Computeriter Electrical Interface

This drawing from the Soroban Computeriter Coder Unit Technical Manual shows the underside of the Computeriter.

The electrical interface to the Soroban Computeriter can be viewed as three mostly-independent blocks:

• A mechanical "coder unit" that encodes printing characters
• Switches that signal non-printing functions (space, carriage return, shift, etc.)
• A "decoder unit" that allows a computer to drive the printing mechanism

Let's look at each of these separately.

 

Coder Unit Interface

Here's the connection diagram for the Coder Unit connections, taken from the CDC 161 Typewriter Unit documentation.

 Each of the TC1 through TC6 contacts encodes one bit of the 6-bit Computeriter character set. Since these are mechanical switches, some will close before others and all are subject to bouncing. The TCC contacts change state after the TCx switches have settled to indicate they can be read. 

Here's a drawing from the Soroban documentation that shows the timing:

 

Non-printing Function Switches

Rather than encode the non-printing functions, the Computeriter uses separate switches to signal when the user presses the keys. These include the space bar, the tab key, the backspace key, the carriage return key, and the shift key. An additional switch signals when the inked ribbon advances, which it does for each character typed.

The function of some of these switches is obvious: press the space bar, and the "Space Cam" switch will change state briefly. Others are not so obvious, like the difference between the "Shift Cam" and the "Upper Case Lever"; by experimentation I determined that the "Shift Cam" switch changes state momentarily when the shift button is pressed or released, while the "Upper Case Lever" indicates the current position of the type mechanism. Since the "Ribbon Feed" and "Tab Cam" switches are not used by the CDC 161 I didn't investigate their operation.

You might ask why these switches are SPDT (single pole, double throw) rather than a simpler single contact SPST switch. The answer is that these are connected to simple Set/Reset flip-flops which serve to debounce the contacts. The first closure of the normally-open contact sets the flip-flop, and all subsequent bounces of that contact do not cause the flip-flop to change state. When the switch returns to its normal position, the normally-closed contact resets the flip-flop.

I mentioned in the previous post that one of the switches is both a logic input and illuminates a bulb. That's the right-hand contact of the DPDT switch at the bottom of the drawing, connected to pin 34 of the DD-50 connector.

This drawing also contains an error. Have you spotted it? It's literally impossible for the common contact of the DPDT switch to connect to the right side of the DD-50P connector, as the switch is part of the Computeriter while everything to the right of the DD-50 connector is part of the CDC 161 electronics. Thus the common contact must be connected to pin 50 of the DD-50.

 

Decoder Unit Interface

The decoder is a mechanism that can activate any of the typewriter keys through seven small solenoids. Six of the solenoids (TM1 through TM6) select which key will be activated, while the seventh (TCM) triggers the action.

A SPDT switch (TBS) indicates that the action has taken place, and the Computeriter is ready to print another character. An additional switch disables printing at the right edge of the platen.