Specifications

4–24 Troubleshooting
d. If the printer prints from the host, but occasionally loses blocks
of data, the most likely cause is the host not responding to “send no
more data” signals from the printer. With a Centronics interface, this
means the host is ignoring BUSY; in RS–232, the host is ignoring the
XOFF character or “Data Terminal Ready” (pin 20). Sometimes the
cable is not conveying the necessary signals. Test this by using a
serial line analyzer and software or test equipment that displays the
data and handshake lines of the printer. A break–out box works, but
will not debug XON/XOFF or other RS–232 data protocols. If the
printer is sending XOFF, the host may not be receiving it if it
requires 1.5 or 2 stop bits.
e. If the printer prints from the host, but occasionally prints double
characters, there is probably a noise problem at the interface or the
host computer is sending an inverted strobe. This problem can occur
on Centronics or Dataproducts interfaces, never on RS–232. What
happens is the strobe signal registers logic 1 more than once for a
certain character. To fix this, change the strobe to trailing edge or
invert the Dataproducts strobe polarity.
Also check that the terminating resistors are present at locations C12
and D12 on the CCB. You can correct noise problems by using a
shielded data cable or by changing the terminating resistors. (Refer
to Appendix E.) The standard terminating resistors are optimized for
high speed data transfer for cables between 1 and 5 meters long; you
can change them to be slower and slightly more immune to noise.
Noise is caused by static, floating logic ground, unshielded cable,
changes in ground voltage from nearby equipment, or capacitively–
or magnetically–induced noise. On very long cables,
capacitively–induced noise from the other signals of the cable
(especially Centronics “ACK” or Dataproducts data request “DRQ”)
can cause false strobes. Unshielded and flat ribbon cables are much
more prone to problems due to increased length. The best solution is
to shorten cable, shield it, and reduce local electromagnetic noise.