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PmodRS485™ Reference Manual
Revised April 12, 2016
This manual applies to the PmodRS485 rev. B
DOC#: 502-310
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 1 of 4
Overview
The Digilent PmodRS485 provides signal and power isolation for high speed communication lines utilizing the RS-
485 communication protocol.
1 Functional Description
The PmodRS485 utilizes Analog Devices ADM2582E to facilitate RS-485 and RS-422 serial communication protocols
between devices in environments with high electrical noise. The ADM2852E provides both signal and power
isolation allowing for accurate data transfer across long distances. Data transmission rates of 16 Mbit/s can be
achieved.
2 Interfacing with the Pmod
The PmodRS485 communicates with the host board via the UART protocol. In order to transmit data, the Driver
Enable line must be pulled to a logic level high voltage state; similarly, in order to receive data, the Receive Enable
line must be driven to a logic level low voltage state.
Truth tables indicating the status of the various pins of the PmodRS485 are provided below:
The PmodRS485.
Features include:
PmodRS485™ Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 2 of 4
Inputs
Output
A-B Voltage
Difference
~RE
RxD
≥ -0.03V
Low or Not Connected
High
≤ -0.2V
Low or Not Connected
Low
-0.2V < A-B < -0.03V
Low or Not Connected
Don't Care
Inputs Open
Low or Not Connected
High
Don't Care
High
High Impedance
Table 1. Receiving.
Inputs
Output
DE
TxD
Y
Z
High
High
High
Low
High
Low
Low
High
Low
Don't Care
High Impedance
High Impedance
Table 2. Transmitting.
Pin
Signal
Description
1
~RE
Receive Enable
2
TxD
Transmit Data
3
RxD
Receive Data
4
DE
Driver Enable
5
GND
Power Supply Ground
6
VCC
Positive Power Supply (3.3V/5V)
Figure 1. PmodRS485 functional block diagram.
Signal
Description
A
Input A
B
Input B
Z
Output Z
Y
Output Z
Table 3. Pinout description table: Pmod Header J2.
Table 4. Screw terminals.
RS485
Transceiver
RE
DE
TXD
RXD
GND
VCC
JP1
Y-output
Z-output
B-input
A-input
PmodRS485™ Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 3 of 4
Multiple PmodRS485 devices can be chained together up to 256 nodes in total. When two PmodRS485s are
connected, JP1 should be loaded on both devices. When more than two PmodRS485s are connected, JP1 should
only be loaded on the two devices at the terminating ends of the wire, and stubs off of the main line should be
kept as short as possible.
RS485
JP1
ZY
AB
RS485
RS485
JP1
ZY
AB
RS485
Shorted jumper
Not Shorted Jumper
RS485
JP1
Z
Y
A
B
RS485
JP1
Z
Y
A
B
Figure 2. Half-duplex communication.
RS485
JP1
Z
Y
A
B
RS485
JP1
Z
Y
A
B
RS485
JP1
ZY
AB
RS485
RS485
JP1
ZY
AB
RS485
Shorted jumper
Not Shorted Jumper
Figure 3. Full-duplex communication.
Any external power applied to the PmodRS485 must be within 3.0V and 5.5V; however, it is recommended that
Pmod is operated at 3.3V.
PmodRS485™ Reference Manual
Copyright Digilent, Inc. All rights reserved.
Other product and company names mentioned may be trademarks of their respective owners.
Page 4 of 4
3 Physical Dimensions
The pins on the pin header are spaced 100 mil apart. The PCB is 1.5 inches long on the sides parallel to the pins on
the pin header and 0.8 inches long on the sides perpendicular to the pin header.