3–24 Troubleshooting
XON/XOFF if possible; this requires the least complex cable.
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. (See page 5–62.) 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.
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