Elliott 503 FACTS

Extract from the booklet Elliott 503 FACTS booklet


The 503 is a fast general purpose digital computer designed for scientific applications. The logic elements use high-speed  circuits based on high frequency transistors and diodes so that the 503 is completely transitorised and compact. The basic computer consists of a central processor, a power unit and a control station. The 503 incorporates a fully autonomous facility for controlling the transfer of data between its central processor and its peripheral equipment.

The design follows normal practice for single-address machines. One operand is contained in the accumulator and the other in the store location specified by the address given in the instruction. Either operand or the result of an operation is stored in this location after the function has been performed. The current instruction is held in the instruction register to which is connected a decoding system for selecting the function to be performed and the store location required. Facilities are provided for double-length working in multiplication and division.

Dimensions and Weights

Height, width and depth in inches, weight in pounds.
Logic Cabinet 78 x 38 x 24 1,000
Main Store Cabinet 78 x 38 x 24 850
Power Cabinet 78 x 38 x 24 1,350
Full Controller Cabinet 78 x 38 x 24  1,000
Half Full Controller Cabinet  78 x 38 x 24  750
Peripheral Services Cabinet 78 x 38 x 24  1,200
Backing Store Cabinet
  (Full + controller)
78 x 38 x 24 1,050
Backing Store Handler
  (32,768 words)
78 x 38 x 24 800
Backing Store Handler
  (16,384 words)
78 x 38 x 24  600
Control Console  35 x 50 x 33  850
Reader Console  35 x 31 x 33  250
Cold Air Unit  48 x 27 x 73  1,350
Magnetic Tape Handler 77 x 23 x 24  740
Line Printer 55 x 58 x 30  1,900
Card Reader/Punch 59 x 27 x 47  950
Elliott Card Reader 50 x 41 x 22  300
Digital Plotter
  (without stand)
10 x 18 x 14  33
High Speed Output Printer 46 x 34 x 26  400
Display Panel 22 x 37 x 5  75
Display Panel and Stand 50 x 40 x 24  110

The basic 503 uses eight-track paper tape input and output and has a main storage capacity of 8,192 words. The core store unit is a block of 4,096 words operating at a cycle time of 3.6μsec. The basic machine has two of these units. The arithmetic unit provides for fixed-point and floating point operations.

The following peripheral units may be attached to the 503 in conjunction with the appropriate logic boards. These are available as optional extras:

    Core Backing Store
    Magnetic Tape Handlers
    Line Printer
    Card Reader/Punch
    Elliott Card Reader
    Digital Incremental Plotter
    High Speed Output Printer


Paper tape input  1,000 char/sec.
Paper tape output    100 char/sec.
Output writer 10 char/sec. 
Backing store 15,800 words/sec.
Magnetic tape speed 42,000 char/sec.
Line printer 1,000  lines/min. (max.)
  (2,000 char/sec. max.)
Card Reader/Punch 300 cards/min.

(400 char/sec.) 
Elliott Card Reader 340 cards/min. 
   (450 char/sec.) 
Digital Plotter  300 steps/sec. 
High Speed Output Printer 100 char/sec. 

503 is a parallel binary computer, fixed-point number representation being such that numbers are held in the range -1 £ x < +1, with two's complement notation for negative numbers, and standard floating point form x = a.2b such that -1 £ a < -½ or a = 0 or ½ £ a < 1 and -256 £ b < 256. The word length is 39 bits.

The main control panel is an array of 13 buttons and/or lamps. To the left of this array is a 39-key word generator. The usual method of controlling the 503 computer is via the Reserved Area Program, in conjunction with the control typewriter and the message button. The majority of 803 programs can be run on the 503, by means of the '803 Operator' program, provided five-track paper tape equipment is available. A loud-speaker, driven from one of the control signals, is also provided.

Two single-address instructions occupy one word. By means of a single B-line placed between the two instructions, the second instruction may be modified by adding to it the content of the location specified in the address portion of the first, wherever this location may be. As each instruction is obeyed, the count in the sequence control register is increased by a half, unless the instruction causes a transfer of control. Selection of the succeeding instruction follows completion of an operation immediately. 


When the RAP is in the store, pressure on the MESSAGE button causes entry to the RAP which then assumes a message will be input via the typewriter.

IN. Causes the computer to read a specially prepared, relocatable binary tape. 
IN;N. Reads a special binary tape whose position in the store is optional. The integer N specifies where the program is to be placed.
NAME.  Transfer control to the program called NAME. 
N.  Transfers to the first instruction in location N.
N;S. Transfers to the second instruction in location N. 
CONT.  Continues a program from the point at which is was left to obey a Manual Interrupt. 
CONT;ERRINT.  Continues a program from the point at which it was left to obey an Error Interrupt. 
LIST. Prints out in "chronological" order the names of the programs in the store beginning with the last to be stored. If a checkable program is incorrectly stores its name is followed by an asterisk. The size of the available store is printed after each list.
CANCEL. Removes last program stored.
CANCEL;NAME.  Beginning with the last program stored, removes all programs as far back as and including the named program. 
RESET.  Clears the main store and resets the RAP pointers.
FREE STORE. Causes the size of the available store to be printed on the output writer. 

The messages printed on the output writer, when action from the operator is called for are:-

NOPROGNamed program is not available.
ERRSUMA sum-check error has been detected on attempting entry to a program.
NOROOMThere is not enough room in the store for the program being input. 
*If this appears after a listed program name that program is corrupt. 

The messages printed on the output writer to help the operator know the state of the computer are:-

?This is printed on a new line to show that manual interrupt has been accepted.
>Entry to a named program has taken place correctly, via the program head. 
UNCHECKThe program is not sum-checkable. 
ENDThe running of a program has been successfully completed and control has returned to the RAP.


ERRINT 1Floating point overflow
ERRINT 2Spare 
ERRINT 3Parity error in main store
ERRINT 4 Attempted use of unavailable peripheral device 
ERRINT 5 Impermissible reference to the reserved area of the store 

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