Quatech SSCF-200 User manual - page 9
Quatech CompactFlash Card User’s Manual
SSCF-200/300
- 9 -
6. Using the SSCF-200/300 Card
6.1 TERMINATION
RS-422 and RS-485 lines should be terminated at the end of the main branch of the receiver with the
cables characteristic impedance. These terminating impedances reduce signal reflections at the cable
end. It is not necessary to terminate the transmitter end of the cable. The most common method of
termination is to install a terminating resistor, typically with a value of 120
Ω
, at a single receiver.
The SSCF-200/300 does not have internal terminating resistors fitted as standard. If required, a
120
Ω
resistor should be connected between the RXD+ and RXD- pins of the DB9 connector. For RS-
422 applications, a terminating resistor will also be required between the CTS+ and CTS- pins.
6.2 FAIL SAFE OPEN CIRCUIT PROTECTION
A transmission line enters an indeterminate state if no drivers are transmitting on it or the line has
been cut. This indeterminate state can cause the receivers to receive invalid data bits from noise
picked up on the cable. To prevent reception of these data bits, the SSCF-200/300 receiver inputs
incorporate 1.2k
Ω
bias resistors (one pulling up on “+” and one pulling down on “-“) that ensure that
the differential voltage on the RXD and CTS pairs is greater than the 200mV input logic “1”
threshold (even in the presence of an external 120
Ω
terminating resistor).
6.3 TXD AND RTS DRIVERS
The RTS signal driver on the SSCF-200/300 is always enabled, regardless of the mode in which the
card is to be used.
The SSCF-200/300 incorporates an auto-tristate feature on the TXD signal driver. The driver is
enabled only when data is in the process of being transmitted. This mechanism can prevent bus
contention in RS-485 systems (caused by multiple transmitters driving the line in opposing states).
The RS-485 transmitter arbitration scheme is beyond the scope of this guide, and is typically
handled by application software.
Note that when multiple characters are transmitted “back-to-back” the output drivers stay active for
the entire duration of the transmission i.e. the drivers do not go in and out of tri-state for each
character in a multi-character block. The auto-tristate feature is always in operation, regardless of
the mode in which the card is to be used.