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Revision: January 7, 2011
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Pmod Crossover Cable Needed to Connect Old and New UART Interfaces
There are two different signal pin assignment conventions for UART Pmod connectors on Digilent
Pmod peripheral modules and microcontroller boards. They are known as the old and new UART
interfaces, and are described below.
Connecting a Pmod with an old UART interface to a microcontroller board with a new UART interface,
or vice-versa, requires using the Digilent Pmod Crossover Cable.
Which UART Interface Do I Have?
The following Digilent Microcontroller boards use the old interface on their UART Pmod connectors:
Cerebot 32MX4
Cerebot II
Cerebot Plus
Cerebot Nano
I/O Explorer
The following Digilent Pmods also use the old UART interface:
PmodRS232
PmodCLS
All other Digilent microcontroller boards and Pmods use the new interface on their UART Pmod
connectors.
Understanding the UART Interfaces
At the time that the pin assignment for UART interfaces was originally designed, the microcontrollers
had separate SPI controllers and UARTs, and there was no relationship between the signals on
either.
Some newer microcontrollers (notably newer PIC32 parts) have serial interface controllers that can be
used as either an SPI controller or as a UART. In order to take maximum advantage of these newer
serial interface peripheral devices, it was necessary to change the signal pin assignments to make the
Pmod UART pinout agree with the Pmod SPI pinouts.
Doc: 514-005 page 1 of 2
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
Connecting UART Pmod Interfaces
The original design (the old UART interface) uses the following signal assignment:
Pin Signal
1 CTS (Clear to Send)
2 RTS (Request to
Send)
3 RXD (Receive Data)
4 TXD (Transmit Data)
The new UART interface uses the following signal assignment:
Pin Signal
1 CTS (Clear to Send)
2 TXD (Transmit Data)
3 RXD (Receive Data)
4 RTS (Request to
Send)
The difference between the old and new UART interfaces is that the signals on pins 2 and 4 are
swapped. The Pmod Crossover Cable swaps pins 2 and 4.
www.digilentinc.com page 2 of 2
Copyright Digilent, Inc. All rights reserved. Other product and company names mentioned may be trademarks of their respective owners.
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