Function And Use Of Serial Ports; Com1 As Serial Console Device; Com1 Through Com4 As Rs-232 I/O; Com3/4 As Rs-422 And Rs-485 Networks - Octagon Systems XE–700 Reference Manual

Function and use of serial ports

COM1 as serial console device

You can use COM1 as a console device to communicate with another PC. For COM1
to be a serial console, the "V" video switch (Switch 4 position 3) must be set to Off.
See the Console Devices chapter for more information.
Note When interfacing the XE–700 SBC to your desktop PC, you must use a null modem
adapter. The default settings are 38.4K baud, 8-bit, no parity, 1 stop bit.
COM1 through COM4 as RS–232 I/O
COM1 is an 8-wire RS–232 interface. COM2 is a dedicated 4-wire RS–232
interface. COM3 and COM4 can be configured through Setup as a 4-wire RS–232
interface. You can connect up to four RS–232 serial I/O devices.
COM3/4 as RS–422 and RS–485 networks
COM3 and COM4 can also be used as RS–422 or RS–485. RS–422 and RS–485 use
differential signaling to communicate between the devices on a network.
Differential signal reduces the effect of environmental noise, allowing
communication over distances up to 1200 meters.
The RS–422 and RS–485 receivers provide an active high (space) condition for
shorted, open, or inactive lines. Note that RTS is used differently by RS–422 and
RS–485. Review the information in the following sections regarding RTS.
RS–422 is a point-to-point configuration. RS–485 is a multi-node configuration that
allows up to 32 nodes on a network. COM3 and COM4 can be configured in BIOS
Setup for RS–422 / RS–485. Refer to Table 4–5 on page 43 for switch settings for
terminating an RS–422/485 network.
RS–422
RS–422 is typically point to point configuration. RS–422 is also specified for multi-
drop (party-line) applications where only one driver is connected to, and transmits
on, a "bus" of up to 10 receivers. The device at the end of an RS–422 network must
be terminated. The XE–700 SBC optionally terminates with a 110 ohm resistor.
Refer to Table 4–5. Figure 4–3 shows a typical RS–422 four-wire interface circuit.
The RTS* signal is used to control the transmitter and receiver in RS–422 mode.
The RTS* signal is controlled by the Modem Control Register bit 1 (MCR[1], which
is offset 0x04 from the UART base address). Writing MCR[1] to 0 (default state)
sets RTS* to an inactive state (RTS* = logic high) which ENABLES both the RS–
422 Transmitter and Receiver. Writing MCR[1] to 1 sets RTS* to an active state
(RTS* = logic low) which DISABLES both the RS–422 Transmitter and Receiver.
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