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Texas Instruments introduced in January 1974 with the SR-50 a product utilizing only the bare minimum of the capabilities of the TMC0500 Building Blocks for Scientific and Programmable Calculators but the subsequent introduction of the SR-52 Programmable and the PC-100 Printer Cradle clearly demonstrated the scalability of this groundbreaking architecture. While the SR-52 included an internal reader for magnetic cards to store and load programs with up to 224 steps, lacked the lower-end SR-56 any means to archive its programs and the user had to enter the program manually after turning the power switch into the On position. With just 100 program steps available for the user of the SR-56, an annoying but acceptable approach. Things changed dramatically when Texas Instruments introduced in May 1977 with the TI-58 its successor offering a maximum of 480 program steps and the calculator had still no external or internal media to archive them. Texas Instruments consequently introduced in March 1979 the TI-58C with Continuous Memory, meaning that both user program and user data are maintained even if the power off condition. Instead of porting the TMC0598 Multi-Register Chips designed in power-hungry PMOS technology used with the TI-58 into modern CMOS technology, the engineering team decided to use a standard SRAM (Static Random Access Memory) manufactured in a power-saving CMOS process and developed with the TMC0590 Continuous Memory Interface Chip a TMC0500 Building Block to translate the signals of the TMC0501E Arithmetic Chip to the common signals of SRAMs with a capacity of 1k*4 Bits. 

The TMC0501E Arithmetic Chip uses a total of nine signals to communicate in the TI-58 with the TMC0598 Multi-Register Chips:

• IDLE, CLK 1 and CLK 2 Outputs are used to synchronize all peripherals connected to the TMC0501E Arithmetic Chip with the 16 States of its Instruction Cycle • EXT Output indicates that the TMC0501E Arithmetic Chip is addressing external memories/registers • IRG Input to receive the 13-bit Instruction Words from the TMC0520/TMC0530/TMC0540/TMC0560/TMC0580 ROM Chips • 4-bit bidirectional I/O Bus I/O 8, I/O 4, I/O 2, and I/O 1 to communicate with Multi-Register Chips and other peripherals

Standard SRAMs use a completely different approach to interface with Microprocessors and use the following signals:

• Bidirectional Data Signals Data I/On-1 – Data I/O0 with n being the width of the Data Bus • Address Signals Am-1 – A0 to address up to 2m Data Words • /CE Chip Enable Input to activate the SRAM and switch from low-power data retention mode to active mode • /WE Write Enable to store the Data Word applied to Dn-1 – D0 into the memory cells addressed with the signals Am-1 – A0 • /OE Output Enable to control the direction of the bidirectional Data Signals

With the TMC0598 Multi-Register Chips manufactured in PMOS technology and powered with V_DD = -10V and CMOS SRAMs powered with V_DD = +5V, the TMC0590 Continuous Memory Interface Chip performs three main functions:

• Converting the TMC0500 communication protocol to the standard SRAM interface • Translating the PMOS levels of the TMC0501E Arithmetic Chip to the CMOS levels of the SRAM • Enabling and disabling the low-power data retention mode of the CMOS SRAM

Texas Instruments used with the first design of the TI-58C a 1k*4 Bits CMOS SRAM manufactured by RCA, the inventor of CMOS technology, with a TI internal part number in a 18-pin DIP (Dual In-line Plastic) encapsulation. We assume the part is actually a MW5114 chip sporting only /CE and /WE signals while the second design of the TI-58C makes use of either a Toshiba TC5047AP or NEC uPD445L chip in a 20-pin DIP housing and featuring in addition to the /CE, /WE, and /OE signals an unused CE2 pin.

The TI-58C uses an additional -5 Volt power supply fed to the GND pin of the CMOS SRAM while its V_DD pin is tied to the V_SS pin of the PMOS devices. When the calculator is turned off, a Germanium diode with a low voltage drop connects the GND pin of the CMOS RAM to the negative battery terminal to provide the voltage necessary for data retention in this state. An additional 470 µF capacitor connected in parallel to the battery keeps the memory safe for a few minutes even in case the battery pack is replaced.

Node	Voltage	PMOS	CMOS	Comments
Battery +	0 V	VSS	VDD
Battery -	-3.75 V		GND	Retention Mode
VDD2	-5.0 V		GND	Active Mode
VDD	-10.0 V	VDD
VGG	-15.8 V	VGG

The only known application of the TMC0590 Continuous Memory Interface Chip is the TMC0591 chip used with both the first and second design of the TI-58C.

QUICK-LINK to TMS0500 Family.

Type	Calculators	Function
TMC0591	TI-58C (1979), TI-58C (1980)	Continuous Memory Interface

Item	Min	Typ	Max	Unit	Comments
VSS		0		V
VDD2	-4.3	-3.75	-3.3	V	Retention Mode
VDD2	-5.5	-5.0	-4.5	V	Active Mode
VDD	-10.5	-10.0	-9.5	V
PHI1, PHI2		225	230	kHz	Opposite phases

Pin	IO	Function	Pin	IO	Function
1	V	Negative Voltage VDD	28	O	SRAM Address A8
2	I	Instruction words	27	O	SRAM Address A9
3	I	Calculating status	26	O	SRAM Address A6
4	IO	ARITH Interface D0	25	O	SRAM Address A0
5	IO	ARITH Interface D1	24	O	SRAM Address A1
6	IO	ARITH Interface D2	23	O	SRAM Address A2
7	IO	ARITH Interface D3	22	O	SRAM Address A3
8	I	Clock Input 2	21	O	SRAM Address A4
9	I	Clock Input 1	20	O	SRAM Address A5
10	IO	SRAM Data I/O3	19	O	SRAM Output Enable
11	IO	SRAM Data I/O2	18	O	SRAM Chip Enable
12	IO	SRAM Data I/O1	17	O	SRAM Address A7
13	IO	SRAM Data I/O0	16	V	Negative Voltage VDD2
14	O	SRAM Write Enable	15	V	Common Voltage

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

© Joerg Woerner, March 02, 2021. No reprints without written permission.