P61

Select P61 to configure Digital Communications Address.

On a network of instruments the address is used to specify a particular instrument. Each instrument on a network should be set to a unique address from 1 to 254.

P62

Select P62 to configure Digital Communications Baud Rate.

The baud rate of a communications network specifies the speed at which data is transferred between the instrument and the master. As a rule, the baud rate should be set as high as possible to allow maximum throughput. This will depend to some extent on the installation and the amount of electrical noise the communications link is subject to, but the instruments are capable of reliably operating at 19,200 baud under normal circumstances and assuming correct line termination.

Although the baud rate is an important factor, when calculating the speed of communications in a system it is often the 'latency' between a message being sent and a reply being started that dominates the speed of the network. This is the amount of time the instrument requires on receiving a request before being able to reply.

For example, if a message consists of 10 characters (transmitted in 10msec at 9600 Baud) and the reply consists of 10 characters, then the transmission time would be 20 msec. However, if the latency is 20msec, then the transmission time has become 40msec. Latency is typically higher for commands that write to a parameter than those that read, and will vary to some degree depending on what operation is being performed by the instrument at the time the request is received and the number of variables included in a block read or write. As a rule, latency for single value operations will be between 5 and 20 msec, meaning a turnaround time of about 25-40msec. This compares very favourably with competing devices, which can often take as much as 200msec to turn around communications transactions.

If throughput is a problem, consider replacing single parameter transactions with Modbus block transactions, and increase the baud rate to the maximum reliable value in the installation.

1200

1200 bps

2400

2400 bps

P63

Select P63 to configure Digital Communications Parity.

Parity is a method of ensuring that the data transferred between devices has not been corrupted. Parity is the lowest form of integrity in the message, it ensures that a single byte contains either an even or an odd number of ones or zeros in the data. In industrial protocols, there are usually layers of checking to ensure that first the byte transmitted is good and then that the message transmitted is good. Modbus applies a CRC (Cyclic Redundancy Check) to the data to ensure that the packet of data is not corrupted. Thus, there is usually no benefit in using odd or even parity, and since this also increases the number of binary bits transmitted for any messages, it decreases throughput.

nonE

No parity (factory default)