Based on a PM clarification that it is a Personal Laserwriter NTR, power down and disconnect the printer cable temporarily. Then set the ten position push button switch to zero, which specifies that the three ports may then be used as follows (according to Apple all three may be used at the same time):
8-pin port: LocalTalk, PostScript batch <----- the usual way to connect, using Localtalk or PhoneNet, or for unshared single Mac to printer (according to MacSecrets Pogue and Schorr, not Apple) , a regular Mac miniDIN printer cable. I just tested both the PhoneNet method and the printer cable method successfully with my PLW NTR set to switch position zero, Laserwriter 8 driver.
25-pin port: Serial interface, 9600, N, 8, 1, Xon/Xoff, PostScript batch
Parallel port: Centronics interface, PostScript batch
Reconnect printer cable, power up and retest.
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Switch position 3 specifies the 8-pin port to be LocalTalk protocol interface but HP IIp printer emulation. This will likely produce the garble with Macs but no electrical interface damage.
Switch positions 1,4,5,6, and 9 can put the 8 pin printer port into serial protocol and in so doing can electrically damage other devices on the LocalTalk network if there are any, according to the printer owner's manual. Nothing said if the Mac computer comm or printer port can be damaged.
Note that the unlabeled switch specifies the 8 pin port only electrical protocol and which particular printer emulation modes for different ports, never port selection, and in all cases you have a choice of three operating interface ports.
