comfilepi:troubleshooting_rs485_communication

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====== 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 [[https://www.raspberrypi.com/documentation/computers/configuration.html#pl011-and-mini-uart|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 stable((https://www.raspberrypi.com/documentation/computers/configuration.html#mini-uart-and-cpu-core-frequency)). 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><code>GOVERNOR="performance"</code></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℃ ((https://forums.raspberrypi.com/viewtopic.php?p=1618604#p1618604)), 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 [[https://www.raspberrypi.com/documentation/computers/os.html#get_throttled|Raspberry Pi documentation]] for information on how to interpret the output. Or inspect the //"Throttled"// output from the following script:

<code bash>
#!/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}"
</code>
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