2.9 ABOUT SHIELDING
Shielding is recommended for both signal and enable wiring especially if long lengths of wire are used, or
when operating in an environment of high electromagnetic noise. Each wire's shield should be
connected at only 1 point, connecting both ends of a shield will produce a closed loop in which noise
has no way to exit. A daisy chain connecting multiple Readers counts as a single wire, and the shields
should be connected together, but not to the Reader, at each drop along its run. Since shield
connections are recommended at the source of the associated signal, SIGNAL shields are connected to
Readers and ENABLE shields are connected to the Interface. If the SIGNAL wires are combined in
the same cable with the ENABLE wires their shields are common and connected to one Reader. There
is an excellent wiring drawing contained in both the Model 1845 and 1885 manuals.
2.10 MATING THE INTERFACE TO A COMPUTER
Communication characteristics, speed, parity, and number of bits per character, must be matched
between the Interface and the connected host. If the Interface is talking at 2400 baud (bits per second)
and the host at 4800, they'll never understand each other. Most hosts can be configured to a number of
different speeds and formats. Some, however, cannot. That's why the Interface can be set to operate
from 110 to 19,200 baud. If your host is stubborn, match the Interface to the host's settings. If your
host is flexible 19,200 baud is recommended with 7 bits per word and 1 stop bit so the Interface can
spend less time communicating and more time looking for Tags. Parity, either even or odd, is
recommended for reliability and RS-422 is more reliable than RS-232 but is generally less available for
a variety of hosts.
PCB address P3 and P4 consist of pins and jumpers. Default communication characteristics are set by
installing or removing the jumpers. Thus shorting together or leaving the pin pairs open. The "removed"
jumpers may be left on a single pin so they won't become misplaced should someone want to change
the configuration at a later date. The position of the P3 jumpers is read at reset so configuration
changes are not registered until a subsequent power up or software reset.
2.10.1 SETTING THE SERIAL TYPE AND APPLICABLE DOCUMENTS
Pins at PCB address P4 are used to select between RS-232 or RS-422 voltage levels and consist of
three pins and a jumper used to short two of them together. With the Interface oriented as shown in
Figure 2-3, place the jumper on the center and right-most pins to select RS-232 operation. If the
jumper is placed on the center and left-most pins RS-422 operation is selected. The P4 jumper,
actually changes voltage references to the communication receivers and drivers and its effects are
registered immediately at connector P2, which should be left unconnected while changing P4..
Following are documents that generally explain serial communications
EIA Standard, RS-232-C August, 1969
EIA Standard, RS-422-A December, 1978
EIA Standard, RS-423-A December, 1978
RFID, Inc. Interface Specification 710-0004-021
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