Monday, May 9, 2022

Planning diagnostic indicators for the power sequencing relays, ready to start again Tuesday morning


Mother's Day holiday was followed by an invitation to ride the new Guardians of the Galaxy ride at Disneyworld EPCOT Monday morning. The ride opens to the public at the end of this month but passholders and some others were given early access opportunities. Limited slots and many requests, but those who were lucky and got in right away when the system opened the reservation line were successful. 

Because of this, I spent Sunday with my wife and will be at Disneyworld on Monday. I am sure the 1130 and my workshop will understand why they are being neglected for a couple of days. 


Inside the power sequencing box of the IBM 1130 are four relays and a contactor, in addition to several terminal blocks and lots of wiring. The contactor connects the incoming 230V power to most of the system when the Power On switch is flipped, under control of some sequencing and verification logic.

Another relay is a time delay unit that waits about 5 seconds before energizing, giving the power supplies time to come to full voltage and stabilize. It controls the generation of a Power On Reset signal to set the 1130 logic to a known good state, and triggers a check that all three main voltages are present. If not, the system drops power and locks out the Power On switch until reset. 

Finally, there are three relays R1, R2 and R3. 

R3 is simple - as soon as the main circuit breaker is switched on while the machine is plugged into an energized 230V outlet, it turns on. That passes the incoming 230V to the stepdown transformer T3 which delivers 115V AC at a convenience outlet to power things like my oscilloscope. This must be available even when the Power On switch is set to the off position thus it energizes immediately.

R1 is connected through the voltage sense SMS card, such that it only energizes if reed relays detect that +6, +3 and -3V power is available. It requires all three to energize R1 and if any of them drop out then R1 will also deactivate.

R2 is connected through R1 and switches on right after R1 closes the contacts. R2 is used to connect the +12V and +48V supplies to the rest of the machine, something that is only done once the three main voltages are determined to be good. 

Upon flipping the Power On switch, the main contactor engages, the fans begin spinning, the stepdown transformer powers the incandescent lamps for the display panel, and the power supplies are provided their input power. 

Once the power supplies develop enough power, the circuit in the SMS card closes and relay R1 will energize. That quickly energizes relay R2 which delivers the final voltages throughout the machine. 

At the end of the 5 second timed interval, the time delay relay TD-1 energizes. If R1 is active, meaning we have good power available to the logic cards, then we come out of initial reset and can begin operating the computer. If, however, R1 is not active, then a special hold circuit is triggered on the SMS card which makes the main contactor drop, removing power to the system.

Relays R1, R2 and R3 are fed with 24VAC, the power sequencing voltage generated by a small transformer inside the power sequencing box. Each relay is wired in parallel across the 24VAC when switched on, thus I can put a suitable LED across the winding terminals to light up when power is present at the relay coil. 

With the peak voltage that will be experienced, I will place a 1/2W resistor of nominally 2,350 ohms in series with a normal LED. The resulting 14ma instantaneous maximum and average under 10ma will power the LED to a reasonable brightness. I will do a quick test right across the 24VAC transformer output before I start soldering these indicators in place. 


  1. Hi Carl. I am following your progress with interest. You have probably already thought of this, but LEDs do not like a reverse bias of more than a few volts. When driving them from AC a reverse-parallel diode is required to protect them. Filament lamps may be a simpler solution in this case.

    1. Excellent point. Going to make a change - incandescent or adding diode - stay tuned