Operation; Configuring Handshaking And Receive-Only Modes; Using Rts/Cts And Automatic Handshaking; Rs-485 Half-Duplex Mode - Campbell SDM-SIO1A Instruction Manual

7.

Operation

7.1

Configuring Handshaking and Receive-Only Modes

7.1.1 Using RTS/CTS and Automatic Handshaking

NOTE

7.1.2 RS-485 Half-Duplex Mode

NOTE
SDM-SIO1A and SDM-SIO4A Serial Input/Output Modules
This section includes more detail on less common modes of operation, provides
details on how to save power use, and shows the internal configuration of the
SDM-SIO1A when configured in RS-485 or RS-422 full-duplex mode.
Handshaking is a method used by RS-232 to ensure communications
equipment is free to receive or transmit data. This interface is often called
RTS/CTS (hardware handshaking) or DTR/DTE.
Automatic handshaking for the SDM-SIO1A is activated through use of the
BaudRate parameter of the SerialOpen() instruction.. When the BaudRate
value is set to a negative number, the SDM-SIO1A will enable automatic
handshaking. It is worth noting that normally setting the BaudRate to a
negative number will enable automatic baud rate detection; this is not the case
for the SDM-SIO1A.
When enabled, the two I/O ports (pins 8 and 11) will operate as RTS and CTS
lines. It is important that the remote equipment supports handshaking as no
data will be sent if handshaking is enabled but is not supported by the
equipment the SDM-SIO1A is connected to.
Once handshaking is enabled, pins 8 and 11 are no longer available for general
use and are dedicated to the auto-handshaking system.
Enabling handshaking will increase active current consumption due to the extra
RS-232 load. It will not affect sleep current, however, as the RS-232 chip is
disabled (sleep is set by calling the SerialClose() function).
For connection diagrams and further information on using handshaking,
consult Section 6.2.1, Wiring Examples for Connecting the SDM-SIO1A
• Auto flow control should only be selected in RS-232 mode.
• When Auto flow control is enabled, the user cannot set or clear the spare I/O
ports (pins 8 and 11).
In RS-485 half-duplex mode, the SDM-SIO1A will wait for approximately
2.5 ms after a character is received before it tries to transmit any data in its
transmit buffer. This is to ensure that a contention does not occur on the data
line which will cause data corruption. There is also a delay of at least the time
to send one character after the last character has been transmitted before the
SDM-SIO1A will listen for new incoming data.
When in either RS-485 half-duplex or full-duplex modes, the user
cannot set or clear the spare I/O ports (pins 8 and 11).
.
(p. 11)
23