ZRT Manual
Page 16 of 37
Rev. J – 11 September 2008
5.2.2
RS232/RS422/RS485 Versions
The RS232/RS422/RS485 versions of the ZRT may be software programmed to have either an
RS232 or an RS422/RS485 compatible interface.
To allow programming without an interface adapter, a radio programmed for RS422/RS485
operation will switch back to RS232 operation if the front panel rotary switches are set to
position 00 to select programming mode. Note that although the ZRT can withstand RS232
voltages on its inputs in RS422/RS485 mode, this may not be the case for externally connected
devices. To avoid any risk of damage to the externally connected equipment, care must
therefore be taken to ensure that programming mode is not selected while connected to
RS422/RS485 terminal equipment.
Pin allocation when set for RS232 operation:-
o- - - - - 9. nc Not Connected
4. DTR: Data Terminal Ready - - - - - - - o
(Internally connected to Pin 6)
o- - - - - 8. CTS: Clear to Send
(Internally connected to Pin 7)
3.
: Transmit Data Input - - - - - - - -
TXD
o
- - - - - 7. RTS: Request to Send
o
(Internally connected to Pin 8)
2.
: Receive Data Output - - - - - - - -
RXD
o
o- - - - - 6. DSR: Data Set Ready
(Internally connected to Pin 4)
5. GND: GROUND - - - - - - - - - - - - - - - o
1.
Data Carrier Detect
- - - - - - - - -
DCD: o
Pin allocation when set for RS422/RS485 operation:-
o- - - - - 9. nc Not Connected
4. DTR: Data Terminal Ready - - - - - - - o
(Internally connected to Pin 6)
o- - - - - 8. *****: Do Not Use
(Internally connected to Pin 7)
3.
: +ve Data In to Radio - - - - - - - -
TXD
+
o
- - - - - 7.
: -ve Data In to Radio
o
TXD -
(Internally connected to Pin 8)
2.
: -ve Data Out from radio - - - - - -
RXD -
o
o- - - - - 6. DSR: Data Set Ready
(Internally connected to Pin 4)
5. GND: GROUND - - - - - - - - - - - - - - - o
1.
: +ve Data Out from radio
- - - - -
RXD +
o
When half-duplex (2-wire) RS422/RS485 operation is programmed, the TXD- and TXD+ signals
are internally connected to the RXD- and RXD+ signals respectively. The 2-wire half-duplex
bus only needs to be connected to the RXD- and RXD+ pins (Pins 1 and 2). Shorting links are
not required in the connector.
When operating in 2-wire RS422/RS485 mode, the radio will output data on to a 2 wire circuit
whenever it is received, which could lead to a bus conflict in conditions of high interference and
prevent a connected terminal from outputting transmit data. To avoid this condition it is
recommended that the message packeting option is turned on at both ends of the radio link.
The DTR and DSR signals are looped back to each other internally in both configurations.
When configured for RS232 operation, the RTS and CTS signals are also looped back to each
other. The radio should in most circumstances operate correctly in applications requiring
RTS/CTS handshake although the it is unable to use CTS to prevent buffer overflow. Overflow
situations can easily be avoided by making the serial port baud rate the same as the radio signal
baud rate, or by ensuring that message sizes do not exceed the buffer size of 1024 bytes.
