Q: I Want to use RS-422 Converters to Extend RS-232. How
do I Connect them?
A: It depends
on the two devices being connected, whether they are DTE or
DCE devices, how they are connected together before adding
a RS-232/RS-422 Converter, how many signal lines are required.
This information is for devices requiring only Receive (Rx)
and Transmit (Tx) signal only, other devices require more
A1: CASE #1:
The Device connects directly to the serial port on the Computer.
The computer serial port is configured as a DTE port, the
device cabling is configured as DCE to connect directly. With
this connection, the first RS-422 converter connects with
a Standard serial cable wired pin #1 to #1, #2 to #2, etc.
The second converter requires a null modem connection between
it and the device, since our converter is configured as DCE
on the RS-232 side, and the device cabling is also DCE. See
the connection diagram for CASE #1.
A2: CASE #2:
The Device connects through a Null Modem (Crossover) Cable
to the serial port on the Computer. This original connection
requires a Null Modem cable because both Computer and Device
are configured as DTE. In this case, the first RS-422 Converter
connects to the Computer with a Standard serial cable wired
pin #1 to #1, #2 to #2, etc. The original Null Modem Cable
is set aside, not used. A new Standard serial Cable is connected
to the device and the second converter. See the connection
diagram for CASE #2.
A3: CASE #3:
In the third case, we want to extend a set of RS-232 connections
between two devices, but we don't know how either port is
configured. Both devices use DB9F (female) connectors. We
have a voltmeter. We know the converters are DCE devices.
When powered, the RS-232 output will have a negative DC voltage
compared to GND. So, we connect a Standard cable to the device,
power it up, measure between Pin #5 of the cable to Pin #3.
If it has a minus voltage (usually between -3VDC to -11VDC),
connect it to the DB9 connector of the converter. This connection
should be okay. If almost no voltage on Pin #3, check Pin
#2. If it has the minus voltage, use a Null Modem Connection
between the device and converter to swap the pin connections.
If neither has a voltage, you need pinouts for the connector/cable.
Make a similar test for the RS-232 connections at the other
end. The voltage should be on the cable pin connecting to
Pin #3. No Connection diagram for CASE #3, it will either
match CASE #1 or CASE #2, but could instead require connections
with Null Modem Connections on both converters if the
original cable is not used.
The pin numbers for the RS-422 connections
are not shown, they may vary according to the model, but the
signal name will be shown on the data sheet.
In the case of DB25 RS-232 connections,
Pin #2 is TD, Pin #3 is RD, and Pin #7 is GND.
Note: These connection figures do not
show power supplies or handshaking lines needed to power "port
powered" converters. The port powered devices usually need
the RTS line and DTR line high (signal level +11 VDC), a power
supply may be required for both ends if the RS-422 transmitter
voltage is less than 4.4 volts TD(B) to TD(A). For pinouts
of RS-232, refer to the FAQ on RS-232 Connections that WORK!.
This information should help you make
the connections needed for the most common connections.