DATAMATH

DATAMATH CALCULATOR MUSEUM

Datamath Calculator Museum DCM-50A Playground KBD102 Keyboard with BCD Encoder/Decoder

Date of introduction:	March 28, 2025	Display technology:	n.a.
New price:		Display size:	n.a.
Size:	3.5" x 7.5" x 0.45" 89 x 191 x 12 mm³
Weight:	2.6 ounces, 75 grams	Serial No:	0001
Batteries:	n.a.	Date of manufacture:	mth 04 year 2025
AC-Adapter:	n.a.	Origin of manufacture:	USA
Precision:		Integrated circuits:
Memories:
Program steps:		Courtesy of:	Joerg Woerner

The Datamath Calculator Museum DCM-50A (PLAYGROUND) is based on the generic DCM-50A Platform to access most features of calculators and certain features of products other than calculators that are not covered by the scope of the DCM-50A (TMS0100, TMS0800, TMS1000, TMS1200 and TMS0900) and the DCM-50A (OTHERS) category. It consists of different printed circuit boards (PCBs) that can be stacked on top of the DCM-50A Platform:

• DCM-50A Playground FRAME: Plug-In Carrier to access all voltages and signals from the DCM-50A Platform
• DCM-50A Playground DIL42: Daughter Board with 42-pin ZIF Socket, Patch Field, Diode Matrices, Push Buttons, SPDT Switches and Pin-Modifiers
• DCM-50A Playground BB400: Daughter Board with Solderless Breadboard (400 connections), Push Buttons and SPDT Switches

While these additions greatly enhance the flexibility of the DCM-50A Platform, is it still hampered by its keyboard design. Designed for compatibility with Texas Instruments single-chip calculator circuit introduced between 1971 and 1975, does it provide the following means to emulate a calculator keyboard:

• 11x4 switch matrix keyboard with patch field for SPST switches (e.g. Constant/Chain) or selector switches (e.g. F/4/2) with diode matrix
• Jumper field to select between digit scanning (D1 to D11) or segment scanning (SA to SG, SDP) of the keyboard
• Jumper to route D10 from TMS0100 and TMS1000 or DK from TMS0800 to keyboard column 10

Most Non-TI single-chip calculator circuits are covered by these specifications, but some manufacturers chose different approaches for keyboard scanning supported by the optional DCM-50A (PLAYGROUND) KBD123 Keyboard with Switch Matrix:

• Mostek's MK50310 Series uses an additional digit output D0 for scanning of a 12 x 3 keyboard matrix
• NEC's µPD946 Series uses an additional segment output SH and a V_SS column for scanning a 10 x 3 keyboard matrix and selector switches

The remaining known "non-scanning" single-chip calculator circuits are using one of three completely different approaches:

• GI's GI 250 and later C-500 Series uses 19 individual keys connected to one of the 24 pins of the calculator chip
• Mostek's MK6010 uses a 4-bit BCD-Encoded input port for the numbers and individual keys for the functions
• NEC's µPD271 uses a 4-bit BCD-Decoded output port for the numbers 1 to 9 and the functions and individual keys for the [0] and [C] keys

To enable the Characterization of these very early and in the History of single-chip calculator circuits important devices, we developed here at the Datamath Calculator Museum the DCM50A Playground KBD102 Keyboard with BCD Encoder/Decoder. It is plugged on top of the DCM-50A Platform and centered around a 10x2 switch matrix keyboard with different assembly options:

• 10 individual number keys available at N1 to N10 and their common pin NC
• 10 optional pull-up or pull-down resistors for each number key, available as NR signal
• 10 individual function keys available at F1 to F10 and their common pin FC
• 10 optional pull-up or pull-down resistors for each function key, available as FR signal
• BCD Encoding/Decoding Diodes with selectable polarity for number keys [0] to [9], available as E8, E4, E2 and E1 signals
• Selector switch for encoding/decoding Zero into BCD '0' or BCD '10'
• 10 optional diodes for a "wired-or" signal NOR indicating a key press of any number key
• 10 optional diodes for a "wired-or" signal FOR indicating a key press of any function key

All pins of the matrix (20 keys, common pins, pull-up/pull-down pins, 4-bit BCD Encoded/Decoded numbers, wired-or'd numbers and functions) are directly accessible on pin headers and can be connected with the matching pins on the DCM50A PG C-500, DCM-50A PG MK6010 or DCM50A PG µPD271 Daughter Boards.

KBD102 Keyboard - C-500 Mode: General Instrument's original PICO1 design, commercially known as GI 250 single-chip calculator circuit, its GI 251 mask-option and various successors known as C-500, C-550, C-560, C-570 and CZL-550 are using up to 19 individual keys connected directly between a so-called KB_EN output and dedicated input pins or multi-functional input/output pins of the calculator chip. The KBD102 Keyboard (C-500 Mode) uses only the optional pull-down resistors and no Encoding/Decoding.

KBD102 Keyboard - MK6010 Mode: Mostek's original MK6010 design, its MK5010 and MK5012 variations and Cal-Tex's CT5001, CT5002 and CT5012 "clones" are using a dedicated 4-bit BCD-Encoded input port for the numbers and individual keys for the functions. All pins are using "negative" logic which is selected on the KBD102 Keyboard (MK6010 Mode) with the polarity of the diodes used as BCD Encoder and the [0] key is encoded as BCD '10'.

KBD102 Keyboard - µPD271 Mode: NEC's µPD271 uses four Timing Signals T1 (2⁰), T2 (2¹), T2 (2³) and T4 (2³) to scan BCD-Decoded numbers between 1 and 9 on its NCOM input line and BCD-Decoded functions on its FCOM input line. The [0] and [C] keys are connected directly to dedicated input pins of the µPD271 chip. All pins are using "positive" logic which is selected on the KBD102 Keyboard (µPD271 Mode) with the polarity of the diodes used as BCD Decoder. Nine green jumper wires connect the BCD Decoder from the Number keys [1]..[9] to the corresponding function keys and two pairs of brown jumper wires and two blue wires connect the [0] and [C] keys directly between V_SS and the corresponding input pins.

Please notice that the DCM-50A Playground KBD102 Keyboard needs to be plugged into the switch matrix connectors on the DCM-50A Platform for proper alignment.

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