Specifications

3–24 Troubleshooting
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 5–56.) 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.
f. If the printer prints garbled data or slews uncontrollably, put the
printer into hex dump mode and analyze the binary data. One
cause of garble is the host interface or cable is not transmitting
all the data. When this occurs, the Centronics or Dataproducts
interface receives a 1 on every unconnected data line.
Uncontrolled slewing is often caused by enabling PI (Paper
Instruction) when the host lacks a PI signal. (PI shows as “p” on
the hex dump.) Some DEC RS–232 and Dataproducts interfaces
only send 7 data bits. In this case, the eigth bit will be received as
one. In RS–232, a common cause of garble is to set the interface
to the wrong baud rate or parity. Sometimes the host sends 1.5 or
2 stop bits; in this case, the printer’s “1–stop–bit” setting will
accept both 1.5 and 2 stop bit data. Sometimes the data may
“fade” or “persist” from one character to the next. This reveals a
problem with Centronics or Dataproducts terminating resistors