Completion of the block transfer is signified by either setting a program flag, or entering the sequence break.
Connection with Sequence Break System
When the TC is connected to the Sequence Break System, the program is automatically interrupted each time an MPS command needs to be given.
Programming is unaffected during reading and a record may be read with no flags set. The TC initiates breaks so that an MPS may be given in time.
Similarly, the break is initiated during writing each time an MPS needs to be given.
Motion Command Summary
| Time before first MPS | Time between MPS's | Time after End of Record to deselect | Flags that may be set | |
| MWA MWB | 3 ms. | 400 us. (longer time causes deselection) | 10 ms. | RWF (if unit is deselected and MPS given, or unit becomes unavailable), Parity, EOT. |
| MRA MRB | 7 ms. | 400 us. (longer time misses information, and rwc set) | 5 ms. | RWF, (if information is missed, or unit becomes unavailable), EOT, EOR, Parity. |
| MBA MBB | — | — | 10 ms. | RWF (if unit becomes unavailable), EOR, EOT. |
CATHODE-RAY-TUBE DISPLAY
The PDP-3 Cathode Ray Tube Display is useful for presentation of graphical or tabular information to the operator. It uses a 16 inch round tube with magnetic deflection. For each In-Out transfer order, one point is displayed at the position indicated by the In-Out Register. Bits 0-9 of the IO indicate the X coordinate of the position, and bits 18-27 indicate the Y coordinate. The display takes 60 microseconds.
An additional display option is a Light Pen. By use of this device the computer is signaled that the operator is interested in the last point displayed. Thus the program can take appropriate action such as changing the display or shifting operation to another program.