Printing with the SR-51/SR-51A?

Back in the days when the PC-100A Printer Cradle was introduced, there was a lot of talk about its compatibility with the SR-51 and SR-51A Scientific Calculators. Although not officially supported by TI, several people were able to use the PC-100A with both SR-51 and SR-51A by putting the PC-100A in SR-52 mode. Some people found that the print mode worked fine with their SR-51s but never could get the trace mode to work, getting an "1" and a "?" on the printout. Here at the Datamath Calculator Museum we developed in 2023 our DCM-0500 (Platform) for reverse-engineering of calculators based on the groundbreaking TMS0500 Building Blocks and we decided to finally solve the mystery about the epic question "Can I print with my SR-51 or SR-51A calculator?" The answer is pretty simple: "Kind of, but it depends..."

In a first step we need to look closer at the four different version of the PC-100 Printer Cradle:

Neither the SR-51 nor the SR-51A calculators are including the firmware for printing in their two SCOM (Scanning Read-Only Memory) Chips and rely instead on the TMC0561 BROM (Bare Read-Only Memory)Chip in the PC-100 Printer Cradle. Consequently are only the original PC-100 and the more common PC-100A Printer Cradles candidates for successful printing with the SR-51 and SR-51A. And yes, the TMC0561 holds the firmware extension for the SR-52 Programmable Calculator while the TMC0569 is responsible for the SR-56 Programmable Calculator.

In a second step we focus on the SR-51 and SR-51A Scientific Calculators:

The overview above lists only the three main types of SR-51 and SR-51A calculators, we observed during the life cycle of the calculators some minor differences that we will focus later on.

The mechanical and electrical interface with the SR-51/SR-51A calculators is accomplished by the key-actuated mounting bracket/connector on the PC-100/PC-100A Printer Cradles which fits into the battery pack opening in the calculator. For a successful printing with the SR-51/SR-51A both a mechanical fit and electrical compatibility is essential. Mounting the SR-51 on the PC-100/PC-100A Printer Cradle is finicky, we tested a few calculators and none fitted perfectly with the mounting brackets and battery contacts.

The 12-pin docking connector uses 11 signals to communicate between the SR-52 or SR-56 calculator and the PC-100 Printer Cradle:

With major differences between the electronics of the three different main types of the SR-51 and SR-51A calculators we analyze each design individually.

SR-51: Comparing the Main-PCBs of four different evolutions of the SR-51 Scientific Calculator manufactured between August 1974 and May 1975, one month before its discontinuation, shows only subtle differences in the layout but holding a big surprise! The early three Main-PCBs have Pin 5 of the Print Cradle connector floating while the SR-51 from the last production month has Pin 5 wired to Pin 14 of the TMC0522/TMC0523 SCOM stack, its D0 Digit Time output. Looking into the circuit diagrams of both the SR-52 Programmable Calculator and the PC-100 Print Cradle we understand that a connection between D0 and the keymatrix input KP of the TMC0501 Arithmetic Chip is used to detect the PC-100. Or in other words: Texas Instruments consciously removed in the first iterations of the Main-PCB design the capability of the SR-51 Firmware to detect a possibly connected PC-100 - explaining why some SR-51 had success with printing and others not.

To avoid interference between sliding or locking switches and momentary push button switches during keyboard scanning, the TMS0500 Building Blocks requires extra diodes for all non-momentary switches. The SR-51 and SR-51A use a sliding [R-D] switch for the selection of radians and degrees while the PC-100 Printer Cradle uses a latching switch for the [TRACE] function. The additional hard-wired connection to detect the presence of a PC-100 Printer Cradle act as an always-on switch and consequently requires a diode inside the SR-51/SR-51A, too. The keyboard mappings of the SR-51 and SR-51A are identical and use keymatrix inputs KS for the [R-D] switch and KP for both the [TRACE] switch and presence detection of the Printer Cradle PC-100, hence requiring two extra diodes.

The Diode D1 for keymatrix input KP is easy to spot on the SR-51 PCB while the Diode D2 for keymatrix input KS is hiding in plain sight - it is actually located on the larger display and keyboard PCB of the calculator.

SR-51A (Version 1): During the analysis of the SR-51 we learned to focus on three prerequisites for successful printing with a SR-51 or SR-51A Scientific Calculator:

The first bullet point is a no-brainer, the SR-51A is using the same housing as the SR-56 which was officially supported to work with the PC-100 and PC-100A Printer Cradle. Check.

The connection between Pin 14 of the TMC0522/TMC0523 SCOM stack (Digit Times output D0) and Pin 5 of the Printer Cradle connector is easily visible on the PCB of the SR-51A (Version 1). Check.

And last but not least are two Diodes D1 and D2 mounted directly next to the TMC0501E Enhanced Arithmetic Chip with traces running from the cathode of D1 to Pin 9 (keymatrix input KS) and from the cathode of D2 to Pin 12 (keymatrix input KP). Bingo

We attached an SR-51A (Version 1) Scientific Calculator on a PC-100A Printer Cradle, pushed the [TRACE] switch and started entering some numbers, calculating the average and variance values and were greeted with a crisp print-out. Quod erat demonstrandum.

SR-51A (Version 2): With the SR-51A (Version 2) sharing its printed circuit board (PCB) with the SR-56, we didn't expect any show-stopper for its successful operating on a PC-100A Printer Cradle. And yes, the SR-50A (Version 2) is using the same PCB, too but disqualifies immediately for two reasons:

We analyzed already the "Clock Generator Patch Area" of the unified SR-50A/SR-51A (Version 2) and SR-56 PCB and learned that the SR-51A uses like the SR-56 its second (TMC0533) SCOM Chip to generate the clock signals and outputs the Digit Time D0 signal with its first (TMC0532) SCOM Chip. Comparing the three PCB assemblies of the calculators reveals many differences and we are not only concerned about the SR-51A missing the gold-plated PC-100 Printer Cradle connector but seeing many more diodes on the SR-56 assembly than on the SR-51A (Version 2) assembly.

Digging deeper into the visual differences, we located a total of nine diodes on the SR-56 PCB but only four diodes on the SR-51A (Version 2) PCB and started to reverse-engineer the schematics of the calculators from the layout of the PCB and its populated components. The Diodes D1 - D9 and Resistors R1, R2 are used for the functionality listed below:

Matching the missing components of the SR-51A (Version 2) PCB with the above listed overview of their respective purposes results in the following list:

And consequently can't the SR-51A (Version 2) be used on the PC-100 Printer Cradle!

Including some minor variations observed during the life cycle of the SR-51 and SR-51A Scientific Calculators between 1975 and 1977 we created a "Compatibility Matrix" for successful printing with a PC-100 or PC-100A Printer Cradle: