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The PC
422/485 Serial Interface is a card you can install in a PC that
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allows that PC to communicate with and control RS-422 or RS-485 devices.
The IC030C-R2 uses a 16550 UART chip, the same chip as the one in the
regular IBM
asynchronous adapter (RS-232 comm port). This means that
®
you can set its data rate, data format, and interrupt control through DOS,
or by programming the UART, the same way you would if it were a regular
RS-232 port. Refer to the IBM Technical Reference Guide for information
on programming the chip. The IC170C uses the 16850 chip, for even better
performance (it features a 128-byte FIFO buffer).
The card can be set to use serial port COM1:, COM2:, or six other
predefined addresses. This makes it compatible with most communications
software and programming languages. In general, the card works the same
way as an RS-232 serial port, except that RS-422/485 drivers and receivers
are built into it rather than RS-232 components.
The RS-422 interface specification supports very-long-distance (up to
4000 ft. [1219.2 m]) point-to-point serial communication with virtually
error-free differential drive characteristics. Figure 2-1, below, shows a
simplified diagram of an RS-422 application.
PC-422/485 CARD
Figure 2-1. An RS-422 (point-to-point) application.
The RS-485 interface specification is compatible with RS-422, but
supports multipoint applications: The output of the PC
Interface, like that of other RS-485 devices, can be in any of three states
(high, low, or off). This allows you to connect and selectively poll multiple
PCs in a multi-drop arrangement, as shown on the next page in Figure 2-2.
RS-485 also provides for two-wire half-duplex applications. You can build
such an application (shown on the next page in Figure 2-3) by connecting
the card's TX+ to its RX+ and its TX– to its RX–. (See the pinout chart in
Table 2-1, also on the next page, for the pinning of these signals.)
2. Introduction
TX
RX
CHAPTER 2: Introduction
RX
TX
422/485 Serial
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RS-422 DEVICE
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