bbc-l1.dot

digraph bbc_l1 {
 graph [fontsize=9 fontname="Verdana" 
 compound=true rankdir=LR splines=true];
 node [shape=box fontsize=9 fontname="Verdana" style="rounded"];
 edge [fontsize=9 fontname="Verdana"];

 subgraph cluster_bbc {
    edge[constraint=false];
    label="BBC Master";
     rank="same"; 
     fd [label="Floppy Disk"];
     cf_ide [label="CF (IDE)"]
     fd -> cf_ide;
 }
 
 ide_usb [label="Physical Transfer",shape=oval]

 // Overall linkage:
 // Invisible edge used to align the boxes:
 fd -> ide_usb [style=invis];
 ide_usb -> cf_usb
 edge[constraint=false];

 // None-aligning edge linking BBC output to the rest:
 //cf_ide -> ide_usb ;
 //ide_usb -> cf_usb [lhead=cluster_pc];
 cf_ide -> ide_usb;//[ltail=cluster_bbc, lhead=cluster_transfer];// [label="Transfer CF Card"];


 subgraph cluster_pc {
    edge[constraint=false];
    label="Modern PC";
     rank="same"; 
     cf_usb [label="CF (USB)"]
     fdi [label="Floppy\nDisk Image"]
     cf_usb -> fdi;
 }

}