In the April 1993 Radio Shack M.O.M., page 13, is an article titled "Don't
Mix Old Tandy Printers With Other's PC Clones." It states that

     "...some ...IBM clones may not have a parallel printer port that will
     tolerate the voltages provided through the printer cable from the
     older Tandy printers to the computer. In these cases, permanent
     damage to the computer system can result when the printer is plugged
     in and turned on."

The article goes on to discuss the fact that the older Tandy printers were
designed to work with the "Centronics Standard Port" which, as the IBM PC
and it's clones became more widespread, was replaced in popularity with
the IBM-style printer port. Thus:

     "To be compatible with computers that were sold in those early days,
     the older Tandy printers supplied +5 volts on pin 18 of the printer
     connector. All the newer Tandy printers have switched to the more
     popular IBM parallel port standard that does not have this 5-volt
     signal present. It is this 5-volt line on pin 18 that causes the
     problem with some of the clone computers."

Apparently, a few PC clones do not have good enough buffering to provide
adequate isolation from "extraneous voltages on the parallel port." 
Unfortunately, Tandy is hardly in a position to test every machine on the 
market. The memo does mention that "[m]ost of the reports seem to revolve
around using Tandy daisy wheel printers with recent releases from Packard
Bell." However, it also points out that "...no exact information is 
available on which of the clones will be damaged." Further, "[t]here is
also no guarantee that the printer will not function with the ...clone
computer."

All that being said, this seems to be the bottom line:

     "Unfortunately, there are no symptoms to indicate that the two are not
     compatible until it is too late. The printer will simply destroy some
     parts of the computer's main logic board without warning. The extent
     of the damage is not determinable and could depend on the clone's
     design and construction."

The memo suggests that "customers who want to attempt to correct the
problem by themselves" can cut line 18 at the printer end of the cable. 
(I.e., either trim off pin 18, or cut the wire in the cable that's attached
to pin 18.) However, it also continues with the warning that "[i]t is still
possible that the clone computer will not like the voltages present on the
other lines and [that they will] still cause damage."

It should be noted here that Tandy can hardly be held at fault for this
situation. Their older printers were built in conformance to an earlier
standard for parallel I/O -- this is reasonable. For that matter, most
modern computers, including Tandy's, have no problem dealing with the
voltages involved. And, of course, the newer Tandy printers do not cause 
any problems.



The following is a list of printers that are designed for use with the old
Tandy parallel port. These printers, and any older models, could 
potentially trigger the problems discussed in this article. When in doubt, 
suspect any Tandy printer that requires cable 26-0227 or 26-0223 to 
interface to a modern IBM parallel printer port.

     DMP 100     DMP 105     DMP 106     DMP 107
     DMP 110     DMP 120     DMP 130     DMP 130A
     DMP 132     DMP 133
     DMP 2102    DMP 2110    DMP 2120
     DMP 300
     DMP 430     DMP 440     DMP 442
     DWP 230     DWP 520
     LP  1000



-- William K. Walker
   North Valley Digital
   740 Alpha Road
   Alpha  OH  45301-0149

   Voice: 513-426-0344
     FAX: 513-426-0344
     CIS: 71066,24
     AOL: wkwalker
     nvdi@mcimail.com