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Apr 8, 2014

Digital's DECSYSTEM-20 - Part 7 - Console


The KY11-C Console was located in the cabinet # 1 on the high right (see next picture).

Cabinet # 1 (front end processor cabinet)

KY11-C Console Panel abowe the PDP-11/40 drawer


The KY11-D Programmer's Console provides the programmer with a direct system interface. The console allows the user to start, stop, load, modify, examine, step, or continue a program. Console displays indicate processor operation and the contents of the address and data registers.

The programmer's console interacts with the processor through a microprogram control located in the processor. The console contains only indicators (light emitting diodes), switches, and the contact bounce filtering circuits for the control switches. Console operation does require certain Unibus operations through the processor: DAT0 for DEP and DAT1 for EXAM. For single-step operation, the processor responds to a Console Bus Request (CBR) whose priority supersedes all other BR priorities. Note that use of the KM11 Maintenance Console option provides a further display of machine states, and allows single microword stepping.

KL10 Panel in detail

The KY11-D Programmer's Console consists of the KY11-D Console Board (5409701) and two cables (BC08R-06) which are used to interconnect the console to the KD11-A processor. Both power and logic signals are provided by these cables that connect to the DATA PATHS (M7231) board and the STATUS (M7235) board. Operating instructions for the console are included in the PDP-11/40 System Manual.

Power switches above the Console Panel in DECSYSTEM-20


The display on the console consists simply of Light Emitting Diodes (LED's) with current limiting resistors; the drivers for these displays are located on the DATA PATHS and STATUS boards of the KD11-A processor. Input signals from the processor are shown at the left of the displays; console notation for the displays is shown in parenthesis near the diode symbol.

A rusty PDP-11/35 before restoration. Connectors J1 and J2 shown on top right. 11/35 was similar to 11/40 but the card cage was rotated 90 degrees

Connectors (J1, J2) for processor interconnection are shown above. These connectors provide for the display signals from the processor as well as the Switch Register data and control signals to the processor.

Panel LED's and switches


The data switch inputs from the Switch Register are simple resistor inputs. The control switches have Set-Reset flip-flops to eliminate contact bounces in addition to a driving gate.

An additional switch for Off, Power, Console Lock are also provided. Its connectors (J3, J4) consist of two quick disconnect tabs to allow direct interconnection to the Power Control Unit.

In earlier versions the panel was connected to the drawer


The BC08R-06 cables are interconnected to the KY11-D console (J1, J2) and the M7231 and M7235 modules according to the instructions on the printed circuit boards and the circuit schematics. Orientation of the shield is specified, and required for proper interconnection. Connection for power control to J3 and J4 is simple as this connector provides only a switch closure, either interconnection of two wires is acceptable.

PDP-11/35 and PDP-11/40 console indicators, switches and toggles

Since the front end processor of the DECSYSTEM-20 was a PDP-11/40 the console of the PDP-11/40 is shown here. Many PDP-11's had similar panels to the PDP-11/40 (and KL10).


Address and Data registers

ADDRESS Register

displays the address of data just examined or deposited. During a programmed HALT or WAIT instruction, the display shows the next instruction address.

DATA Register

displays data just examined or deposited. During HALT, general register R0 contents are displayed. During Single Instruction operation, the Processor Status word (PS) is displayed.

Status lights


lights when the processor clock is running. It is off when the processor is waiting for an asynchronous peripheral data response or during a RESET instruction. It is on during a WAIT or HALT instruction.


lights when the processor has control of the bus.


lights when the UNIBUS is used.


lights when in console mode (manual operation). Machine is stopped and is not executing the stored program.


lights when the CPU is executing program instructions in User mode.


lights when the ADDRESS Register display shows the 16-bit Virtual Address.


Load address or data using switches

SWITCH Register

used to manually load data or an address into the processor. Switch position "up" = '1' ,and "down" = '0'.

Switches to the right were function switches


transfers contents of the Switch Register to the Bus Address register. The resulting Bus Address is displayed in the ADDRESS Register, and provides an address for EXAM, DEP, and START. The LOAD address is not modified during program execution. To restart a program at the previous Start Location, the START switch is activated.


causes the contents of the location specified by the Bus Register to be displayed in the DATA Register. If the EXAM switch is depressed again, the contents of the next sequential word location are displayed (Bus Address is incremented automatically). If an odd address is specified, the next lower even address word will be displayed. If a non-existent memory address is specified, no UNIBUS operation will be completed, and contents of the Switch Register address (777570) will be displayed in the DATA Register.


causes the processor to continue operation from the point at which it has stopped. The switch has no effect when the CPU is in the RUN state. If the program has stopped, this switch provides a restart without a System Reset.

Mostly momentary function switches


ENABLE: allows the CPU to perform normal operations under program control. HALT: causes the CPU to stop. Depressing the CONT switch will now cause execution of a single instruction.


if the CPU is in the RUN state, the START switch has no effect. If the program had stopped, depressing the START switch causes a System Reset signal to occur; the program will then continue only if the ENABLE/HALT switch is in ENABLE.


deposits contents of the Switch Register into the location specified by the Bus Address. If the DEP switch is raised again, the Switch Register contents (which were probably modified) are located into the next word location (Bus Address is incremented automatically). If an odd address is specified, the next lower even address word will be used. If a non-existent memory address is specified, no UNIBUS operation will be completed, and contents of the Switch Register address (777570) will be displayed in the DATA Register.

PDP-11/70's panel was similar to the PDP-11/40

The following video is most informative about how the panel was used.

Video about how to enter a program into the computer's memory and run it

Running Programs

When running any program, the program must first be loaded into the core memory either manually or via the automatic loading programs (bootstrap loader or absolute loader). Once the program is in storage, it can be run at any time by loading the starting address of the program (refer to appropriate program documentation) into the Switch register, depressing the LOAD ADRS switch, and then depressing the START switch. The user also must make certain that the ENABLE/HALT switch is in ENABLE and that the appropriate external devices are on-line (connected to the computer).

Inside the panel

The program can be manually stopped at any time by setting the ENABLE/HALT switch to HALT. It can be restarted from that point by returning the ENABLE/HALT switch to ENABLE and depressing the CONT switch. It can be started anew by reloading the starting address and depressing the START switch.

PDP-11/40 in good shape

A program can be altered during operation, or new data introduced, through the Switch register. This console register has a bus address that the processor can reference in its instruction sequence. The information transferred may be treated as data or used to alter program flow.

PDP-11/40 modules sticker, fans and power supply connections

Because of the speed of the computer, console indicators are of limited value while the computer is running. Console indicators are used primarily during manual operation, single instruction operation, or during the maintenance mode. During manual operation, the console indicators reflect the console operations of LOAD ADRS, EXAM, and DEP. During maintenance operations, the console indicators display various data functions of the processor as the maintenance module is used to step through the program a microword at a time. Use of the maintenance module is described in the KD11-A Processor Maintenance Manual, EK-KD11A-MM-001.

See references for more information about how to use the console.

[The next part of this series will cover connectors at the back of the cabinets.]


/1/ PDP-11/40 System Manual

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