The rear five racks of ERNIE, numbered from 1 to 5 from left to right, hold more of the many DC power supplies required to run ERNIE's valves and transistor circuits, the fuses and power distribution, the all-important redundancy table, the serial number generator and the teleprinter drive units.
Virtual ERNIE - virtualcolossus.co.uk ©2019
The rear racks hold most of the larger 200v and 50v DC power supplies required for the valves and cold-cathode tube counting circuits as well as the lower powered -6v, +8v and 25v units which are used for all of the transistor based circuitry. Again, with redundancy and reliability in mind, the designers have included complete spare units for each type ready to replace with just a switch of the cable.
On the bottom of the second rack are the circuit breakers which can be also tripped using the emergency stop button on the main console removing all power from ERNIE. One circuit breaker is for the A.C power input while the second is for the 80v D.C input from the backup batteries.
ERNIE has his own A.C power supply generators (of which there is of course a full spare which runs ready to take over should the first one fail) along with a D.C battery backup.
ERNIE A.C and D.C power supplies - images © BT Heritage
Each of the D.C power supplies passes its output to the mains distribution panel on this rack which has a standard GPO No.4 detector mounted with a switch allowing the operator to choose which of the 18 power supplies to display.
The serial number generator consists of five decade counters (although only four decades were in use when ERNIE was initially created). A serial number is printed against each bond number generated to keep track of which order the numbers were created in, with the higher value prizes being allocated first.
Serial Number Generator showing a value of 0237 - virtualcolossus.co.uk ©2019
The generator consists of chains of cold-cathode tubes with each digit having a separate counter (units, tens, hundreds and thousands) which not only allows a pulse to increment the counter but also gives a visual display of the current value.
Each cold-cathode tube requires a voltage of 100V to fire so to initialize the counter, a 100V start pulse if applied to the first counting tube storing digit 0. This also applies a +50V bias the the next tube in sequence (1). AS the next +50V counting pulse is applied to all code-cathode tubes, this raises the tube with the +50V bias to the required 100V which then fires, biasing the next tube along (2) and also extinguishing any other tubes. This process continues until it gets to the ninth tube which biases a passing-on tube which fires on the tenth pulse and operates the succeeding counter in the same way (the tens in this case) plus firing the 0 digit tube ready to start the sequence again. A set of auxiliary cold-cathode tubes (on the left) convert the d.c. condition of the counters to the pulse condition required by the character distributor when reading off the serial number digit by digit, starting with the thousands and working down to the units.
Again, as with many other systems, ERNIE has a complete spare Serial Number Generator ready for use.
The redundancy table enables ERNIE to suppress the printing of bond numbers which are invalid for the draw, for example, where a range of bonds have yet to be sold. This greatly reduces the workload by staff when manually finding the bond photocopies within the filing system and also shortens the time for the draw by quickly moving on to a more likely valid bond number.
The first draw held in June 1957 included a total number of 48.4 million bond units but ERNIE can choose from a possible 720 million numbers! (24 x 30 million). While it was not physically practical to store all 48 million sold numbers within ERNIE, the redundancy table was able to reject all except 118.1 million in the June 1957 draw leaving the final invalid bonds to be discounted by hand.
The redundancy table works using just the first four digits of the bond serial number generated, the first four digits being the 10 million digit, the denomination (£1, £2...£500), the 1 million digit and finally the 100,000 digit. ERNIE can therefore reject numbers only based on 100,000 and up. The final output from the redundancy table is a simple "number is redundant" pulse which signals the master control to erase the current number from primary store and to continue generating another number. Should no redundant pulse be generated, the number will be transferred to the secondary stores ready for printing, the table is set to fail safe in that if a fault occurs, it is likely to cause the acceptance of an ineligible number rather than reject a valid one.
The table was physically adjusted by inserting or removing transistors in the correct place and strapping connections between sections. This was done once a month calculated from supplied tables showing the upper total of each denomination sold plus a small number of possible groups of numbers missing within those sold.
The basic layout of the first two sections is a matrix of copper strips, each row and column consisting of two strips each insulated from one another. The junction where each section crosses is arranged so that a transistor can be fitted, component down, into a hole drilled into the board with the three wires sticking upwards (see diagram below).
Redundancy Matrix Closeup Ref: EX 60123, Research Report No. 14221 - E.R.N.I.E. Design of Redundancy Table. - image © BT Heritage
The grid is arranged with a pulse signal being fed into one of the strips from the top and from the left dependant on the digits of the serial number being checked. These meet at a specific cross point which is wired each month to the required output.
If a transistor is to be fitted, the base is connected to the left most of the vertical strips and the emitter to the lower horizontal strip. The collector can then be connected to one of two tags on either the horizonal or vertical strip.
There are three basic options for each cross point: connecting to the vertical strip means that the result of this check will be a redundant signal whereas connecting to the top horizonal strip passes the result on to the next section for further investigation. The third basic option, the default one, is to leave the transistor out altogether - this means that this result will not return a redundant signal meaning the number chosen is acceptable and can be printed.
The redundancy table can deal with two types of redundancy - terminal redundancy, which refers to numbers exceeding the highest numbered bond sold for any denomination, and intermediate redundancy, which refers to unbroken blocks of 100,000 numbers within the issued range of numbers.
A button is available to disconnect the output of the redundancy table allowing all numbers generated to be printed for testing purposes and to allow setup of the cyclic tester
Layout of redundancy table Ref: EX 29985, Research Report No. 14221 - E.R.N.I.E. Design of Redundancy Table. - image © BT Heritage
Section 1 comprises of a matrix comparing the first two digits of the bond serial number. These are the 10 million number and the denomination code. The section is split into two boards (for ease of construction) and are the two boards shown on the top left-hand side of the redundancy table.
The first 10 million number is split out and a pulsed signal arrives at one of the strips running vertically down this section. For the initial draw, ERNIE was set to only generate numbers up to 30 million, but space was allocated to expand this for future to 60 million numbers which is why there are numbers 0 - 5 arranged across the top of the board but only the 0 - 2 range have transistors wired in.
The denomination pulse arrives on one of the strips from the left hand side (marked with letters A - Z). The denominations for £1 to £10 (A-K) being on the top part of section 1 and the £20 to £500 on the bottom section.
For example, suppose the highest numbered bond sold for denomination A (a £1 bond) for this months draw is 15,470,976. The first operation is to compare the bond denomination with the first digit, which in this case could be one of the following \'0A,1A & 2A\'. To set this on the section 1 table, we firstly find the cross point where the top column is 0 and where row is set to A (the first row down). We can see that every bond number under 10,000,000 has been sold so we want every number from 0-9,999,999 to be in the draw. This is achieved by leaving the space empty - remember, the default is to assume the number is valid and to print it. Next, we have to look at column 1 compared with the same row A, this will be all the numbers from 10,000,000 - 19,999,999. As we have just over half of the bonds sold in this batch and half not sold, we have to pass this further into the redundancy table to check the millions or 100,000 values to find if this number is redundant. To do this, we fit a transistor into the 1A position and wire the collector to the strip running horizontally towards section 2 on the right. Only one transistor should be set into this position on each row so that we know which of the first two digits are being compared when checking section 2 onwards.
For the final 20,000,000 - 29,999,999 bond numbers we can see that none of the numbers in this range have yet been sold which means we can set all numbers starting 2A as redundant. This can be done by fitting a transistor into column 2 (as we did in column 1), but this time, we wire the collector to the vertical strip on the right. Any transistors connected to this strip will result in a redundant pulse being sent to the master control. This will then delete the number from the primary store and begin the search for a new bond number.
We also have two special options available, the first is for when we have more than one 10 million column to connect to the second section in a single denomination. There are three extra rows available at the bottom of the top part of the second section which can be wired directly to cover this possibility. The second way of wiring directly is to wire the collector straight down to a special auxiliary section which can cope with a few "special cases". Virtual ERNIE: To set a transistor simply click or touch on the cross point where you would like to change the transistor. Each click will loop around the following settings: no transistor, redundant, next section or spare. Alternatively, you can right-click or long-press to pull up a menu which you can select exactly which mode you want to set. Note that you can only set which of the three spare lines to connect to by using the menu and it is not currently possible to connect to the special auxiliary table in this version.
Section 2 is very similar to Section 1 but larger as the columns across the top cover all ten of the million (third) digit values. The columns are listed as the letters that appear on the bond (Z=0, B=1, F=2 ... W=9). The grid of copper strips and the method of attaching in transistors at the junctions is the same although there are a few additions to take account of many more times where more than one million group requires extending to the next section. Section 2 is the two larger boards to the right of section 1.
Since only one transistor per line can be wired to the horizontal strip that extends to the next section, the GPO added removable bridges which could be attached across each row of the top section. These bridges are fitted with a row of turret tags so that the collector of a transistor can be connected to one and then wired to the third section. These bridges were removed later in ERNIEs life when then number of intermediate blocks of numbers unsold dropped. Virtual ERNIE: To hide the bridges so you can see and change the transistors, click the eye icon.
In this section, we are comparing the signal from section 1 (where we checked digit 1 and 2, the 10 million and denomination) and splitting out the third number which is the one million digit. Continuing our example from section 1, where we were setting up for a denomination A with 15,470,976 sold bonds, we can see that the only value which will be extended into this section will be 1A so we know that all the transistors we setup on the A row will be checking 1A against our million column. Again, as before, we can miss out a transistor to represent a completely sold million of bonds, attach the collector to the horizonal strip to extend to the next section or connect to the redundant strip to show a value that should be removed from the draw. We also have the option to connect a transistor up to the bridge (shown on Virtual ERNIE as a blue coloured transistor wire) and therefore onto the third section that way or lastly, to wire directly onto one of the auxiliary or special third section boards. Our example value for the third digit is for 5 million so we would leave out the transistors for 0-4 (Z,B,F,K & L) as 10-14 million are definitely sold. Next, we would wire the N column transistor (5 million) to the next section tag to allow ERNIE to check further digits and lastly, wire all further transistors to the redundant strip.
The tags to the right of the second section are wired to the third section so that connections between second and third sections can be made with short cross-strappings. As the third sections are the last to be checked and no longer need to be extended to a further section, they no longer need to have separate lines for each row. This means that the third sections are a fixed set of transistors which check specific ranges of 100,000 values and we can wire multiple lines to each. There are 9 possible rows to connect to on Section 3 so we have nine tags on a board next to section 2 which are permanently wired up direct to each of these rows. There are multiple groups of tags to make it easier to wire from the nearest section 2 extension strip. Virtual ERNIE: Click or touch a transistor connection point to switch between no transistor, redundant connection, extension to the third section via the fixed strip, extend to the third section via the bridge (shown with a blue wire) and finally to extend to the auxiliary or special tables. Right clicking or long press should bring up a menu where you can select these options more easily plus if you select a bridge or aux/special connection, you will get further options to specify which part of the section to connect to.
To understand how the tags between the second and third sections are configured, it is necessary to understand the layout of the third section itself first. As we no longer require separate denomination lines as we are not passing on to a further section, the transistors for the final third section are set out in a fixed pattern which one or more of the section two rows can be connected to. The transistors are laid out with firstly nine transistors representing the redundancy of 100,000-900,000 values (1-9), the next has eight transistors for 200,000-900,000 and so on down to a single transistor for 900,000 only being made redundant. There is no 0 value as making redundant the full set of numbers from 0-9 can be done with a terminal redundancy on the second section. This triangle layout is arranged into a rectangle to save space on the physical table (see animation below).
Section 3 layout animation showing how the transistors are laid out.
Therefore, for our example, 15,470,976, we are looking to make redundant everything over 500,000 so we want to connect our extension of the A denomination from the section 2 table to the section 3 with values 5-9 which means that any fourth digit with a value of anywhere above 5 will get a redundant signal and be deleted. To do this on ERNIE, the tag at the end of the A denomination strip on section 2 would be wired up to the 5 tag on the group of tags leading to the third section. Virtual ERNIE: To change the wiring from section 2 to the section 3 tags, click on the endpoint of the wire you wish to change just as it comes onto the section 3 tag board. Each click selects the next tag along in a cycle. For example, to change the A denomination, click on the wire at the top left of the third tag section just to the left of the number 1.
This deals with redundancies at the top of the 100,000 bond numbers, but if we have several bonds at the beginning of this batch of 100,000 - we need to connect up to the Auxiliary Section 3. This table sits just below the standard Section 3 table and has the same layout but is wired to run from only 0 being redundant (0-99,999) up to everything between 0 and 8 (0-899,999). To connect to these requires manually strapping from the collector of the relevant section 2 transistor. Virtual ERNIE: This can be achieved on Virtual ERNIE by right-clicking a transistor and selecting Fit Transistor->Aux/Special then right-clicking a second time and selecting the "To 3rd section Aux" dropdown to specify which part of the auxiliary table to connect to.