Document Reference No.: ES_000011
ES-R-2x01-M RS232 to RS422 / RS485 Converter User Guide
Version draft
Clearance No.: ES#*
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©2009 EasySync Ltd.
8
JP1 Links
RS422/RS485 Function
1
2
Fit for 120R termination between TxD- and TxD+
3
4
Fit for 750R pull-up bias on TxD+
5
6
Fit for 750R pull-down bias on TxD-
7
8
Fit for 120R termination between RxD- and RxD+
9
10
Fit for 750R pull-up bias on RxD+
11
12
Fit for 750R pull-down bias on RxD-
13
14
Fit for 120R termination between CTS- and CTS+
Table 2.4 – JP1 Termination and Bias Configuration Jumper
The factory setting is for no JP1 links to be fitted.
The need for termination depends on the baud rate and the propagation delay of the cable. A reasonable
rule of thumb is that no termination is required if the cable return trip delay is less that the bit time / 6.
For example, 1000m of cable with a propagation delay of 70% (of speed of light) will have a return trip
delay of 6.7us. The bit time for a baud rate of 9600 is approximately 100us therefore it is reasonable to
assume that any edge reflections have reduced to an insignificant level when the receiver samples the
data in the middle of the bit time.
Termination is only fitted to the receiving end of the link if it is required.
In a RS485 two-wire interface, termination may be required on both ends of the link, as each end has a
receiver, and is activated by fitting a link between JP1 pins 1 and 2.
In a RS485 four-wire interface termination may be required on the receiver and is activated by fitting a
link between JP1 pins 7 and 8. No link would be fitted across JP1 pins 1 and 2.
Bias resistors may be necessary to set a defined logic level at a receiver when the connected transmitter
is not enabled. Bias resistors are not required for a RS422 interface as the transmitter is always on.
The RS422/RS485 transceivers used on the ES-R-2x01-M feature fail-safe circuitry to guarantee a logic-
high receiver output if the receiver inputs are not connected. Therefore it will not normally be necessary
to fit any of the bias resistor options.
2.1.4
Slew Rate Configuration
Jumper JP2 selects the slew rate limit on the RS422/RS485 driver. Select the lowest value that is
consistent with the intended baud rate. The factory setting is to link pins 2 to 3 to select a slew rate that
supports up to 10M bits/s. Note that the maximum data rate of the ES-R-2x01-M is limited to 1Mbps by
the RS232 transceivers.
JP2
Slew Rate Limit
Link 1 – 2
500k bits per second
Link 2 – 3
10M bits per second. Factory Setting.
No link fitted
115k bits per second
Table 2.5 – JP2 RS422/RS485 Slew Rate Configuration Jumper
Preliminary
