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Fairchild Semiconductor developed early in the 1970s a family of micro-programmed MOS/LSI (Metal–oxide Semiconductor/Large Scale Integration) processor blocks called PPS 25 (Programmed Processor System - 25 Digits) to bridge the gap between simple electronic calculators and microcomputers. A minimal PPS 25 design uses six Micromosaic chips interconnected by 4-bit data buses and various control signals, corresponding directly to the block diagram of a computer based on the Harvard architecture.

At the core of the system is the CPU, composed of the 3805 Arithmetic/Logic Unit and the 3806 Control Unit. The 3808 and 3809 Data Memories each provide three dynamic 25-digit shift registers, while the 3810 Program Memory provides 256 Words * 12 Bits of storage. The 3803 and 3807 Input Device chips support up to 32 keys and 16 switches each, and the 3811 Output Device drives displays of up to 16 Digits.

The basic keyboard device is the 3807, it is generating a 5-bit code for each key that makes a transition from an inactive state to an active state. This 5-bit code is stored on the 3807 Input Device until a command occurs in the program, instructing it to transmit the 5-bit code to the 3806 Function and Timing Unit. In addition the 3807 Input Device is scanning up to 16 static switches. The 3803 Input Device extends the keyboard capabilities beyond 32 keys.

The 3807/3803 Input Device chips are combining the following features in one 40-pin package:

• Four Scanning Outputs D1-D4 for up to 32 keys and 16 switches • Eight Key Matrix Inputs R1-R8 • Eight Switch Matrix Inputs S1-S8 • Alarm Input Lock • Direct Key Output KN • Serial Micro Instruction Input MI • Anti-bounce Feature • Two-key rollover (legato)

QUICK-LINK to PPS 25 Building Blocks.

Type	Year	Function	Products	Comments
3807	1973	Input Device	Centurion Industries Multiputer CPD-15, CPD-35 Cybernetic Systems Mathiputer CPD-15, CPD-35	40 pin Ceramic or Plastic DIP

Item	Min	Typ	Max	Unit	Comments
VSS	4.75	5.0	5.25	V	TTL, DTL compatible
VDD		0		V
VGG	-9.5	-10.0	-10.5	V
VOH1	2.4		VSS	V	1 TTL Load
VOL1	0		0.4	V	1 TTL Load
VOH2	VSS-1.0		VSS	V	MOS Load
VOL2	0		0.5	V	MOS Load
VIH1	VSS-1.0		VSS	V	DTL/TTL compatible
VIL1	VDD		VSS-4.2	V	DTL/TTL compatible
VIH2	VSS-1.0		VSS	V	Clock Input
VIL2	VGG		VSS-14.0	V	Clock Input
Ext. CK		400		kHz	Two-phase clock
CP1 Width				us	Active low
CP2 Width				us	Active low
CP1 to CP2 Delay				us	Between pulses

 

PPS 25 - 3803/3807 Keyboard Device
Pin	IO	Function	Pin	IO	Function
1	O	Scan Output D2	40	O	Scan Output D3
2	O	Scan Output D1	39	O	Scan Output D4
3	V	Positive Voltage VSS	38	O	Alarm Input Lock
4	V	Negative Voltage VGG	37	O	Key Output KN
5	V	Common Voltage VDD	36		N.C.
6		N.C.	35	I	Switch Input S1
7	I	Key Input R5	34	I	Switch Input S2
8	I	Key Input R4	33	I	Switch Input S4
9	I	Key Input R6	32	I	Switch Input S3
10	I	Key Input R3	31	I	Switch Input S5
11	I	Key Input R7	30	I	Switch Input S6
12	I	Key Input R2	29	I	Switch Input S7
13	I	Key Input R8	28	I	Switch Input S8
14	I	Key Input R1	27	I	SYNC
15		N.C.	26		N.C.
16	I	TEST	25		N.C.
17		N.C.	24		N.C.
18	I	Micro Instruction Input MI	23	I	Clock P2
19		N.C.	22	I	Clock P1
20		N.C.	21		N.C.

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

© Joerg Woerner, May 8, 2026. No reprints without written permission.