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SPROGS Paper No 38: The Hewlett-Packard Plotter

R E Thomas

October 18 1973

Although this paper deals with the inclusion of the HP plotter into the SPROGS system, it should be relevant to its general use on the 1906A.

1.General characteristics

The plotter is Teletype compatible, and has provision for connecting the typewriter terminal as well. However, its modem interface will not allow it to be driven over long distances (eg to the offices at ACL) without some form of driver circuit. Before setting the device, the user must install the paper and pen (removing any protective cap) and set the origin scale to correspond to the size of paper used.

2 Operation

The basic commands for line drawing are set out here. There are some others for point plotting; these are described in the HP leaflet.

(a) All lines are terminated by linefeed carriage return (this HP has been modified to follow ICL procedure). No command is obeyed until these control characters have been received. All other control characters are ignored.

(b) No commands are recognised until PLTL is met

(c) Drawing commands consist of an X and a Y coordinate (up to 4 digits each, unsigned) separated by at least one space. These coordinates are absolute. e.g.

 12 9999
451  638
 72 5500

A suitable FORTRAN format is 

  2 (1x,I4)

(d) The pen can be moved without drawing a line by following the Y coordinate with ↑ e.g. 

 12 9999↑

A suitable FORTRAN format is 

  2 (1X,I4), 1H↑

(e) Any spurious characters in these commands will cause a data error, and the line will be ignored. Lines longer than 6 inches will not be straight.

(f) The command PLTT terminates plotting.

3 Using the plotter from SPROGS

The SPROGS system will write a lineprinter file to be drawn subsequently, in order to avoid having a large graphics package on-line to the plotter. DVOUT will issue PLTL and ENDSPR will issue PLTT. PLOT will be ignored. TODXY will draw the appropriate vector, splitting lines into segments less than 6" inches long, and minimising the commands needed for any intensity overstriking by drawing lines in both directions.

There is some problem associated with commands such as ADVFLM and COLOUR which could have meaning for the plotter but require operator intervention. It seems that this can only be achieved by placing markers in the file, and using a small program on-line to the plotter to interpret this file. A possible method is outlined below.

4 Drawing on the plotter

4.1 Simple case

Here, both Teletype and plotter are on-line together. The user issues a LF command for the desired file.

4.2 Both devices not on simultaneously

In this case it is necessary to swap over plugs, although this is likely to cause the 1906A to detect a line error and force a logout. The problem can be minimised by using a listing macro: 

      DP 0, CONNECT PLOTTER 
      WAIT 30 SEC 
      LF file 
      WAIT 30 SEC

4.3 Compound file

The plotter file itself contains operator commands. This is only possible if the Teletype is also connected. The file listing is done by a small on-line program which issues suitable commands for the operator.

The suggested sequence in the file is 

      PLTT 
      message 
      PLTL

where message could be 

      ADV FLM or COLOUR 3

The program, on detecting **, would issue

 
      DP 0, message 
      QN 0, ARE YOU READY

and wait for the reply 

      AN YES 
      CU
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