Centronics parallel ports are now widely used to connect everything but printers to PCs. In most cases, the user must configure the application software for the right port (LPT1: or LPT2:). However, designing in automatic port recognition is a simple matter. The connection of an output line to any spare input pin of the DB25 connector is all that is necessary on the hardware side (Figure 1.)

Listing 1—Plug-and-play centronics connector

10 DEF SEG=0:KEY OFF:CLS 20 S1=PEEK(&H408)+256*PEEK(&H409):E1=S1+1 'read LPT1: address 30 S2=PEEK(&H40A)+256*PEEK(&H40B):E2=S2+1 'read LPT2: address 40 OUT S2,0 'LPT2: test 50 IF (INP(E2) AND 64) <> 0 THEN 90 60 OUT S2,128 70 IF (INP(E2) AND 64) <> 64 THEN 90 80 GOTO 150 'device detected on LPT2: 90 OUT S1,0 'LPT1: test 100 IF (INP(E1) AND 64) <> 0 THEN 140 110 OUT S1,128 120 IF (INP(E1) AND 64) <> 64 THEN 140 130 GOTO 160 'device detected on LPT1: 140 PRINT"device not detected":GOTO 180 150 S=S2:PRINT"device connected to LPT2:":GOTO 170 160 S=S1:PRINT"device connected to LPT1:":GOTO 170 170 PRINT"base address =";S;"(decimal) 180 REM (c)1996 Patrick GUEULLE

A few lines of software can easily send a signal to every available parallel port and particular data line (D₇, in this example) and then determine whether or not the signal reaches the expected status line (ACK, in this case). Listing 1's short GWBASIC listing reads the base addresses of LPT1: and LPT2: from the BIOS area to cope with possible variations between true PC compatibles and certain clones. You can test additional parallel ports in the same way, and you can also simply rewrite the program in C, Pascal, or even assembly language.

At least two input lines (ACK and BUSY) and any D₀ through D₇ output pin lend themselves quite well to this application. Therefore, this method makes it possible not only to detect the particular parallel port on which the device connects, but also to identify any one of several devices, each of which is coded by a unique position of the strap. In the Listing, only the decimal values of 128 (for D₇) and 64 (for ACK) need to be adapted, because they act like "masks" for the corresponding input and output lines.

Note that this arrangement does not prevent the device from making normal use of the data line involved in the process. Full bytes can still transmit via D₀ through D₇ of the parallel port by means of direct OUT instructions. Only the ACK line is no longer available, preventing the device from using the true Centronics handshaking protocol. If this restriction causes trouble, you might prefer to use another status line, such as PE (paper end). (DI #1888)