Rockwell Automation 2755-DM9, -DM9E High Speed Decoder - Manual (Page 85)

Chapter 6
Communicating With a Host
6–3
As shown in Figure 6.1 the RS-232 interface uses pins 2, 3, 4, 5 and 20 of the
HOST port connector. Three of the communications lines, pins 4 (RTS), 5
(CTS), and 20 (DTR), are optional flow control lines
.
Depending upon how
the decoder is configured (HOST PROTOCOL configuration parameter,
described in Chapter 4), these modem control lines are enabled or disabled:
•
RS-232: no flow control.
•
RS-232 XON/XOFF: XOFF character from the host suspends
transmission and the XON character resumes the transmission. Both the
XON and XOFF characters are removed from a message by the receiving
device.
•
RS-232 RTS/CTS-1: enables the RS-232 (RTS and CTS) control lines for
flow control.
•
RS-232 RTS/CTS-2: enables the RS-232 (RTS and CTS, ) control lines
for flow control. Use this mode when the decoder is communicating with
a 2760-RA or -RB communications controller using modem controls.
The following are descriptions of the three RS-232 modem control lines:
DTR (Pin #20) The Data Terminal Ready signal is sent by the decoder
(RTS/CTS-1, -2) and informs the host device that the decoder is on-line
and capable of receiving data from the host. The DTR
line will remain on while the decoder is on.
Note: The DTR line will be asserted (turned on) regardless of the
communications interface being used.
RTS (Pin #4) The Request To Send signal is sent by the
(RTS/CTS-1) decoder and informs the host device that the decoder is ready
to send data. When the host receives an RTS signal (pin #4)
from the decoder, the host must assert CTS to inform the
decoder to begin sending data. When the decoder stops
sending an RTS, the host must stop sending a CTS before the
decoder can assert the RTS again.
RTS (Pin #4) In this mode, the RTS signal (pin #4) is sent
(RTS/CTS-2) by the decoder to inform the host that the decoder can accept
data. The host should only send data when RTS is asserted.
CTS (Pin#5) The Clear To Send signal is sent by the host
(RTS/CTS-1,-2)device and informs the decoder that the host will accept data.
The decoder will only send data if the CTS line is on.
Refer to Figure 6.2 for an illustration of how to connect a host device to the
decoder using the RS-232 interface.
RS-232 Interface