“SDO is responsible for telling the drive how to work, while PDO is responsible for keeping the drive working continuously.”—— Essence of understanding from a bald engineer

If you have understood SDO, then congratulations, you are just one step away from “making the motor really move” — PDO.

PDO is the soul of the entire CANopen control. The speed, position, and current commands you write must be implemented through PDO for real-time control.

This article will thoroughly explain:

What is PDO?
What is the difference between TPDO and RPDO?
What does mapping actually map?
Why do some drives “not respond when speed is written”?
What is the impact of SYNC on PDO?
How to configure a usable PDO?
How to achieve multi-axis synchronization for AGV/robot arms?

After reading, you will fully master PDO.

🟧 1. What is PDO? Explained in one sentence

PDO (Process Data Object) is used for high-speed real-time transmission of “process data”.

It is specifically designed for “real-time control”:

✔ Speed command✔ Position command✔ Current command✔ Actual speed feedback✔ Actual position feedback✔ Status word✔ Control word

These are all “process data” and must use PDO.

🟧 2. There are two types of PDO: TPDO (Upload) vs RPDO (Download)

Type	Direction	Function
TPDO	Slave → Master	Feedback: Position, Speed, Status word
RPDO	Master → Slave	Commands: Control word, Speed, Position

The COB-ID for these two is:

TPDO1: 0x180 + NodeID
RPDO1: 0x200 + NodeID

For example: Node 8

TPDO1 = 0x188
RPDO1 = 0x208

These are the two you will most commonly encounter when capturing messages.

🟧 3. The core concept of PDO: Mapping

Many people get stuck on Mapping, but it can be explained in one sentence:

Mapping is telling the drive: In these 8 bytes of PDO, which objects do you want to place? In what order?

For example, if you want:

RPDO1 Byte 1~Byte 2: 6040 (Control word) RPDO1 Byte 3~Byte 6: 60FF (Speed)

Then:

6040 maps to 16bit
60FF maps to 32bit → Total 48bit (6 bytes)

Leaving 2 bytes empty.

Mapping is about these.

🟧 4. Steps to configure PDO mapping (Standard Process)

This is the most common pitfall with PDO,you must first disable → map → then enable.

Taking RPDO1 as an example:

✔ Step 1: Disable PDO

0x1400 sub1 = 0

✔ Step 2: Clear mapping

0x1600 sub0 = 0

✔ Step 3: Write mapping object list

For example:

6040: 16 bit → Write 0x60400010
60FF: 32 bit → Write 0x60FF0020

0x1600 sub1 = 0x60400010 0x1600 sub2 = 0x60FF0020

✔ Step 4: Write mapping count (sub0)

0x1600 sub0 = 2    # 2 mapping objects

✔ Step 5: Re-enable PDO

0x1400 sub1 = COB-ID (e.g., 0x200+NodeID)

This process must be memorized,

90% of PDO configuration failures are due to incorrect order.

🟧 5. PDO Transmission Types

There are 3 transmission modes for PDO:

✔ 1) Synchronous (requires SYNC)

Transmission Type = 1~240

The master sends SYNC (0x80) at fixed intervals:

For example, every 10ms:

SYNC → RPDO → TPDO → Synchronous occurrence

AGV dual-wheel and multi-axis robotic arms must use synchronous mode.

✔ 2) Asynchronous (event-triggered)

Transmission Type = 255

Send immediately when data changes:

TPDO: Speed change, Position change
RPDO: Execute immediately after the drive receives it

Suitable for: Single-axis or asynchronous scenarios.

✔ 3) Periodic Trigger (event count)

Configure event count, for example:

Transmission Type = 5 → Trigger PDO only after receiving 5 SYNCs.

Suitable for: Saving bandwidth, low-frequency status updates.

🟧 6. Impact of SYNC on PDO (Key)

If your drive is in synchronous mode:

No SYNC → The motor will not move
SYNC cycle too slow → Motor action is jerky
Unstable SYNC → Jitter, delay, desynchronization

Common issues with AGV dual wheels:

“The angles of the left and right wheels do not match”
“Inconsistent speed leads to deviation”

90% of the time it is because:

SYNC is not using hard timing
SYNC DP is missing, delayed, or jittery
PDO configuration is asymmetric

🟧 7. Practical: Configuring a usable RPDO (Speed Control)

Objective: The master writes to Node 8 via RPDO1:

Control word (6040)
Target speed (60FF)

✔ Configuration Example (SDO Writing)

# Disable RPDO1 node.sdo[0x1400][1].raw = 0 # Clear Mapping node.sdo[0x1600][0].raw = 0 # Map 6040 (16bit) node.sdo[0x1600][1].raw = 0x60400010 # Map 60FF (32bit) node.sdo[0x1600][2].raw = 0x60FF0020 # Write sub0 = 2 node.sdo[0x1600][0].raw = 2 # Enable RPDO1 (COB-ID) node.sdo[0x1400][1].raw = 0x200 + 0x08

# Send speed command (PDO) pdo = node.rpdo[0] pdo['Controlword'].raw = 0x0F pdo['Target velocity'].raw = 1000 pdo.transmit() → The motor immediately starts rotating.

🟧 8. Practical: Configuring TPDO (Speed and Status Word Feedback)

Typical mapping:

6041: 16bit (Status word)
606C: 32bit (Actual speed)

Similarly for Mapping.

🟧 9. How to achieve AGV / Multi-axis synchronization?

The master periodically sends SYNC (high-precision timer)
All drives are configured for synchronous mode
All RPDO / TPDO mappings are consistent
All motors update commands at the same SYNC moment
Read TPDO feedback for real-time synchronization status

The control of AGV’s steering wheel uses this principle.

🟧 10. Engineer’s Pitfall Summary (Very Important)

❌ 1) Mapping without disabling PDO

→ Guaranteed failure

❌ 2) sub0 not cleared to 0 first

→ Also guaranteed failure

❌ 3) Asymmetric mapping of RPDO and TPDO

→ Different angles during synchronization

❌ 4) Unstable SYNC

→ Severe deviation, jitter

❌ 5) PDO length exceeds 8 bytes

→ Directly reports 0604 0042

❌ 6) Incorrect object mapping (e.g., mapping 6041 into RPDO)

→ The motor will directly alarm

🟦 Next Article Preview (Part 7)

How to correctly configure PDO (Disable → Clear sub0 → Write mapping → Write sub0 → Enable)