The circuit for selecting and energizing the E relays is shown in [Fig. 12]. We call this circuit the Select-Receiving-Register circuit. For example, suppose that the P8 and P2 relays are energized. Then this circuit energizes the E10 relay. The E10 relay closes the contacts between the C1 relay coils and the bus; and so it connects the C1 register to receive the next number that is sent into the bus. This kind of circuit expresses a classification and is sometimes called a pyramid circuit since it spreads out like a pyramid. A similar pyramid circuit is used to select the sending register.
We shall need a relay for moving the input tape a step at a time. We shall call this relay the MI relay, for moving input tape. We also need a relay for moving the program tape a step at a time. We shall call this relay the MP relay for moving program tape. Here then is approximately the total number of relays required:
| Relays | Name | Number |
|---|---|---|
| I, S, C, O | Input, Storage, Computer, Output | 30 |
| P | Program | 4 |
| E | Entrance | 13 |
| MI | Move Input Tape | 1 |
| MP | Move Program Tape | 1 |
| Total | 49 |
A few more relays may be needed to provide more contacts or poles. For example, a single P1 relay will probably not have enough poles to meet all the need for its contacts.
Fig. 13. Latch relay.
Each cycle of the machine will be divided into 5 equal time intervals or times 1 to 5. The timing of the machine will be about as follows:
| Time | Action |
|---|---|
| 1 | Move program tape. |
| Move input tape if read out of in last cycle. | |
| 2 | Read program tape, determining the receiving register. |
| Read through the computing circuit setting up the C5 register. | |
| 3 | Move program tape. |
| Energize the E relay belonging to the receiving register. | |
| 4 | Read program tape again, determining the sending register. |
| 5 | Transfer information by reading through the |
| Select-Sending-Register circuit and the | |
| Select-Receiving-Register circuit. |
In order that information may remain in storage until wanted, register relays should hold their information until just before the next information is received. This can be accomplished by keeping current in their coils or in other ways. There is a type of relay called a latch relay, which is made with two coils and a latch. This type of relay has the property of staying or latching in either position until the opposite coil is impulsed ([see Fig. 13]). This type of relay would be especially good for the registers of Simon.
If any reader sets to work to construct Simon, and if questions arise, the author will be glad to try to answer them.