Connecting older PLCs to industrial networks


Thread Starter

Colin T. Marsh

I would appreciate feedback to resolve a technical argument on older classic PLCs vs current PLCs / software PC/PLCs.

I believe early Allen-Bradley PLCs, such as PLC2s, required a 1771-KF(?) interface module which connected the PLC's serial (programming?) port to the original Data Highway network. This
interface module apparently "polled" the PLC, read back status data (containing information generated by PLC ladder logic), and from this data was able to determine if the PLC had a message to send. If so, the interface module then generated a read command to the PLC, got the message data, reformatted it and passed it on to the network.

I believe that when current classic A-B PLC5s (for example) or even PC/PLCs want to communicate, then under ladder logic program
control they can simply send a (DF1) message out of their serial ports, i.e. without having to wait to be polled by an external device. If required an interface module (such as a 1770-KF2) would connect the PLCs serial port to a DH+ network.

Q1: Why did the older A-B PLCs have to be "polled"?
Q2: Is the current PLC (PC/PLC) method an "interrupt" operation?
Q3: Are "polling" and "interrupting" equivalent?
Q4: Which method came first and why?
Q5: What are the significant advantages / disadvantages / differences between one method and the other?
Q6: When is one method preferable to another?.


Anthony de la Rosa

classic PLC5's did not have serial ports, only DH+ ports so the only way to connect is via a number of DH capable adapters. This includes the 1784-KT card, PCMK card and the old 1771-KF2 serial to DH box.