The PDP-16 Register-Transfer Modules
and
The PDP-16/m "Subminicomputer"

The PDP-16 was a collection of register-transfer-level modules used for designing unique (or relatively low production volume) systems. The control mechanism of these systems was created by connecting together modules representing the blocks in a flow chart. The data path of these systems was created by connecting together data, memory, and transducer (I/O) modules. Cross-connections allowed the control mechanism to control the data path and branch in response to information coming back from the data path.

The PDP-16/m was a canonical configuration of PDP-16-series modules that was really a small microprogrammable computer. The control mechanism trivially interpreted a microprogram stored in a braided-wire ROM, in which a wire for each word of memory passed through or around a core for each bit in a word, indicating whether or not that bit in that word was a 0 or a 1. (I believe that this was the same basic technique as used in the PDP-9, although that wasn't modifiable by customers.)

From [DEC 1971]:

Figure 1-1: Factors Suggesting Use of PDP-16:

Factor	Logic module system	PDP-14	PDP-16	Full-scale minicomputer (PDP-16, PDP-8/e, OR PDP-11)
Type of computation	Boolean (single-bit) functions	Boolean functions	Binary arithmetic commands and subroutines-multiply, binary to BCD, etc.	Same as PDP-16
Day-to-day program changes	No	No	No	Yes
Number of identical systems per year	1-10	1 or more	1 or more	1-10
Memory requirements	<20 bits	1K including control steps	1-16 16-bit words typical, up to 256 16-bit words (read, write, and/or 1K 16-bit read-only) for special applications	>1K, 12 to 16 bits
Control program steps	<20	<1000	20-250 (excluding looping and memory accesses)	>150
Cost	<$1000	>$3000	$800-$4000	>$3000

Table assembled from information in [DEC 1971] and [Bell et al. 1972]: PDP-16 modules:

	English Name	PMS Name	Generic Name	DEC Module Number	Description
	Evoke units	K(evoke)	KEV16	M7310
	Two-input OR gates	K(serial merge; 4 input)	KOR16-A	M1307	10 2-input NOR gates for merge of control flow or branch input expansion.
	Four-input OR gates	K(serial merge; 2 input)	KOR16-B	M1103	6 4-input NOR gates for merge of control flow or branch input expansion.
		K(subroutine call)	KSR16	M7315	6 subroutine return units. One start pulse circuit.
		K(branch 2-way)	KB16-A	M7312	6 2-way branch units.
		K(branch 8-way)	KB16-B	M7314	2 8-way branch units.
	Bus sense	K(bus sense and termination)	KBS-16	M7304
	Bus terminator		KTM-16	M962
		K(bus control and termination combined)	KBS16-A	M7332
		T(lights and switches)	KBM16X	M7334
		K(clock)		M401
		D(NOT)		M111
		D(NOR)
		D(decoder; 4 input)		M161
	Arithmetic control	K(function encoder)	KAC-16A	M7330
	Arithmetic registers	DM(arithmetic register unit)	KAR16-A	M7331
		DM(gpa; control)	KAC16	M7300
		DM(gpa; registers)	KAR16	M7301
	General-purpose interface	T(general purpose interface)	DB-16A	M7311
	Output interface	T(output interface)	DB16-B	M7316
	Input interface	T(input interface)	DB16-C	M7317
	Serial data interface	T(serial interface)	DC16-A	M7313
	Constants generator		MR16-A		4 16-bit constants, set via jumpers on the board.
	Transfer register	M(transfer)	MS16-A	M7305	16-bit register with the ability to read or write the whole word or either byte.
	Byte register	M(byte)	MS16-B	M7320	16-bit register with the ability to read the low 4, 8, 12, or 16 bits and with the ability (via jumpers on the board) to read with bits interchanged
	Scratch-pad memory	M(scratchpad; 16 words)	MS16-C	M????18	16 words of 16 bits. There is a separate signal to control reading and writing of each word.
	Scratch-pad memory	M(array; 256 words)	MS16-D	M7319	256 words of 16 bits. The address must be loaded from the bus before the data can be read or written.
	Scratch-pad memory		MS16-E		1024 words of 16 bits. The address must be loaded from the bus before the data can be read or written.
	Flags	DM(flag)	KFL16	M7306	3 1-bit flags (D flip/flops).
	No-op unit	K(no-op)	KNP-16	M7321	Provide a 100-200ns delay in a control signal.

Every PDP-16 system must have a KBS-16 and a KTM-16.

The backplane contains a 32-bit data bus used to transfer data between modules.

The KAC-16 and KAR-16 are used together, in adjacent slots of the backplane, so that an edge connector can join them on the handle-end.

[Editor's notes: The M-series modules (named for their maroon handles) were largely built from 7400-series TTL SSI/MSI ICs. Some of the above modules, such as M111, were standard modules with uses outside of PDP-16 systems, and consist of a relatively small number of ICs on a single-height, single-width module; as such, one may conclude that they were fairly wasteful of a level of packaging, trading off space and cost for ease of assembly of the system. Compare this aspect of the PDP-16 modules to the Universal Control Modules. ]

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