Texas Instruments SR-51A (Version 2)

Date of introduction:  June 1975 Display technology:  LED-stick
New price:  $224.95 Display size:  10 + 2
Size:  5.8" x 3.2" x 1.3"
 147 x 81 x 32 mm3
Weight:  8.5 ounces, 240 grams Serial No:  080781 B
Batteries:  BP1A Date of manufacture:  wk 18 year 1976
AC-Adapter:  AC9130, AC9130A Origin of manufacture:  USA (LTA)
Precision:  13 Integrated circuits:  TMC0501, TMC0532, TMC0533
Logic:  Sum-of-Products Displays:  DIS234H
Memories:  3    
Program steps:   Courtesy of:  Joerg Woerner

The SR-51A was introduced shortly after the SR-51 to reduce manufacturing costs. Later during production, probably with the introduction of the SR-56, the SR-51A was revised again. The hardware of both the SR-50A and SR-51A was redesigned to accommodate the TMC0501 Arithmetic Chip, one (SR-50A) or two TMC0530 SCOM (Scanning and Read-Only Memory) Chips (SR-51A and SR-56) and none (SR-50A and SR-51A) or one TMC0599 Multi-Register Chip (SR-56). Here at the Datamath Calculator Museum we refer to the TMC0530 based SR-50A and SR-51A calculators as "Version 2", they can easily be recognized with a bold "B" trailing their serial numbers printed on the label on the back of the calculators.

Basically the same printed circuit board (PCB) was used with different chip populations to manufacturer three different calculators priced between US$ 109.50 and US$ 224.95, and thanks to the power of marketing the most complex design priced at US$ 179.95 and already featuring the revolutionary Algebraic Operating System (AOS™) introduced in September 1975 with the SR-52 Programmable calculator while the SR-51A priced at US$ 224.95 was still relying on the outdated Sum-of-Products (SOP) Logic tracing back to the SR-50.

Calculator Logic Arithmetic
Register Chip
SR-50A (V2) SOP TMC0501 TMC0531   US$ 109.50
SR-51A (V2) SOP TMC0501 TMC0532
  US$ 224.95
SR-56 AOS TMC0501 TMC0537
TMC0599 US$ 179.95

Dismantling the featured SR-51A Version 2) calculator with Date code LTA 1876 and manufactured in April 1976 in Lubbock, Texas and comparing with an SR-51A (Version 1) calculator manufactured in November 1975 reveals some unexpected changes. While the earlier calculator 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 TMC0533 SCOM Chip replacing the TMC0523 of the original SR-51 and SR-51A (Version 1) featuring an integrated Clock Generator for the TMC0501 Arithmetic Chip and the optional TMC0599 Multi-Register Chip used with the SR-56.

Other areas of the PCB didn't change that dramatically, we still identify the two SN27882 display drivers known from the previous designs of the calculator and the small Display Module soldered directly to the Main-PCB.

Comparing the PCBs of the three siblings SR-50A (Version 2), SR-51A (Version 2), and SR-56 (in TI internal documents often dubbed SR-51P for Programmable) reveals some very subtle differences, too. Right below the power supply module for the internal calculator voltages VDD (-10.0 Volts) and VGG (-15.8 Volts) you'll find a 50 and a 51 marking. Depending on the calculator model three configurations are populated:

• SR-50A: A 150 kOhm resistor soldered into the left (50) position connects the CLK pin and VGG of the TMC0531 to enable its internal clock generator and providing the two
   non-overlapping clock signals with 180° phase difference and a frequency around 180 kHz to the TMC0501 Arithmetic Chip
• SR-51A: A 150 kOhm resistor soldered into the right (51) position connects the CLK pin and VGG of the TMC0533 to enable its internal clock generator and providing the two
   non-overlapping clock signals with 180° phase difference and a frequency around 180 kHz to the TMC0501 Arithmetic Chip. The clock generator of the TMC0532 is disabled and it
   generates the D0 signal instead for possible PC-100 Printer Cradle compatibility
• SR-56: A 30 kOhm soldered into the right (51) position connects the CLK pin and VGG of the TMC0538 to enable its internal clock generator, a 39 pF capacitor between the CLK pin
   and VSS greatly improves the stability of the clock frequency. The clock generator of the TMC0537 is disabled and it generates the D0 signal for PC-100 Printer Cradle compatibility

In the lower-left area of the PCBs you'll notice a small symbol for a diode but only the SR-56 has actually a diode soldered into the position, both the SR-50A and SR-51A are using a piece of wire, instead. From a technical point of view the diode is necessary for operation of the calculator on a PC-100 Printer Cradle. While the Key input matrix line KP of the TMC0501 Arithmetic Chip is used in stand-alone calculator mode for various keys (D1, D2, D3, D4, D6, D7, and D13), doubles it in the PC-100 Printer Cradle application to signal three additional conditions or actions:

• D0: Presence detection of PC-100 Printer Cradle
• D12: [PRINT] key detection
• D15: [TRACE] switch detection

The SR-56 uses a second set of diodes and current limiting resistors for the charging circuitry of its battery pack to accommodate for the increased power consumption of the calculator due to the TMC0599 Multi-Register Chip compared to the SR-51.

Last but not least features only the SR-56 gold-plated contacts for the PC-100 Printer Cradle. Learn more about Printing with the SR-51 / SR-51A.

As of today we know two revisions of the consolidated PC used with the SR-50A (Version 2), SR-51A (Version 2) and SR-56 but noticed only one minor change in the "Clock Generator Patch Area". The unused wiring positions in the lower area of the PCB were removed to free up some space for the slightly larger power supply module. The picture on the right compares the relevant part of the PCB from two SR-51A calculators manufactured in April 1976 and November 1976.

Comparing the Constant ROM Content with 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 TMC0532A Chip of the featured SR-51A (Version 2) with our TMS0500 Platform after recording their ROM Images showed no differences. Looking into the constants programmed into the TM0533A SCOM Chip reveals as expected values like 2.54 (inches to centimeters), 0.3048 (feet to meters) and 0.9144 (yard to meters) known already from the TMC0523A-5 Chip located in an SR-51 calculator manufactured in March 1975.

The SR-51A is featured in the Texas Instruments Deutschland GmbH leaflets ER-1975 and ER-1976 dated 1975 resp. 1976.

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© Joerg Woerner, January 6, 2024. No reprints without written permission.