Serial Communications (16C452)
The ZT 8809A provides fully buffered RS-232-C serial data and
control lines via two connectors, supplying the
±
12 V swing needed to
meet RS-232-C driver requirements. In addition, RS-422/485 drivers
are on board, available for use at COM2. Refer to Appendix A for the
proper jumper selections. The RS-232-C buffers meet the RS-232-C
standard for signal conditioning for the recommended 50-foot cables.
Longer cables are permissible provided the resulting load capacitance
does not exceed 2500 picofarads. The ZT 8809A also implements the
capacitive signal conditioning as required by the RS-232-C standard.
Refer to the table on page 3-7 for an overview of the different serial
communications standards.
RS-232-C vs. RS-422/485
Serial port 2 is configurable for either RS-232-C or RS-422-A/
RS-485 operation. Both types of serial drivers and receivers are on
board. Use jumpers W14-W19, W21-W22, and W29-W32 to select
between the two types.
As shown in the table on page 3-7, RS-232-C is a single-ended serial
communications protocol that allows one driver and one receiver on
each line, with a maximum of 50 feet separation and a maximum
speed of 20K bits per second. RS-422-A was defined to increase the
distance of separation and the maximum bit rate. It allows for a
maximum separation of 4000 feet and a maximum bit rate of 10
megabits per second. Differential drivers and receivers accomplish
this increase in distance and speed and also allow for an increase in
the number of serial receivers to 10.
RS-485 is similar to RS-422-A, with the additional capability for 32
drivers and/or receivers. The ZT 8809A uses the same serial drivers
for both RS-422 and RS-485 applications. For RS-485, the use of
multiple drivers requires control of the drivers’ output enable; the
printer port signal INIT may be used for this control. Refer to the
description of jumper W13 in Appendix A, and to Chapter 9
regarding the printer port.
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