Modbus-TCP frequently times out

[MarkRivers]

When the ISCO 65D pump is polled at 10 Hz with 4 Modbus port drivers (1 for coils and 3 for registers) it is timing out every few minutes. This is typical output.

2021/02/26 15:16:07.766 drvModbusAsyn::doModbusIO port SP1_Reg_In_1 error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:16:07.846 drvModbusAsyn::doModbusIO port SP1_Reg_In_3 error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:16:07.876 drvModbusAsyn::doModbusIO port SP1_Bit_In error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:16:15.795 drvModbusAsyn::doModbusIO port SP1_Reg_In_2 error calling writeRead, error=gse-isco1:502 timeout: Resource temporarily unavailable, nwrite=6/6, nread=0
2021/02/26 15:17:15.968 drvModbusAsyn::doModbusIO port SP1_Reg_In_3 writeRead status back to normal having had 20 errors, nwrite=6/6, nread=94
2021/02/26 15:17:16.008 drvModbusAsyn::doModbusIO port SP1_Bit_In writeRead status back to normal having had 20 errors, nwrite=6/6, nread=20
2021/02/26 15:17:16.348 drvModbusAsyn::doModbusIO port SP1_Reg_In_1 writeRead status back to normal having had 17 errors, nwrite=6/6, nread=202
2021/02/26 15:17:17.168 drvModbusAsyn::doModbusIO port SP1_Reg_In_2 writeRead status back to normal having had 15 errors, nwrite=6/6, nread=126
2021/02/26 15:20:22.735 drvModbusAsyn::doModbusIO port SP1_Reg_In_2 error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:20:22.815 drvModbusAsyn::doModbusIO port SP1_Reg_In_1 error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:20:22.845 drvModbusAsyn::doModbusIO port SP1_Bit_In error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:20:30.769 drvModbusAsyn::doModbusIO port SP1_Reg_In_3 error calling writeRead, error=gse-isco1:502 timeout: Resource temporarily unavailable, nwrite=6/6, nread=0
2021/02/26 15:21:10.888 drvModbusAsyn::doModbusIO port SP1_Reg_In_2 writeRead status back to normal having had 13 errors, nwrite=6/6, nread=126
2021/02/26 15:21:10.968 drvModbusAsyn::doModbusIO port SP1_Reg_In_3 writeRead status back to normal having had 11 errors, nwrite=6/6, nread=94
2021/02/26 15:21:11.318 drvModbusAsyn::doModbusIO port SP1_Reg_In_1 writeRead status back to normal having had 14 errors, nwrite=6/6, nread=202
2021/02/26 15:21:12.138 drvModbusAsyn::doModbusIO port SP1_Bit_In writeRead status back to normal having had 14 errors, nwrite=6/6, nread=20
2021/02/26 15:24:34.655 drvModbusAsyn::doModbusIO port SP1_Reg_In_1 error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:24:34.735 drvModbusAsyn::doModbusIO port SP1_Reg_In_3 error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:24:34.765 drvModbusAsyn::doModbusIO port SP1_Bit_In error calling writeRead, error=asynManager::queueLockPort queueRequest timed out, nwrite=0/6, nread=0
2021/02/26 15:24:42.681 drvModbusAsyn::doModbusIO port SP1_Reg_In_2 error calling writeRead, error=gse-isco1:502 timeout: Resource temporarily unavailable, nwrite=6/6, nread=0
2021/02/26 15:25:42.857 drvModbusAsyn::doModbusIO port SP1_Reg_In_3 writeRead status back to normal having had 20 errors, nwrite=6/6, nread=94
2021/02/26 15:25:42.897 drvModbusAsyn::doModbusIO port SP1_Bit_In writeRead status back to normal having had 20 errors, nwrite=6/6, nread=20
2021/02/26 15:25:43.287 drvModbusAsyn::doModbusIO port SP1_Reg_In_1 writeRead status back to normal having had 17 errors, nwrite=6/6, nread=202
2021/02/26 15:25:44.087 drvModbusAsyn::doModbusIO port SP1_Reg_In_2 writeRead status back to normal having had 15 errors, nwrite=6/6, nread=126

This is not acceptable and needs to be tracked down.

Note that this happens even when no Modbus write operations are being performed.