Article 2107 of dec.notes.technology.dechips:
Title: PC64 parallel port loopback
Reply Title: (none)

From the Alpha SRM console driver for the parallel port:

/*++
 *
 *  Support has been added for a parallel port loopback connector. This allows
 *  for the exer to be run in a read/write mode on the port. A global flag
 *  has been added to indicate the presence of a loopback connector. The
 *  loopback connector has the following configuration:
 *
 *    1  --->  13      Strobe---->Select
 *    2  --->  15      DB0------->Error
 *    10 <---  16      Ack<-------Init
 *    11 <---  17      Busy<------Selectin
 *    12 <---  14      Paperend<--Autofeed
 *
 *    From the registers the connection is:
 *
 *     DB0(Data Reg, Bit 0)----->  ERR (STR bit3)
 *
 *      CTR Bit             STR Bit
 *
 *      STB  0  ---invert-->SLCT 4
 *      AFD  1  ---invert-->PE   5
 *      INIT 2  ----------->ACK  6
 *      SLIN 3  ----------->BUSY 7
 *
 *  The driver checks for a loopback by checking if the ERR bit follows
 *  data bit 0, and sets the flag. This check is done in the open routine
 *  and is reset back in the close routine. If a loopback is detected the
 *  driver no longer checks for busy, or any error conditions. Note that
 *  if no loopback is connected the read portion of the driver is invalid.
 *
 *--
 */

stephen