MIT UNIBUS XGP interface

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UNIBUS XGP interface (at the top of its controlling PDP-11, at right) along with the XGP (left)

The MIT UNIBUS XGP interface was a one-off UNIBUS peripheral which drove a Xerox Graphics Printer, the earliest raster printer (and thus the ancestor of the ubiquitous laser printer).

It used DMA to transfer rasters from the PDP-11's main memory to the XGP. A raster was 1700. bits long; in the PDP-11 memory, that took 108. words (1728. bits). (Some calculation took the raster length to be 1728. bits.) The raster buffers contained 3 additional words; these apparently were a 2-word header (below), and an "initial image mode word". 

"Each [scan] line consists of a header, specifying how long it is, which line it is to be printed upon, and options .. These two words of header are followed by the line in the format required by the XGP interface.
The first PDP-11 word in each line is the number of PDP-11 WORDS .. used for this scan line, including itself and the rest of the header. [The buffer length appears to be in byte-swapped order.] ..
The second PDP-11 word is the scan line upon which this line is to be printed. [It is not clear if this in interpreted by the hardware as well, or just by the driver in the PDP-11.] It is allowable to have non-sequential lines. The unspecified lines will be blank. If the line numbers are not ordered, they will come out as close as possible together, and the internal line count will be reset. .. The top line on a page is numbered 1. ..
Paper cutting may be specified by setting the sign [high bit] of the second PDP-11 word. The rest of the [scan line] so marked will be ignored"

It is not yet clear if this last is interpreted by the hardware, or the driver in the PDP-11.

To print a scan line, load the Memory Address Register (below), set the FGO and FMOT bits in the Control Register (ditto); FDIE as well, to get an interrupt when done. The interface's interrupt vector is 0370.

The XGP at MIT was one of a number provided to various entities by Xerox in 1972; each had to produce its own hardware computer interfaces. MIT and Carnegie-Mellon both used PDP-11's to drive their XGP's, but it is not known how much there was any design connection between the two interfaces (although the similarity of the specifications of the two makes it plausible that there was some knowledge exchange).

MIT connected theirs to a PDP-11/20 (there was at one point some confusion about exactly which model of PDP-11 it was, some thought it was a PDP-11/05; the discovery of the image confirms that it was an -11/20). That was connected to the ITS KA10 MIT-AI via the Rubin 10-11 interface.

Device registers

Register Abbreviation Address
Control Register XCR 772100
Memory Address Register XMAR 772102
Status Register XSR 772104
Cut Register XCUT 772106

772100: Control Register (XCR)

FERR Unused FMOT Unused FDONE FDIE Unused FCUTI FCUT FGO
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

The header of the driver (below) indicates that FCUTI in in this register, but this is believed to be an error. The code definitely indicates that this bit is in the XCUT register.

772102: Memory Address Register (XMAR)

BA15 <---> BA01 Unused
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

772104: Status Register (XSR)

FOS FSYN FOR FNXM FRDYC FBCB Unused Status FRDY Status Unused FACT
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

772106: Control Register (XCUT)

Unused FCUTI Unused
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00

Data encoding

The data pointed to by XMAR is DMA'd from memory and decoded by the interface in various modes. For each fresh line, the interface starts off in character mode.

In character mode, the first byte says how many bits of data follow. Subsequent bytes are shifted to the video flip-flop controlling the CRT beam, as many as necessary to reach the bit count. A 0 byte escapes character mode. The next byte decides what mode to switch to: 0 enters vector mode; 1 stops the printer and sets DONE; 2 enters image mode. Anything else is an error.

In vector mode, alternating bytes say how many pixels of white or black to draw. Two zeros in succession escapes back to character mode.

In image mode, bytes are imaged to pixels verbatim. There is no escape except an error condition. Normally this will be an overscan error when the beam reaches the right margin.

External links

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