Bradley Smoker PLC-5 user manual Application Guidelines, Number of Controllers

Communicating with Devices on a DH+ Link 9-17

The following figure shows the effect of a personal computer on message response time under various configurations.

	40%
	35%	X	X
	30%	+		X	X
Effect			+			X
	25%						X
				+	+
on
	20%					+			X
Response							+
									+
Time	15%											X
(%)	10%											+				X
																+							X
	5%																						+
	0%
		1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18	19	20	21	22

Number of Controllers

X50 W 100 W + 250 W 500 W

W=Words

Number of PLC-5 Processors

Application Guidelines

Consider the following application guidelines when configuring a DH+ link for your system.

•Minimize the number of DH+ nodes to achieve acceptable response times. Keep in mind the size and frequency of messages exchanged between devices.

•Limit the number of nodes on your network when you are trying to achieve fastest control response time. You can establish separate DH+ networks to bring-on additional stations. Use a bridge to connect the DH+ links.

•When you connect a computer to the link for operator interface or a third-party serial device to the DH+ link, select the fastest possible serial interface communication rate.

•Do not add or remove nodes from the network during machine or process operation. If the network token resides with a device that is removed, the token may be lost to the rest of the network. The network is automatically re-established, but it could take several seconds. Control would be unreliable or interrupted during this time.

•A DH+ link has a 90 s timeout period; however, you can include watchdog timers in logic programs for DH+ transfer of data (to provide an orderly shutdown if failure occurs).

•When possible, do not program controllers online during machine or process operation. This could result in long bursts of DH+ activity that could increase response time. See chapter 9 for more information.

•When possible, add a separate DH+ link for programming controllers to keep effects of the personal computer from the process DH+ link.

Publication 1785-UM012D-EN-P - July 2005