TEXAS TI-74 BASICALC & 8K RAM Cartridge  teardown

ANSI BASIC programmable and scientific calculator with 8KByte internal constant memory (7.5KByte available to the user), extended to 16KByte with a 8KByte constant memory removable cartridge.



Power supply:
4 x AAA alkaline battery cells, or 6.3V DC external supply to pin-2 of the I/O port.



BASIC programability
Close compliance with ANSI BASIC with enhanced feature tailored to a portable calculator.
Numeric or BASIC line less expressions are executed in immediate mode.

Radix-100 floating point numbers using 8-Bytes per number. 1-Byte for exponent and 7-Byte for up to 14-digits BCD mantissa.
 
CI-7 Audio Cassette Interface for data and programs off-line storage.
PC-324 24 column thermal printer. 

The 8KByte RAM cartridge can be set as Constant Memory for data and programs, or set for 8KByte Mass Storage device.

Optional 32KByte ROM Cartridges with applications from several suppliers.
Optional EPROM Cartridges for low volume applications.  

Scientific calculator mode
13 digit accuracy, logarithmic, hyperbolic and trigonometric, 2-var statistics, and probability functions. 10 user registers. Alphanumeric error messages.

And a removable cartridge with 8KByte RAM.

Main hardware components listing.

On the CPU PCB:
- C70009 CMOS SoC: 8-bit TMS70C46 microcomputer, 4KByte PROM, 128-Byte RAM, 8-bit timer/counter with 5-bit pre-scale register, from Texas, DIP54, dated as 1985
- HN61256PC93: 32KByte CMOS PROM 4uS max access time, DIP28, from Hitachi Japan, (c) Texas, dated as 8707
- HM6264LP-15: 8KByte CMOS SRAM 150nS max access time, DIP28, from Hitachi dated as 8703
- SI7660CJ: CMOS Switched Capacitor Voltage Converter (typ. 12Khz), DIP8, dated as 8706
- RC4193N: Switching Regulator (typ. 75Khz), DIP8, dated as RC706

On the Keyboard/Display PCB:
- HD44780 LCD Controller, FP-80B, from Hitachi
- HD44100 LCD Driver, FP-60A, from Hitachi


The internal 8KByte constant memory uses a regulated power supply that is maintained when the console is powered off.


The single line 31 5x7 dot matrix characters LCD display with 14 annunciators requires a total of 1280 pixel elements driven at 1/5 bias, 1/16 duty cycle multiplexed drive, with 16 column drive connectors and 80 segments drive conenctions.

Texas TI-74 8KByte RAM constant memory removable cartridge details

The constant memory cartridges housing a HM6264 8KByte CMOS SRAM memory uses a 3-Volt Lithium battery able to retain data up to 5 years when removed from the console.

Texas TI-74 8K RAM removable cartridge total teardown & schematic diagram

The year is 2016. This Texas TI-74 8K RAM Cartridge internal details are about non existent.
Until now, that is. I did a complete tear-down on this one and have draw a schematic diagram as well to be shared with the rest of the world.


This small box sports a sturdy and solid build.
Having no visible screws I decided to remove the metal plate. To make this task easier  I have used a hot air drier to soften the glue a little and a x-acto knife to lift the metal plate.

 

A small metal spring is used to connect the internal shield to the external metal plate.

This is important as this metal plate is connected to the TI-74 calculator internal shield when the cartridge is inserted to comply with radio frequency regulations.

The 30-pin external cartridge port interface protocol is very well documented in the publicly  available Texas TI-74 Technical Data Manual.
Get one copy while it still is available.

Next I removed the plastic label  looking for screws. No luck.

So the only way was to force the cover to lift a little on both sides of the case.
In this particular case, the best choice for this job was to use a large screwdriver applying a little twisting force at the slot one side at a time.

The cover started to lift and came out easily, although the four cover latches were destroyed in the process.  This was caused by the very rigid case that was not flexing at all to allow a cleaner job. 

The PCB assembly is totally shielded by a tin plated aluminum foil.
 

 Gently lifting the PCB assembly at the connector side and then pulling it out worked nicely.

 Unwrapping the shield. The soft glue didn't offer special resistance.

Label printed on the connector:
"Made in Japan. 
HL61023-15D J 642"

Constant memory is achieved thanks to a Sony CR-2430 3Volt Lithium cell battery that may have last up to 5 years.
This battery was depleted at 0.370Volt after more than 34 years, but at least it didn't leak, so the electronics is intact and fully operational as long as the cartridge is kept inserted into the calculator to keep receiving the 5Volt for RAM retention from the calculator batteries.

The battery cell is soldered to the PCB. It is a 2-tagged horizontal PCB mount type.

Side views.

Battery cell unsoldered and removed.

All parts together now.

TI-74 8K RAM Cartridge schematic diagram

This a partial diagram but it covers all the PCB components.
The only missing details are the SRAM address and data lines but this is easy to figure out when needed.

Component list:
Double sided PCB.
IC1: HM6264LFP-15 8KByte CMOS SRAM   150nS.
D1, D2: 1N270 BKC (100V, 500mAp, Vf 300mV)
R1: 1K8 1/4W axial
R2: 3K3 SMD
R3: 150R SMD (PCB bottom side)
C1: 100nF 50V SMD
C2:  1nF 50V SMD
BATT: CR-2430 3V Lithium
30-pin connector.