Allen Bradley PLC-5 Classic Controller Complete Guide

Allen Bradley PLC-5 Classic Controller System Design and Maintenance Guide

Overview of the Classic PLC-5 Controller Family

The Allen Bradley 1785 series PLC-5 programmable controller is a classic platform in the field of industrial automation, with the "Classic" series including four models: PLC-5/10 (1785-LT4), PLC-5/12 (1785-LT3), PLC-5/15 (1785-LT), and PLC-5/25 (1785-LT2). Despite the popularity of enhanced, Ethernet, and ControlNet processors, there are still a large number of production lines worldwide that rely on these classic controllers for stable operation. For maintenance engineers, understanding the core characteristics, system architecture, and fault handling mechanisms of these processors is key to ensuring the continuous operation of outdated systems.

1.1 Comparison of Model and Core Parameters

Processor model, catalog number, maximum user memory, maximum I/O points, program scanning time, K-word, remote I/O port, DH+port

PLC-5/10 1785-LT4 6K word 512 dots 2ms (discrete)/8ms (typical) none (adapter only) 1

PLC-5/12 1785-LT3 6K word 512 dots 2ms/8ms 1 (adapter only) 1

PLC-5/15 1785-LT 6K can be expanded to 14K 1024 points (complementary up to 2048) 2ms/8ms 1 (scanner or adapter) 1

PLC-5/25 1785-LT2 13K can be expanded to 21K 1024 points (complementary up to 2048) 2ms/8ms 1 (scanner or adapter) 1

All classic PLC-5 processors occupy the leftmost slot of the 1771 I/O rack, support any 1771 series I/O module (up to 32 points/module), share the same programming software and instruction set, and support ladder diagram and sequential function diagram (SFC) programming.

Engineering Tip: PLC-5/10 and/12 do not have remote I/O scanner function and can only serve as adapter slaves. If you need to scan remote racks as the main station, PLC-5/15 or/25 must be selected.

1.2 Functional specification formulation before system design

Before starting hardware selection and programming, it is recommended to first complete the functional specification document. This document should include:

Control strategy (centralized or distributed)

Process flowchart and sequence of events

Environmental and safety requirements

Expected input/output conditions, start/emergency stop procedures, alarm and fault handling logic

System behavior under abnormal operating conditions

A complete functional specification is not only the basis for selection, but also the standard for final program acceptance. It is recommended to use SFC to describe the sequential control process and implement the specific logic using a ladder diagram.

Key elements of hardware selection

2.1 I/O module selection and density decision

When selecting an I/O module, it is necessary to comprehensively consider the on-site signal type, voltage range, isolation requirements, noise tolerance, and backplane current consumption. For digital modules, density selection follows the following principles:

8-point module: suitable for traditional replacement or occasions that require independent fuse output

16 point module: Balancing cost and space, requiring the use of special wiring arms to achieve independent melting

32 point module: Minimize the number of modules and rack space to the greatest extent possible, with the lowest cost per point

The analog module needs to pay attention to voltage/current range, resolution, and single ended/differential input. Special functional modules (such as encoders, ASCII, weighing, barcode readers) should be selected according to specific applications.

Important: Some main modules (such as 1771-M1 stepper controller, 1771-M3 servo controller) cannot share the backplane with other main modules in the same rack, so each rack can only install a maximum of two such main modules, and must comply with the compatibility combination in Table 2. C of the manual.

2.2 Power selection and backplane current calculation

The power module must meet the total backplane current requirements of all modules within the rack. The calculation formula is as follows:

Summarize the backplane current of all I/O modules (column A)

Add 3.3A (maximum) for the PLC-5 processor itself or 1.2A (column B) for the adapter module

Reserve current for future expansion modules (column C)

After obtaining the total current, refer to Table 2. K (processor rack) or Table 2. L (remote I/O rack) in the manual to select the appropriate power module. Common power sources include:

1771-P4S (120V AC, 8A output)

1771-P6S (220V AC, 8A output)

1771-P7 (120/220V AC, 16A output, external installation)

Important note: The same rack cannot be powered by both external power and slot power modules simultaneously, as they are incompatible. In addition, if there is a power module installed in the rack, the rack configuration plug must be correctly set (located between the two leftmost slots), with the default position being "N" (indicating that the power module is not in use).

2.3 Memory module and battery selection

The classic PLC-5 processor supports EEPROM non-volatile storage and CMOS RAM expansion:

EEPROM: 8K characters (1785-MJ) suitable for all models; 16K characters (1785-MK) are only applicable to PLC-5/25

CMOS RAM: 4K words (1785-MR) or 8K words (1785-MS) suitable for PLC-5/15 and/25

The battery uses 1770-XY (AA size lithium thionyl chloride, Tadiran TL5104）。 At 60 ° C and 100% power outage time, the average lifespan is about 329 days; Up to 2 years at 25 ° C. If the system is powered off for a long time and memory needs to be maintained, it is recommended to regularly replace the battery or use EEPROM to store programs.

2.4 Complementary I/O Configuration

Complementary I/O is a unique feature of PLC-5/15 and/25: it assigns the I/O group address of one rack to another physical rack, and the modules in both racks perform opposite functions (input and output pairing). For example, the input module of the main rack corresponds to the output module of the complementary rack, and both use the same input/output mapping table. This configuration can be used to increase the number of I/O points (up to double) or achieve status display, but it is not recommended for redundant control.

When configuring complementary I/O, it is necessary to follow:

Cannot pair input module with input module (they will compete for the same input image bit)

The output module can be paired with the output module (simultaneously controlling actuators and indicators)

The local rack of the processor does not support complementary I/O

The combination of 32 point module and 1 slot addressing, 16 point module and 2 slot addressing cannot use complementary I/O

Detailed explanation and selection strategy of addressing mode

The classic PLC-5 processor supports three addressing modes, each of which determines the relationship between the slots in the I/O rack and the I/O mapping table.

3.1 2-Slot Addressing

Each I/O group corresponds to two physical slots, and each group is assigned a 16 bit input word and a 16 bit output word. Suitable for 8-point modules, but cannot use 32-bit modules. If using 16 point modules, they must be installed in pairs (input modules in even slots, output modules in odd slots), otherwise a slot must be left vacant. The 8-point module can be mixed and placed arbitrarily.

3.2 1-Slot Addressing

Each physical slot corresponds to an I/O group (one word input, one word output). You can mix 8-point and 16 point modules freely. The 32 point module must be installed in pairs in adjacent even/odd slots (slot 0 and slot 1). If not paired, one of the slots must be vacant.

3.3 Half Slot Addressing (1/2-Slot Addressing)

Each physical slot corresponds to two I/O groups (two word inputs, two word outputs), and each 32 point module occupies one word. The 8-point and 16 point modules can also be used, but it will result in a higher waste of I/O points. This mode does not support forced access to the high-order words corresponding to empty slots or low-density modules.

3.4 Rack Number Allocation Rules

Regardless of the addressing mode, an I/O rack is always composed of 8 I/O groups. The relationship between rack and chassis size is shown in the following table:

Number of chassis slots: 2 slots for addressing, 1 slot for addressing, half slot for addressing

4 slots, 1/4 rack, 1/2 rack, 1 rack

8 slots 1/2 rack 1 rack 2 racks

12 slots 3/4 racks 1.5 racks 3 racks

16 slots, 1 rack, 2 racks, 4 racks

The default rack number for the local rack of the processor is 0, which can be changed to 1 through bit 2 of the processor status word S: 26. The remote I/O rack number cannot conflict with the extended local rack number. For example, if an 8-slot expansion local rack is configured as group 0-3 of rack 2, the remote I/O rack cannot be configured as group 4-7 of rack 2.

Communication system configuration

The classic PLC-5 processor supports two main networks: DH+(Data Highway Plus) and remote I/O.

4.1 DH+Network

DH+uses token passing protocol with a transmission rate of 57.6kbps and can connect up to 64 sites. Each site can send a maximum of 271 bytes of data during the token holding period. DH+links can use daisy chain or trunk/branch topology. The maximum length of mainline cables is 3048 meters (10000 feet), and the maximum length of branch lines is 30.5 meters (100 feet).

Performance influencing factors:

Number of nodes: Each non message node occupies approximately 1.5ms of token holding time, while message nodes occupy up to 38ms

Message size and quantity: Messages exceeding the single packet limit (271 bytes) require multiple token polling

Target node location: Response time is affected by the token passing order (see manual figure 5.3 and 5.4 for comparison)

Internal processing load of the processor

If programming is required on the DH+network, it is recommended to set up a dedicated programming DH+link separately to avoid affecting the real-time performance of the process control network. In addition, do not add or remove nodes online or perform online programming during production operation, as it may cause token loss and network reconstruction (which may take several seconds).

4.2 Remote I/O Link

The remote I/O link uses Belden 9463 dual axis cable (1770-CD), supporting 57.6kbps (up to 3048 meters), 115.2kbps (1524 meters), and 230.4kbps (762 meters). All devices must use the same baud rate.

Scanner mode vs. adapter mode:

Scanner mode: PLC-5/15 or/25 serves as a remote I/O master station, scanning I/O data in remote racks and updating it synchronously with local I/O. Each remote I/O rack transmits one data block per scanning cycle.

Adapter mode: PLC-5/12,/15,/25 can serve as slave stations, accepting remote I/O scans from upper level controllers (such as PLC-5/250 or PLC-3) and processing their local I/O. The adapter mode processor and the upper computer can exchange discrete data of 4 or 8 words, or exchange up to 64 words of data through block transfer.

Block transmission: used for data exchange between analog modules or intelligent modules. In adapter mode, the BTW of the adapter processor must respond to the BTR of the upper computer (and vice versa). In scanner mode, block transfer requests are queued, and each remote I/O scanning cycle completes a maximum of one block transfer per rack address. If the program scanning time is much longer than the remote I/O scanning cycle, multiple block transfers may be completed within each program scan.

Engineering Warning:

If the adapter mode processor uses half slot addressing and is located in a 16 slot rack, block transfer functionality cannot be used - because the I/O image of rack 3 is occupied by local I/O and cannot be used for block transfer communication.

At the end device of the remote I/O link, switch 1 of SW3 must be set to ON (terminal resistance). If there are certain special devices on the link (such as some frequency converter adapters), a 150 Ω terminal resistor may need to be used instead of the standard 82 Ω, but this will limit the number of link devices to 16 and reduce the maximum baud rate to 115.2kbps.

Program development and troubleshooting

5.1 Application of Sequential Function Diagram (SFC)

SFC is a powerful language for describing sequential control processes. Each step corresponds to a control task (associated with a program file), and each transition condition corresponds to a logical judgment. For processes with multiple branches, parallel paths, or complex state machines, SFC is more intuitive than pure ladder diagrams.

SFC Structure Selection Guide:

Independent machine status → Steps+Transition

Single sequence event chain → Simple path

Multi channel selection branch → Select branch (scan from left to right, execute the first true condition)

Parallel path → Simultaneous branching (up to 7 parallel paths)

SFC programming precautions: The chart can be reset using GOTO jumps, global subroutines, macros (compression steps), and SFR instructions. A processor can only have one main program file (which can be SFC or ladder diagram).

5.2 Data Table File Structure

The PLC-5 memory is divided into a data area and a program area. The data table file includes:

Output image file (O0) - fixed size (PLC-5/10/12/15 is 32 words,/25 is 32-64 words)

Input image file (I1) - Same as above

Status File (S2) - Fixed 32 Word

Binary file (B3) - maximum 1000 words

Timer (T4), counter (C5), control structure (R6), integer (N7) - default file number, up to 1000 structures

Floating point number (F8) - up to 1000 floating point words

ASCII (A) and BCD (D) files - file number 3-999

The address format supports logical addresses (such as N23:0), I/O mapping addresses (I: 017/17), indirect addresses (N [N7:6]: 0), index addresses (# N23:0, offset taken from S: 14), and symbolic addresses (programming software functionality).

5.3 Fault Handling Mechanism

When the PLC-5 processor detects a major fault, it will immediately interrupt the current program. If a fault program has been configured (file number specified in S: 29), the processor will execute the fault program once. If the fault program clears the fault (by resetting the fault bit in S: 11), the processor returns to the original program to continue execution; Otherwise, the processor enters a fault mode.

Main fault code example:

12-19: Instruction operand type error, missing operand

20-23: Indirect address element number too large/negative/accessing undefined file

30-32: Subroutine nesting too deep or invalid jump

34-36: Timer/counter negative value, PID set value exceeding limit

74-79: SFC file error, too many activation steps, file type error

Power on protection: Setting bit 1 of S: 26 can prevent the processor from running directly after power failure recovery, but instead scan the faulty program first, and let the faulty program decide whether to allow startup. If the fault program does not clear bit 5 of S: 11, the processor will enter fault mode after the fault program ends.

Remote rack fault recovery:

When a remote I/O rack fails, the processor sets global fault bits (the lower 8 bits of S: 7, S: 32, S: 34) and partial rack status words (integer files specified by S: 16). There are three types of recovery strategies:

User generated main fault: When a fault position is detected, execute JSR to jump to the fault program and execute orderly shutdown

Reset input image table: Use the rack disable bit to force the input image bit to a safe value

Fault area programming: Use MCR area to power off non holding outputs to prevent misoperation

Performance optimization and scan time calculation

6.1 Composition of System Throughput

Throughput refers to the total time it takes for the input signal to change to the corresponding output action, consisting of the following components:

Input module delay (depending on module type, typically 20ms)

I/O backplane transmission time (approximately 1-2ms/full rack)

Remote I/O scan time

Processor program scan time

Output module delay (typical 8.8ms)

6.2 Optimization of Remote I/O Scanning Time

Remote I/O scanning time=(number of racks) × (scanning time per rack @ baud rate)+additional overhead for block transmission

Scanning time per rack: approximately 10ms at 57.6kbps, approximately 2.5ms at 115.2kbps, and approximately 0.7ms at 230.4kbps.

Additional cost of block transmission=(word count x milliseconds per word)+fixed cost. For example, transmitting 10 words at 115.2kbps requires 10 × 0.14+2.5=3.9ms.

Optimization suggestions:

Allocate time critical I/O to separate channels (PLC-5/15 and/25 have two remote I/O channels? Actually, there is only one remote I/O port, but it can be expanded through the 1785-BCM backup module? The manual specifies that PLC-5/25 only has one remote I/O port. But it can be optimized by separating local, extended local, and remote racks

Use a configurable scan list to exclude or reduce scanning frequency for racks that do not require quick updates

Increase baud rate (requires all devices to support and cable length to meet requirements)

Place modules that do not require frequent block transfers locally or expand local racks

6.3 Estimation of Instruction Execution Time

The execution time of different types of instructions varies significantly. For example:

False condition XIC instruction: approximately 1.4 μ s

MVM instruction with true conditions (shielded transmission): approximately 258 μ s

FAL file arithmetic instruction: 98+W × (42.5+N) μ s, where W is the number of processed elements

Indirect addressing and file addressing will increase additional time (approximately 0.8 μ s/address over 256 words, as well as additional overhead for indirect file or element numbers). Using program constants (integers or floating points) is faster than using data table addresses, but modifications require editing the program.

Typical troubleshooting cases

Case 1: Processor PROC LED flashes red

Possible reasons: Invalid memory or EEPROM loading failure. Check if the EEPROM is installed correctly and if bits 13/14 of S: 1 are set correctly (it is recommended to set "EEPROM transfer at power up" to bits 13=1 and 14=1). If it still cannot be restored, try clearing the memory and downloading the program again.

Case 2: All outputs of the remote I/O rack remain in their final state

Check the switch 1 setting of the adapter module (1771-ASB). If switch 1 is turned on, the output will remain in its final state in case of communication failure; If power-off safety is required, it should be set to OFF. At the same time, check whether the fault handling program detects the fault through the global fault bit and performs a safe shutdown.

Case 3: Block transmission always times out or reports errors

Confirm that the rack and group number settings of the block transmission module match the parameters in the programming instructions

For PLC-5/15 or/25 in adapter mode, check if half slot addressing is used and located in a 16 slot rack - block transfer cannot be used at this time, and an adapter image file must be created as a replacement

Check if the terminal resistance of the remote I/O link is correct

In scanner mode, please note that each remote I/O scanning cycle can only perform one block transfer per rack. If there are multiple BT instructions sent to the same rack in the program, they will queue up

Case 4: DH+communication response is extremely slow

Check if there are offline nodes in the network - token loss may cause network reconstruction. Use programming software to view the activity node table (S: 3-s: 6 characters). If there are a large number of PLC-5 processors in the network and each one frequently sends large blocks of messages, consider moving the programming terminal to an independent DH+link or reducing the frequency of sending non essential messages (such as using timed triggering instead of continuous triggering).

System upgrade and replacement considerations

When the classic PLC-5 controller faces a shortage of spare parts or insufficient performance, the following solutions can be considered:

If only the processor needs to be replaced: same series upgrades (such as upgrading from/10 to/25) usually only require replacing the processor module, reloading the program memory, and basic compatibility of I/O configurations

If you need to keep the 1771 I/O rack: you can choose to use the ControlLogix platform with the 1771-ASB adapter, or use the 1756-DHRIO module bridge

If complete system modernization is required: migrate to ControlLogix or CompactLogix platforms, utilize PLC-5 program conversion tools, and retest I/O timing

However, regardless of the solution, the hardware configuration, addressing mode, special function module settings, and fault handling logic of the existing system should be recorded in detail before conversion to ensure seamless replacement of the new system.