Multiple printers are connected to separate serial ports (that is, one printer on COM1, another on COM2, etc.). Jobs are sent to all of the printers, but one of the printers is turned off. It is possible that none of the printers will print until the printer that was turned off is turned on.

The reason for this is that the "Concurrent Access" option in the printer configuration is set to "N". All printers that have the "Concurrent Access" set to "N" are managed by a single port sharing structure. If this single port sharing structure is acquired by the printer that is turned off, none of the other jobs will proceed until the port sharing structure is released. Because the printer is turned off, the thread driving the printer will not release the structure.

Most serial port printers should have the "Concurrent Access" option set to "Y". This will allow each port to have its own port sharing structure, avoiding this problem. The only case where this will not work is for standard serial ports combinations of COM1/COM3 or COM2/COM4. If you are using COM1 and COM3 (or COM2 and COM4) at the same time, both of the printers connected to these ports must have the "Concurrent Access" set to "N". This is because COM1 and COM3 share an interrupt, as do COM2 and COM4. Multi-port serial cards such as the DigiBoard and RocketPort cards should have the ports set "Concurrent Access" to "Y". The hardware on these cards and the device drivers make sure that there are no interrupt conflicts.