A Battery Pack to the required voltages of the calculator electronics.

The secret sauce of the revised design is the new TMC0538 SCOM Chip replacing the TMC0521 Chip of the original SR-50A (Version 1) featuring an integrated Clock Generator for the TMC0501 Arithmetic Chip and the TMC0599 Multi-Register Chip used with the SR-56. We assume that the extra TMC0599 Chip didn't fit on the limited real estate of the SR-50A (Version 1) PCB and we for this reason don't know an SR-56 (Version 1).

The only change we discovered between this SR-56 (Pre-Launch) and other SR-56 calculators is the use of a different display driver, we identified two SN97311 chips already known from the SR-52 instead the SN27882 chips usually located with SR-50A and SR-51A calculators from the same time frame.

Comparing both the Instruction ROM and Constant ROM Content of several sets of TMC0537 and TMC0538 SCOM Chips uses with SR-56 calculators manufactured in February 1976, May 1976 and December 1976 with our TMS0500 Platform after recording their ROM Images showed no differences. An additional TMC0537 SCOM Chip manufactured in December 1975 for Quality Control Purposes matched these TMC0537 Chips retrieved from the three SR-56 calculators, too. Comparing the programmed constants frequently used with computing algorithm of trigonometric functions like sine, cosine, or tangent of an SR-50 manufactured in May 1974 still using the original TMC0521-2 SCOM Chip with the TMC0537NL A7606 Chip of the featured SR-56 (Pre-Launch) showed as expected no differences.

We experienced a much bigger surprise when we were analyzing the Constant ROM Content of the TMC0538NL A7606 Chip and discovered Keycodes for the "Special Functions" of the SR-56 instead the Constants for the "Conversion Keys" of the SR-51A.

Analyzing the six programs totaling 127 program steps, we identified the routines for Polar/Rectangular Conversions [P→R], [R→P], Mean and Standard Deviation Calculation [Mean], [S.Dev.] and Data Entry [∑+], [∑−].

Fellow collectors - if you own a SR-56 please report us the serial number and date code on the back of the calculator for our SR-56 Database.

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

© Joerg Woerner, January 13, 2024. No reprints without written permission.