Serial USB-RS485 failing to read on Pi but not on Ubuntu laptop?

Question

I am reading values from a Battery Management System using two, serial USB-RS485 adaptors (ftdi). I am doing this over MODBUS using the minimal modbus library and python:

# Bank A
# port name, slave address (in decimal)

instrument_A = minimalmodbus.Instrument('/dev/ttyUSB0', 1)

# Bank B
instrument_B = minimalmodbus.Instrument('/dev/ttyUSB1', 1)
# Port on which to send via serial to CAN Shield Arduino
port = '/dev/ttyACM0'
# Initialize CAN Shield Arduino Serial Line
ard = serial.Serial(port, 115200, timeout=5)
// Read something from BMS
socA = instrument_A.read_register(40008)

See picture below

I am reading these values repeatedly, and then sending them over another serial/UART to an arduino which is then controlling some things..

Now this all works absolutely fine on my laptop, which runs ubuntu - but it only works on the Pi for a few reads, after which it fails.

Is the raspberry pi not capable of doing this many serial concurrent processes at once?

Edit:

I have now included the whole python file, as it has been pointed out that the question was not as clear in terms of what is running concurrently etc.

#!/usr/bin/python
# -*- coding: utf-8 -*-

import minimalmodbus
import serial
import syslog
import time
import math

# Some constants which are decided by us, and are sent to the inverter - not coming from the BMS
CONST_BAT_CHARGE_VOLTAGE    = 585  # 58,5V
CONST_BAT_DISCHARGE_VOLTAGE = 44 * 10
CONST_BAT_CHRGE_CURR_LIM = 50*10   # Battery Rack DC Charge Current Limitation, integer
VAR_BAT_DCHRG_CURR_LIM = 200*10   # Battery Rack DC Discharge Current Limitation, integer



# Looping constants

running = 1
counter = 0

# Some constants which are decided by us, and are sent to the inverter - not coming from the BMS
# Voltage at which battery need be charged. 58,5V

CONST_BAT_CHARGE_VOLTAGE = 585

# Voltage at which the battery stops discharging

CONST_BAT_DISCHARGE_VOLTAGE = 44 * 10

# Battery Rack DC Charge Current Limitation, integer

VAR_BAT_CHRGE_CURR_LIM = 0 * 10

# Battery Rack DC Discharge Current Limitation, integer

CONST_BAT_DCHRG_CURR_LIM = 200 * 10
CONST_BAT_RCK_AVG_SOC = 75  # Battery Rack, average module SOC Value, integer
CONST_BAT_RCK_AVG_SOH = 98  # Battery Rack, average module SOH Value, integer

# Bank A
# port name, slave address (in decimal)

# instrument_A = minimalmodbus.Instrument('/dev/ttyUSB0', 1)
instrument_A = minimalmodbus.Instrument('/dev/cu.lpss-serial1', 1)
# Bank B

instrument_B = minimalmodbus.Instrument('/dev/cu.lpss-serial2', 1)
# Port on which to send via serial to CAN Shield Arduino

# port = '/dev/ttyACM0'
port = '/dev/cu.usbmodem14421'
# Initialize CAN Shield Arduino Serial Line

ard = serial.Serial(port, 115200, timeout=5)


################### FUNCTIONS ###################
# Convert integer to binary string

def int2bin(i):
    if i == 0:
        return '0'
    s = ''
    while i:
        if i & 1 == 1:
            s = '1' + s
        else:
            s = '0' + s
        i /= 2
    return s


# Rounds up to the the ceiling

def ceiling(x):
    n = int(x)
    return (n if n - 1 < x <= n else n + 1)


# To determine the number of bits within a type of object

def bitLen(int_type):
    length = 0
    while int_type:
        int_type >>= 1
        length += 1
    return length


# Convert an integer into the parts of its most and least significant bits

def toHex(x):
    LSB = x & 0x00FF
    MSB = x >> 8
    return (LSB, MSB)


# Get battery data

def getBatteryData(InverterAddress):

    # Initializing send buffer, this buffer is populated with integers - ready for transformation into a byte array

    sendBuffer = []
    if InverterAddress == 0x351:
        sendBuffer.append(1)
        sendBuffer.append(CONST_BAT_CHARGE_VOLTAGE)
        sendBuffer.append(VAR_BAT_CHRGE_CURR_LIM)
        sendBuffer.append(CONST_BAT_DCHRG_CURR_LIM)
        sendBuffer.append(CONST_BAT_DISCHARGE_VOLTAGE)
    elif InverterAddress == 0x355:
        sendBuffer.append(5)

        # Battery Rack A, Average module SOC Value, integer

        socA = instrument_A.read_register(40008)
        socB = instrument_B.read_register(40008)
        soc = ceiling((socA + socB) / 2 / 10)
        sendBuffer.append(soc)
        # print "SOC For Bank A"
        # print socA
        # print "SOC For Bank B"
        # print socB
        # Battery Rack, average module SOH Value, integer
        # print "Average SOC Value, both banks"
        # print soc
        sohA = instrument_A.read_register(40009)
        sohB = instrument_B.read_register(40009)
        soh = ceiling((sohA + sohB) / 2 / 10)
        # print "Average SOH Value, both banks"
        # print soh
        sendBuffer.append(soh)
    elif InverterAddress == 0x35A:
        sendBuffer.append(0x65)
        sendBuffer.append(0)
        sendBuffer.append(0)
    else:
        sendBuffer.append(0)

    byteSendBuffer = bytearray()
    for i in sendBuffer:
        (LSB, MSB) = toHex(i)
        byteSendBuffer.append(LSB)
        byteSendBuffer.append(MSB)

    # print "The following is the byte array to be sent to the Inverter"

    for byte in byteSendBuffer:
        print byte
    return byteSendBuffer


while running:
    print 'Starting read and serial write cycle'

    time.sleep(0.3)
    ard.write(getBatteryData(0x351))
    ard.write(getBatteryData(0x355))
    ard.write(getBatteryData(0x35A))
    # try:
    #     print "Attempting read and send..."
    #     ard.write(getBatteryData(0x351))
    #     ard.write(getBatteryData(0x355))
    #     ard.write(getBatteryData(0x35A))
    # except:
    #     print "Failure..."
    #     pass

    time.sleep(0.3)

    print 'Ending read and serial write cycle'

Answer 1

Ended up solving it by setting:

minimalmodbus.CLOSE_PORT_AFTER_EACH_CALL = True

Found this in the minimalmodbus API docs