Instruction manual

3–31Troubleshooting
Symptom Instruction
Printer prints data sent from the host, but
occasionally prints double characters,
random ! or * characters, or loses blocks
of data.
1. 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” signal (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 breakout
box works, but will not debug XON/XOFF or other
RS–232 data protocols. If the printer is sending XOFF
with one stop bit, the host may not be receiving it if the
host requires 1.5 or 2 stop bits.
2. a) 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 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 strobe polarity. (Refer to the
User’s
Guide
.)
b) Also check that the terminating resistors are present
on the controller board (page 5–35). You can correct
some noise problems by using a shielded data cable or
by changing the terminating resistors. The standard
terminating resistors are optimized for high speed data
transfer for cables between 1 and 5 meters long; you
can change them for slower operation, which will provide
slightly more noise immunity.
c) Noise is caused by static, a 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”) 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. Also, make sure that both
the printer and the host computer are properly grounded.