VERIFIED WRITE LOGIC AND WRITE CLOCK OUTPUT
The 2310 drive is formally called the 13SD and had the project name Ramkit. It is installed inside an IBM 1130 as its internal disk drive, as well as in a standalone cabinet labeled the IBM 2310, with one or two drives per cabinet.
The 13SD produces a 720 KHz clock that it sends to the controller logic in the 1130 when the drive is switched to write mode. The controller logic controls a Write Data line based on the state of the clock signal. The clock pulse is sent by a constant 1 emitted during one half of the clock, whereas during the other half of the clock cycle the data value of 1 or 0 is transmitted.
Any time the Write Data line is 1, a flipflop reverses state which flips the magnetic orientation at the head. This writes a pulse on the disk surface. I verified the 720KHz signal was correct and that the flipflop toggles any time the Write Data line is set to 1.
The recording scheme on this disk drive, like many, alternates clock pulses with data. Each half of the 720KHz clock cycle is called a bit cell. The first bit cell is always a 1, so it always records a pulse by flipping the magnetic field. The second bit cell records a pulse if the data bit is 1, otherwise it does NOT flip the field and no pulse is written.
When writing a sector, the controller will send a preamble of a fixed duration of 0 data bits. That is, every clock cycle has a pulse in the clock bit cell and no pulse in the data bit cell. When reading, the drive will synchronize itself during the preamble so that it can tell the difference between clock and data bit cells. This will be important when data bit values of 1 are read later, as it comes in from the read head as a stream of pulses that must be separated. This synchronization is analogous to a phase locked loop.
The drive, because it is in step with the alternation of clock and data bit cells that were written previously, can steer the clock pulses out one signal wire (Read Clock) and steer a pulse out a second signal wire (Read Data) when the data bit cell had a pulse. This helps the controller logic in the 1130 know that the absence of a pulse is a data value of 0, since the next clock pulse arrives with no pulse incoming on the Read Data line. On the other hand, if a pulse arrives on the Read Data line then we know the data value was 1.
CHECKED SIGNAL VOLTAGES PRESENTED TO RK-05 EMULATOR HARDWARE
The RK-05 emulator has interface chips to read signals from the 13SD drive and from controller logic in the 1130. It also has chips to write signals to the drive and the controller logic. It acts as a relay station for many signals. For example, when the 13SD produces a Sector Mark pulse, it is passed along through the emulator to the 1130 controller logic.
Reading and writing activities involve passing the 720KHz clock through to the controller logic, but not passing the signals to activate read or write on to the 13SD. Instead, the emulator produces a stream of pulses on Read Clock and Read Data signal lines to the controller logic, just as would have been emitted by the drive if its heads were really reading the disk cartridge. These come from DRAM in the emulator which holds the data that would be on a 2315 disk cartridge.
For a write operation, the Write Data line from the controller logic, emitted in coordination with the 720KHz coming from the disk drive, will stop at the emulator. The incoming data bits will be captured and used to update the DRAM image of a disk cartridge.
When the disk drive first spins up, the emulator transfers the 1MB of content for a 2315 cartridge from an SD card to the DRAM where it is available for reading and writing. When the drive is switched off, the updated contents of DRAM are written back to the SD card. The format of the file on the SD card matches that used by the IBM 1130 simulators, which makes it relatively easy to create or update virtual 2315 cartridges.
We have all the signals and behavior we need from the disk drive in order to work, in concert with the emulator, to perform exactly like the 13SD drive with a physical 2315 cartridge for each disk image on the SD card.
PUT ALL COVERS BACK ON
I put everything back together, removing temporary wires, and installing all covers. The drive is ready to insert inside the IBM 1130. It is bolted to four spots on the frame, sitting on the right side of the 1130 as viewed from the front. A swinging door on the front of the 1130 is opened to gain access to the drive. A blue handle is pulled down to allow a 2315 disk cartridge to be pushed into the drive or removed.
The handle is then returned to its upright position and the Run switch on the drive is turned on. A lamp on the main panel of the 1130 indicate when the drive is unlocked, allowing the blue handle to be pulled down. The drive must not be spinning, otherwise the drive is locked. Another lamp on the main panel indicates when the drive is able to be accessed - File Ready - which turns on about 100 seconds after the Run switch is activated with a cartridge inside.
The drive has a set of wires delivering 115VAC to power the motors. Another set of wires feeds the +3, -3, +6 and +48V DC power to the rear of the machine. A signal cable is normally connected from the 1130 controller logic to the backplane of the disk drive. However, this cable connects to the disk emulator, and a cable from the emulator is plugged into the13SD drive instead. Lastly, wires connect to the Run switch, Disk Unlock lamp and File Ready lamp. These will all be attached once the drive is bolted into place.
PREPARED TO LIFT INTO THE 1130 WHEN I GET A HELPER
The drive weighs quite a bit and is bulky. It has to be lowered into the frame of the 1130 until the four bolts enter holes in support beams for the disk drive. The weight is therefore about two to three feet away from the body as the drive is lowered and raised in the frame opening, which introduces quite a lever arm that amplifies the effect of the weight on the people holding it.
I will have to wait until someone relatively strong accompanies me to the workshop so that the two of us can lift the disk drive off the rolling lift table and insert it into its place inside the 1130 frame. Once is is sitting on its supports, I can do all the other work solo.
DESIGNING INTERFACE BOARD AND CABLING TO USE RK-05 EMULATOR AND DRIVE
The RK-05 Emulator that I am leveraging as the core of my Virtual 2315 Cartridge Facility has two forty pin IDC style ribbon cables, one of which will interface with the IBM 1130 controller logic and the other will interface with the 13SD disk drive. Neither the 1130 nor the 13SD use IDC 40 pin connectors, so I will build an interface board to connect everything together.
The signal interface from the 1130 controller logic is a cable with an SLT card connector on the end. This has the 24 pins of any SLT card and fits into one card slot on the backplane of the 13SD. Normally that cable with connector would be plugged into the 13SD.
My board will feature two of the IDC 40 pin connectors, a set of header pins that match the card connector from the 1130 controller logic, and will have another cable with SLT card connector that is plugged into the 13SD. This puts the emulator in between the 1130 controller logic and the drive. A few additional connections will provide access to the Run switch and the state of the Disk Unlock lamp.
Those two latter connections are either at 48V or 0, depending on their logical condition. The voltages are not safe to connect to the emulator. The interface board will have two level shifter circuits to convert the 48V signal into a TTL level output in order to feed it into the emulator.
I have yet to finalize the shape of the interface board, nor the exact location where it will be mounted inside the IBM 1130.
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