Contemporary computers do not have RS-232 ports, but USB ports instead. However, there are many USB to RS-232 adapters available.
If the adapter is to be used with a device which uses the Modbus protocol or will be installed in a system which uses the Modbus protocol, the adapter must either be specified for use with Modbus or must be tested on a Modbus device for proper communications functioning.
Selection criteria for Data Acquisition products:
MAQ20-COM2, SLX300-10
These devices have USB ports. If an additional communications channel is needed, a USB RS-232 adapter can be used. The adapter in this case does not need handshake signals included.
These devices both use the Modbus protocol.
Selection criteria for Data Communications products:
DCP485, LDM70, LDM85, LDM422, LDM485
These devices are externally powered and use transmit data, receive data, and ground for communications, and use various handshake signals to control particular functions. Refer to the specific user manual in the Media Attachments area to the right of this Content main body for usage instructions.
DCP35, LDM35, LDM80
These devices are powered from the handshake and the transmit and receive data signals.
The adapter in this case should have the handshake signals included.
If the adapter does not have handshake signals included, use it for the transmit and receive data signals and power the device handshake inputs from separate positive and negative power supplies.
Since the adapter could be installed between two computers and either one could initiate a data transfer, the adapter should be able to allow not only the USB side but also the RS-232 side to initiate the data transfer.
LDM30
This device is externally powered and uses only transmit data, receive data, and ground for communications. RTS is connected to CTS internally. DTR, DSR, and RLSD (DCD) are connected to each other internally.
The adapter in this case does not need handshake signals included.
If there is a system requirement for handshake signals, then choose an adapter with handshake signals included.
Selection criteria for Signal Conditioning products:
SCM9B-A1000/A2000
These devices are externally powered and use only transmit data, receive data, and ground for communications. Handshake signals CTS, DSR, and RLSD (DCD or CD) are internally biased to their asserted state or high state, a positive voltage.
The adapter in this case does not need handshake signals included.
If there is a system requirement for handshake signals, then choose an adapter with handshake signals included.