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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