When RS-485 communication comes and goes, the first reaction is often to replace the gateway or replace the equipment. In field implementation, Tongwei Smart has seen that the same intermittent interruption also occurs when termination resistors, bias, and reference ground are not handled for the bus as a whole. The gateway is the access node between the equipment and the platform. It cannot replace line design. These settings should be recorded during construction and acceptance, so that later remote diagnosis has a basis.
Configure termination resistors only at the two ends of the bus
The two ends of the bus are usually fitted with termination resistors according to the cable characteristics and the equipment instructions. A common engineering value is 120 ohms, used to reduce reflection after the signal reaches the cable end. One at each end, not one added on every device.
Stacking several termination resistors in the middle and at the ends makes the equivalent load heavier, and the driver may not be able to drive it. With no termination at all, a short line at low speed can sometimes still communicate. After the distance becomes longer and the baud rate is raised, edge reflection may bring CRC errors and occasional timeouts.
After power-down, use resistance for a quick judgment
After power is removed and the line is confirmed safe, a multimeter can measure the resistance between A and B. This is only a quick judgment. It cannot replace a check against the manual. Some equipment also carries internal bias, protection, or switchable termination, and the measured resistance will depart from the typical value.
- When each end has one 120 ohm termination, the parallel result is about 60 ohms.
- Close to 120 ohms may mean only one termination is connected.
- Close to an open circuit may mean there is no termination.
- Close to 0 ohms, check for a short circuit first.
Do not enable bias repeatedly across several devices
Bias is also called pull-up and pull-down, or fail-safe. When no device on the bus is transmitting, it holds A and B at a defined idle level and reduces line drift and false triggering. Bias is usually configured at only one suitable position. It cannot be turned on at every device.
Gateways, converters, and controllers may all include their own bias. A mistake that appears easily on site is to enable the bias of several devices at the same time, so the bus load becomes too heavy and the level is pulled off. Before commissioning, first confirm which end has termination, which device has bias enabled, and whether the equipment is controlled by a switch or a jumper. Do not turn everything on just because the labels say termination, matching, or bias.
A differential signal still requires reference ground and shielding to be handled
RS-485 transmits a differential signal, but differential does not mean the reference can be ignored. When the ground-potential difference between devices is large, communication becomes unstable, and in severe cases the interface can also be damaged. Do not short protective earth, the shield, and the signal reference line at will. Whether a signal ground is needed, whether an isolated interface is used, and whether the shield is grounded at one end or at both ends should all be confirmed according to the equipment manual.
Links that cross buildings, cross power-distribution systems, or run a long distance outdoors also need surge protection, isolation, and lightning protection considered separately. If the fault appears only when a variable-frequency drive, a contactor, or high-power equipment starts, include interference rejection, routing spacing, shielding, and isolation in the same judgment.
A suggested check order
- Power down and check the topology and termination.
- Check the termination and bias switches, and confirm they have not been enabled more than once.
- Measure the resistance between A and B, and interpret the reading against the equipment instructions.
- After safety is confirmed, power up and observe communication quality.
Frequently asked questions
- Does turning the termination resistor on at every device make the bus more stable?
- No. Termination should be configured as one at each end of the bus. Adding termination again on intermediate devices increases the bus load.
- If the measured resistance is not 60 ohms, is the wiring certainly wrong?
- Not necessarily. Bias, protection, and switchable termination inside the equipment all affect the reading. 60 ohms is only a quick reference for one 120 ohm termination at each end. The manual and the switch state still have to be checked.