Interfacing Absolute Position Decoders to PLCs and Microcomputers

Microcomputers and PLCs are sequential logic devices. In contrast to a real-time hardware logic, which can perform many operations at the same time, a PLC can perform only a single operation before proceeding to the next logical step. The figure describes the logical operation of a PLC, which is cyclical in nature. During the I/O scan, the PLC looks at the input terminals and activates the outputs based on the ladder logic. During the processor scan, the new input data is processed by the Central Processing Unit (CPU) according to the ladder program and the outputs are updated during the next I/O scan. This cycle repeats again and again.

The expression “garbage in, garbage out” fits very well with the PLC. If the input data is invalid or incorrect, the corresponding machine operation will also be incorrect. Therefore, it is very important that when the PLC reads the decoder input during the I/O scan, the decoder data is valid and free of any ambiguities.

There are two main inherent characteristics of electronic devices that could cause wrong decoder data into the PLC:

a) PLC Reading the Changing Bit Pattern:

As we all know, a BCD, binary, or gray code number is composed of various bits that change state when decoder position passes from one number to the next. Inherently, in Gray Code only 1 bit changes state when changing from one number to the next, while in BCD or Binary data more than one bits may change for each number change. Let us consider the example of changing decoder position from 199° to 200°. In a BCD code, for this 1° change of position, 6 bits will change state, i.e., 100, 80, 10, 8, and 1 bits go LOW and 200 bit will go HIGH. And, due to the reaction time of electronic components, all these bits do not change state at the same time. At a given time when PLC reads the data, some bits might have gone LOW while others may still be HIGH. Therefore, while reading the above changing bit pattern the PLC is liable to read a wrong number.

b) Reaction Time of Input Modules:

PLC I/O modules, even the TTL compatible ones, have lengthy and inconsistent time delays when they change their logic state. This inconsistency gets further compounded by long wiring runs between the decoder and the PLC, and also the limited current drive capability of the decoder outputs. In the above example, when the input to the I/O modules goes from 199° to 200°, the output may stay at 000 for a time, depending on the I/O module reaction time.

An I/O scan during this time (2 to 10 ms in typical installations) will read false data to the PLC. The solid line is the field side of the I/O module and the dashed line is the PLC side. During the switching time (TS), the decoder information as seen on the PLC side is 0, which is invalid.

Even dedicated microprocessor controls with faster scan times are faced with the above two problems, though to a lesser degree. In microprocessors the TS is in microseconds (μs) and software can be designed to ignore inconsistent data. If your microprocessor does not have this software provision or if you are using a PLC, the hardware synchronization described below must be used to assure the integrity of the incoming decoder data.

PLC Synchronization (PC-Handshake):

Whenever the PLC scans the decoder input, it must see stable data. In order to ensure this, the PLC gives a data transfer command and a predetermined time later the PLC synch circuit stabilizes the data for the PLC to read. This time is adjustable on some Autotech units (2 μs to 30 ms), whereas it is fixed on others (50 μs, 100 μs, etc.). The variable time feature, when available, can be used to provide the most fresh data to the PLC.

For example, the time interval between the data transfer and read commands might be 12 ms and say the time delay is set at 5 ms. After 5 ms of the data transfer command the stable data is available to the input modules of the PLC and when the PLC commands the data to be read 12 ms later, it is stable and valid.

Microcomputer Synchronization (Microfreeze):

The Microfreeze can also be called as transparent PC-Handshake. This feature is particularly useful when interfacing data directly to a microcomputer where speed of operation is much higher. In this case the decoder position data is continuously updated at full speed. The data are frozen for 100 μs +/- 10% after a delay of 10 μs from either transition edge of data transfer command. The microcomputer can read stable data during these 100 μs and it automatically unlatches.

Software Filtering:

The problem of synchronizing BCD data to a PLC can also be addressed by software filtering. Software filtering is usually done in one of the following two ways:

A window is created around the last correct reading based on the known operating speed of the decoder. If new position is outside of this window, the data is rejected.
Three samples of position data are taken, of which two must agree before data is accepted. Either of these approaches will increase the scan time of the PLC. Since scan time is an important factor in system speed and resolution, the software approach is usually not a viable approach.
Notes:
1. The synchronization problem does not exist when using Gray Code absolute decoders because only one bit changes state at a time.
2. The synchronization process described above does not result in faster machine operation. The system resolution and permissible decoder speed will still be limited by the PLC scan time. As a rule of thumb, a PLC with 16.67 ms (One AC cycle) scan time will permit 1° resolution at 10 RPM (The rule of 10:1:1).
3. The Synchronization issues do not apply to Networkable Resolvers/Encoders because of built-in handshake.