There are three reasons why the DS1 LED, the board monitor alarm, will be illuminated. First, upon completion of the self test (please refer to the Functional Test (Hardware) section of the PC21 User Guide), secondly, if the RAM Speed Selection Jumpers are not set properly, and lastly, if there is a hardware failure on the PC21. The last option requires the board be returned for repair.
Jumpers JU3 and JU4 on the PC21 determine the RAM speed selection for the firmware. Only one of the two jumpers should be shunted at any time. The manual states in the Functional Test (Hardware) section, that JU3 should be set for factory default. This is NOT necessarily true. JU3 should be jumpered for older PC21 boards. JU4 should be jumpered on newer boards.
The quickest way to determine if you have a possible jumper conflict is if the DS1 LED immediately goes red on power up. If the board is in self test mode (see above), there will be a short delay (about 2 seconds) before the light illuminates, which is proper operation. If the first condition exists, power down the PC21, move the jumper to the other position, and power back up the PC21. If it still immediately illuminates, then there is a hardware failure.
The most common way this jumper gets set improperly is an end user changes the JU3 or JU4 jumper to match his current system when replacing a PC21. This jumper should remain as set by the factory regardless of the old boards setting.