Internal and Real I/O Assignment of PLCs

It is an important step in the implementation and programming organization stages. The I/O assignment tables documents and organizes all of the work done so far. It shows which PLC inputs are connected and which PLC outputs drive what output devices. Here is also where internals are assigned, including counters and timers.
These are the actual coil and contact representations used in the ladder program. If electromechanical relay diagrams exist (e.g. modernization of a process or machine), it is possible to identify real I/O by simply circling the devices, and then assigning I/O addresses (see example 1).
Table 2 contains an I/O assignment table for real inputs or outputs. Table 3 provides an I/O assignment table to internals. These assignments can be extracted directly from logic gate diagrams and ladder symbols used to describe logic sequences. These assignments can also be extracted from the electromechanical diagram’s circled elements. The PLC model that was used determines the numbers used to address I/O. These addresses can be represented as octal or decimal. The table’s description section lists the field devices that correspond with each address.
I/O address assignment table for real inputs and outputs

Table 2: I/O address assignment table for real inputs and outputs.

I/O address assignment table for internal outputs

Table 3: I/O address assignment table for internal outputs.

The input/output connection diagram should be closely followed by the table of address assignments (see Figure 6). Although I/O representations can vary between users due to industry standards, inputs and outgoings are usually represented using squares or diamonds. Part of the documentation package is the I/O connection diagram.
Partial connection diagram for the I/O address assignment in Table 2

Figure 6. Partial connection diagram for the I/O address assignment in Table 2.

During I/O assignments, users should group the inputs and outputs. This will enable simultaneous monitoring and manipulation of multiple I/O. If 16 motors are to be started in a sequence, they must be grouped together. This will allow monitoring of the I/O registers associated the 16 grouped I/O point will reveal their starting sequence. Because I/O systems are modular, it is important that all inputs and outputs be assigned simultaneously. This will prevent you from assigning an input address to an outgoing module, and vice versa.

Sample Problem 1

Study the circuit shown in Figure 7. (a) Identify the real inputs and outputs by circling each, (b) Assign the I/O addresses, (c) Assign the internal addresses (if required), and then (d) draw the I/O connection diagram.
Electromechanical relay circuit

Figure 7: Electromechanical relay circuit.

Assume that the PLC used has a modularity of 8 points per module. Each rack contains 8 modules slots. The master rack is number 0. As long as the right module is used, inputs and outputs can be assigned any address. The PLC determines if an input module or an output module are connected in a slot. Internals begin at address 10008.

Solution to Sample Problem 1

(a) Figure 8 Shows the circled input and output connections. It is important to note that the temperature switch TS3 has been circled twice, even though it is only one device. Only one of the devices is mentioned in the address assignment. Also, only one device is wired to an input unit.
(b) Table 4 illustrates how inputs and outputs are assigned. It assigns all inputs as well as all outputs. There are no I/O locations left for future use.
Identification of real I/O (circled)

Figure 8: Identification of real I/O (circled).

I/O address assignment

Table 4: I/O address assignment.

(c) Table 5 presents the output assignments and a description for each internal. It is important to note that the control relay CR2 does not count as an internal as it is identical with the output rung for PL1. Every contact associated with CR2 is replaced by 020 (the address for PL1) when the control program has been implemented.
Internal output assignment

Table 5: Internal output assignment.

(d) Figure 9 shows the I/O connection diagram of the circuit in Figure 7. This diagram is based upon the I/O assignment in part (b). Only one of the temperature switches (the normally open TS3 switch) is a connected input. Each switch’s logic programming should be based upon a normally open condition.
I/O connection diagram

Figure 9: I/O connection diagram.

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