The PDP-11 Maintenance Panel was introduced with the 11/20. It comprised
of two single height boards, a lamp driver and a lamp/switch board.
Through the use of the four switches, the processor clock could be
interrupted and single stepped through the ISR and BSR timing states of
the 11/20. Timing state numbers, Condition Codes and certain other flags
were displayed on the maintenance panels 16(?) lamps. Of course, having
the machine frozen in suspended animation also facilitated the
troubleshooting of the internal circuits by oscilloscope.
On the 11/40 there were two slots into which a maintenance panel could
be plugged. In the first slot you could single step the processor clock
and cycle through the microcode program. The processor could be made to
stop at a specific microcode address (MPC) set in the front panel
switches. The panel could be plugged into a different slot and perform
the same function for the Floating Point Processor. If you thought that
the PDP-11 had wide and complex micro-instructions, you should have
seen the FPP!
The panel also performed the same function on the 11/05 (no floating
point) and was also used on the 11/70. A good technician would have been
able to determine a fault on an 11/40 and 11/70 using nothing more than
the maintenance panel, front panel, and knowledge of the inter-relationships
of the processor's data paths.
[I am unclear whether the Maint Panel was used in later PDP-11s. I seem
to remember that certain of the micro-stepping and micro-trapping
functions were incorporated into the calculator-style front panels of
the 11/04, /34, /44 and /60. And I draw a total blank re: the later
Micro-PDP-11's. Perhaps someone can fill in some detail here? -MMcC]
The front panel on the PDP-11 went through some major revisions. The
original 11/20, 11/15, and later the 11/05 and 11/10, 11/40 and 11/35,
11/45, 11/50, 11/55, 11/70 and 11/74 were equipped with what the
purists regard as the "real" front panels. Subsequent machines (mostly
the LSI-11s) had either chicklet-calculator octal panels, or just a
trio of switches of various purposes (always including Enable/
Halt). These latter had Console ODT, instead.
The 11/20 panel is laid out roughly as (not to scale):
+-------------------------------------------------------+
| ADDRESS REGISTER RUN BUS FETCH EXEC |
| +---+---+---+---+---+---+ +---+---+ +-----+----+ |
| |ooo|ooo|ooo|ooo|ooo|ooo| | o | o | | o | o | |
| +---+---+---+---+---+---+ +---+---+ +-----+----+ |
| DATA SRC DST ADDR |
| +-+---+---+---+---+---+ +---+---+ +----+ |
| |o|ooo|ooo|ooo|ooo|ooo| |ooo|ooo| | ooo| |
| +-+---+---+---+---+---+ +---+---+ +----+ |
| SWITCH REGISTER L E C H S T D |
|\|/ +---+---+---+---+---+---+ +-+-+-+-+-+-+ +-+ |
| O |ttt|ttt|ttt|ttt|ttt|ttt| |m|m|m|t|t|m| |m| |
| +---+---+---+---+---+---+ +-+-+-+-+-+-+ +-+ |
+-------------------------------------------------------+
The small circles are incandescent lamps (i.e., prone to burning out).
The odd thing at the lower-left is the keyswitch. The 10 o'clock
position is OFF, straight up is POWER, and 2 o'clock is PANEL LOCK.
The lower-case `t's are toggle switches. The lower-case `m's are
momentary-contact switches. They are all large paddles, coloured
alternately magenta and orange. The left bank of switches are used to
set the address and data registers. The action switches on the right are:
- L LOAD ADDR, copies the switch register to the address register
- E EXAM, loads the data register from the memory location indicated
in the address register
- C CONT, continues the processor from a HALT
- H ENABLE/HALT, enables the processor to run if up, stops it if down
- S S-INST/S-CYCLE, selects the style of single-stepping
- T START, resets the system and starts the processor
- D DEP, deposits the contents of the switch register in the
memory location indicated by the address register. This switch
must be pushed up to operate, unlike any of the others.
The way ENABLE/HALT, CONT, and START work are (this is true of an
11/70 as well, and presumably of everything above an 11/40):
- ENABLE/HALT: If in the HALT position the machine will HALT at the
proper moment, depending on the S-INST/S-CYCLE. If in the ENABLE
position, the S-INST/S-CYCLE is ignored. The 11/40 also halts
the bus arbitrator, which has the side-effect of killing off any
in-progress DMA transfers. The 11/45 and 11/70 do the right thing,
other models have not been reported on as yet.
- CONT will just start the processor at where the PC points.
- START will issue a bus reset before starting.
If you want to do a bus reset without running the machine, you switch
to HALT, and depress START. I don't think it executes anything at that
time, which would imply that START can do less than CONT, which will
single-step in this case.
The 11/05 economized on the front panel. LEDs replaced lamps and there
was only a single row of them. They were multiplexed, and whether they
displayed data or address depended on the function being performed.
Action switches remained similar to the 11/20 with the possible
omission of the single step. [Help needed here ...]. The switches were
small and made of white plastic.
The other machines mentioned above had serious front panels. Great big
triangular wedge shaped keys; two rows of data and address lines; and
function lights. The 11/40 had:
Run, Bus, User,
Processor, Console, Virtual
The 11/45 and 11/55 had:
User I Super I Kernel I Prog Phy
User D Super D Kernel D Cons Phy
Data Paths u-Address FPP/CPU
BUS Register Display Register
...and a pair of rotary switches
to select between the two sets of displays!
The 11/70 display was similar if slightly different in detail. Of course
it had 22 of those nice big Data/Address switches!
With the advent of the 11/04, 11/34, and 11/60, two consoles were offered.
One could go the minimalist approach and purchase the cheapie and get a
Boot and Halt switch along with a Run lamp. Not exactly loved by those
that needed to interact with the machines intimately! Alternatively one could
purchase the more fully featured (KY11-B ?) panel with the chicklet keys
and calculator face. Many technicians had their own personal chicklet
panels for troubleshooting those machines ordered with the minimal
panel.
The 11/60 was shipped with only the calculator-style panel. I seem to
recall that DEC might have relented and done likewise with later 11/34
models.
More recent PDP-11s seem to have nothing more than a power switch,
although I believe a button or two and a couple of LED's might lurk
under the covers if you know where to look for them. | 
