Instrument Connectivity > Instrument Installation and Wiring

NOTE: If communications cannot be established between SpecView and the connected instruments first check the instrument manufacturer's manual for correct wiring instructions. For further assistance please contact the instrument manufacturer. They may have test programs that will check your converter and connections.

Serial comms - USB (or old style COM ports RS232 9-pin) to RS422/RS485

If the intention is to use RS422 or RS485 serial comms wiring to connect the instrument, then an external converter from USB (or RS232) to either RS422 or RS485 (as appropriate) must be used.

When purchasing a converter note that there are very low cost converters available, however, these are typically not optically isolated and are therefore are not suited to an industrial environment.

SpecView recommends converters made by B&B Electronics.

The reason B&B devices are recommended is because they are optically isolated and have 'Automatic Send Data Control', this automatically switches from transmit to receive and so works very well with SpecView.

For connecting instruments which have a 2-wire RS485 connection:

It is best to use a 2-wire converter, otherwise in order to use a 4-wire converter a dip-switch to set ECHO to OFF is required. This is because to convert 4-wire to 2-wire the Transmit has to be looped to the Receive (in accordance with the converter manufacturer's instructions), which causes requests to be echoed back up the wire and conflict with the reply.

RS422 and RS485 allow multiple instruments to be daisy-chained together, here is an example of this:

wiring

A suggested cabling specification is Belden 9533, which is a 3-core shielded (screened) cable.

Please note that RS422/485 connections should be terminated correctly,

Particularly if the cable run is more than about 5 metres then it is likely to require a termination resistor on the end.

See the instrument manufacturer's instructions, as to whether a resistor is required to terminate the daisy-chain.

In order to prevent signal interference on the serial comms it is necessary to use cables which are shielded.

(The shield is a wire-mesh inside the outer plastic coating of the cable.)

The shield of each section of cable making up the daisy-chain needs to be connected (twisted) together and then earthed at one end - typically at the instrument end, by connecting to the outer-casing of the instrument rack or housing which should itself be correctly earthed.

The signal-grounds (GND) need to be connected all the way down the chain and NOT to Earth.

Note: GND and Earth are not the same thing: GND is 'signal ground' whereas Earth is the same as the 'Earth' pin on a 3 pin plug.

We recommend using internal COM ports and external optically isolated converters.

We strongly caution against using internal 422/485 cards instead of a converter.

The number of controllers that can be connected to SpecView is only limited by the available addresses on the controllers. Most instruments go to 99 addresses and SpecView supports up to 49 COM ports as standard and can be increased to 40 ports.

RS422 and RS485 would need repeaters if there are more than 16 or 32 instruments respectively. Please see the instrument manufacturer's instructions or the converter's manual.

RS232

Some computers have built in standard RS232 (9-pin) ports specified as:

Pins	Code	Description	Input/Output
1	DCD	Data carrier detect	Input
2	RXD	Receive data	Input
3	TXD	Transmit data	Output
4	DTR	Data terminal ready	Output
5	GRND	Ground	N/A
6	DSR	Data set ready	Input
7	RTS	Request to send	Output
8	CTS	Clear to send	Input
9	RI	Ring indicator	Input

If the intention is to use RS232 wiring to connect the instrument, then the wiring and the connection to the instrument will be specified by the instrument's manufacturer.

RS232 is point-to-point, therefore it has a maximum of a single instrument connection.

Each serial port MUST operate with its own interrupt unless special interrupt sharing cards and software are used.

Serial ports using 16550 UARTS or above are strongly recommended, especially if BAUD rates above 9600 are going to be used. They are required for instruments that use block mode data reading.

For more information also:

Instrument Settings

RS422 (EIA422-A) Instruments

RS485 (EIA485) Instruments

Ethernet and TCP/IP

Troubleshooting Instrument Communication

Connections for SpecView Networking