Ensuring Reliable RS-485 Communication

On the CPi-A, CPi-B, and CPi-S panel PCs, the RS-485 port utilizes the Broadcom SoC's mini-UART. The mini-UART has a number of limitations that need to be considered to ensure reliable communication. Please see the Raspberry Pi documentation for detailed information on the mini-UART.

To avoid the limitations of the mini-UART altogether, consider the CPi-C panel PCs, as their RS-485 port does not utilize the mini-UART, and is not subject to its limitations.

The mini-UART's clock is dependent on the SoC's core clock. This means that, to ensure reliable communication, the SoC's core clock frequency must be stable¹⁾. To ensure the SoC's core frequency is stable, please implement all of the following:

Ensure core_freq=250 (Bullseye) or core_freq_fixed=1 (Bookworm) is added to the /boot/config.txt file to ensure the core frequency is configured to a predictable setting.
Install cpufrequtils with sudo apt install cpufrequtils, and change the contents of /etc/default/cpufrequtils to <html>
```
GOVERNOR="performance"
```
</html>. The default is ondemand, which should actually be fine for most use cases, but performance will provide extra insurance. Reboot and confirm with cat /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor.
Ensure the core frequency does not throttle due to high temperatures. The SoC should not begin throttling unless the core temperature reaches 80℃ ²⁾, so to ensure reliable communication, the SoC's core temperature must remain lower than 80℃.

To check the current core temperature run vcgencmd measure_temp and ensure it remains below 80℃.

To check the current core frequency run vcgencmd measure_clock core and ensure it remains stable.

To check if the system has been throttled run vcgencmd get_throttled. See the Raspberry Pi documentation for information on how to interpret the output. Or inspect the “Throttled” output from the following script:

#!/bin/bash
 
#Flag Bits
UNDERVOLTED=0x1
CAPPED=0x2
THROTTLED=0x4
SOFT_TEMPLIMIT=0x8
HAS_UNDERVOLTED=0x10000
HAS_CAPPED=0x20000
HAS_THROTTLED=0x40000
HAS_SOFT_TEMPLIMIT=0x80000
 
#Text Colors
GREEN=`tput setaf 2`
RED=`tput setaf 1`
NC=`tput sgr0`
 
#Output Strings
NO="${GREEN}NO${NC}"
YES="${RED}YES${NC}"
 
#Get Status, extract hex
STATUS=$(vcgencmd get_throttled)
STATUS=${STATUS#*=}
 
echo -n "Status: "
(($STATUS!=0)) && echo "${RED}${STATUS}${NC}" || echo "${GREEN}${STATUS}${NC}"
 
echo "Undervolted:"
echo -n "   Now: "
((($STATUS&UNDERVOLTED)!=0)) && echo "${YES}" || echo "${NO}"
echo -n "   Run: "
((($STATUS&HAS_UNDERVOLTED)!=0)) && echo "${YES}" || echo "${NO}"
 
echo "Throttled:"
echo -n "   Now: "
((($STATUS&THROTTLED)!=0)) && echo "${YES}" || echo "${NO}"
echo -n "   Run: "
((($STATUS&HAS_THROTTLED)!=0)) && echo "${YES}" || echo "${NO}"
 
echo "Frequency Capped:"
echo -n "   Now: "
((($STATUS&CAPPED)!=0)) && echo "${YES}" || echo "${NO}"
echo -n "   Run: "
((($STATUS&HAS_CAPPED)!=0)) && echo "${YES}" || echo "${NO}"
 
echo "Softlimit:"
echo -n "   Now: "
((($STATUS&SOFT_TEMPLIMIT)!=0)) && echo "${YES}" || echo "${NO}"
echo -n "   Run: "
((($STATUS&HAS_SOFT_TEMPLIMIT)!=0)) && echo "${YES}" || echo "${NO}"

¹⁾

https://www.raspberrypi.com/documentation/computers/configuration.html#mini-uart-and-cpu-core-frequency

²⁾

https://forums.raspberrypi.com/viewtopic.php?p=1618604#p1618604