Macintosh Plus: Serial Printer Damages SCSI Port

PROBLEM DESCRIPTION: When a serial printer is plugged into the SCSI DB-25 port of a Macintosh Plus, the logic board is damaged and the logic board must be replaced.

CAUSE: Serial printers with DB-25 connectors commonly use the RS-232 electrical interface standard. The DB-25 port on the Macintosh Plus is a SCSI port, not an RS232 port. SCSI is a high speed parallel interface and is not compatible with RS-232. The chips used by the SCSI port in the Mac Plus may be damaged by the RS-232 signals from a serial printer.
CURE: Replace the Mac's logic board. To use a serial RS-232 device with the Mac Plus, use Apple's mini-circular 8 to DB-25 peripheral cable.

If the above cure doesn't fix the problem, refer to the Tech Procedures to obtain the general troubleshooting for this product.
Published Date: Feb 19, 2012