gw read: Add support for --gen-tg43 to enable TG43 (pin 2) signal

This is enabled from track 60 on read to enable the drive's
postcompensation filter.

Some drives including, but not limited to, BASF 6104 have a PCF with a
better filter for reading 8-inch floppies that haven't been written
with TG43. Normally PC controllers don't assert the TG43 signal and
have trouble reading disks written without TG43 signalling.

Reference
http://www.bitsavers.org/pdf/basf/80307-048-BASF-6104-technical-manual.pdf
page 34

"Read operation Low input switches the postcompensation filter (PCF)
into the read chain when installed. It is recommended to activate the
PCF from CYL 60 to CYL 76. The LO I/- signal has no effect during read
operation on a drive without PCF. "

Author: Per Mårtensson

src/greaseweazle/tools/read.py

@@ -62,6 +62,9 @@ def read_with_retry(usb: USB.Unit, args, t) -> Tuple[Flux, Optional[HasFlux]]:
 
     usb.seek(t.physical_cyl, t.physical_head)
 
+    if args.gen_tg43:
+        usb.set_pin(2, cyl < 60)
+
     flux = read_and_normalise(usb, args, args.revs, args.ticks)
     if args.fmt_cls is None:
         print(f'{tspec}: {flux.summary_string()}')
@@ -95,6 +98,8 @@ def read_with_retry(usb: USB.Unit, args, t) -> Tuple[Flux, Optional[HasFlux]]:
             if retry != 0:
                 usb.seek(0, 0)
                 usb.seek(t.physical_cyl, t.physical_head)
+                if args.gen_tg43:
+                    usb.set_pin(2, cyl < 60)
             seek_retry += 1
             retry = 0
         retry += 1
@@ -239,8 +244,11 @@ def main(argv) -> None:
                         help="do not overwrite an existing file")
     parser.add_argument("--pll", type=track.PLL, metavar="PLLSPEC",
                         help="manual PLL parameter override")
-    parser.add_argument("--densel", "--dd", type=util.level, metavar="LEVEL",
+    densel_group = parser.add_mutually_exclusive_group(required=False)
+    densel_group.add_argument("--densel", "--dd", type=util.level, metavar="LEVEL",
                         help="drive interface density select on pin 2 (H,L)")
+    densel_group.add_argument("--gen-tg43", action="store_true",
+                        help="generate TG43 signal for 8-inch drive on pin 2 from track 60. Enable postcompensation filter")
     parser.add_argument("--reverse", action="store_true",
                         help="reverse track data (flippy disk)")
     parser.add_argument("file", help="output filename")
@@ -280,14 +288,15 @@ def main(argv) -> None:
         if args.format:
             print("Format " + args.format)
         try:
-            if args.densel is not None:
+            if args.densel is not None or args.gen_tg43:
                 prev_pin2 = usb.get_pin(2)
+            if args.densel is not None:
                 usb.set_pin(2, args.densel)
             with open_image(args, image_class) as image:
                 util.with_drive_selected(
                     lambda: read_to_image(usb, args, image), usb, args.drive)
         finally:
-            if args.densel is not None:
+            if args.densel is not None or args.gen_tg43:
                 usb.set_pin(2, prev_pin2)
     except USB.CmdError as err:
         print("Command Failed: %s" % err)