Legacy Serial Port Requirements

This section defines PC 98 requirements for legacy serial ports. Legacy ports are not recommended for PC 98 systems, but if implemented, such ports must meet the requirements defined in this section.

10. Legacy serial port is implemented as 16550A UART or equivalent and supports 115.2K baud
Required

A 16550A buffered Universal Asynchronous Receiver/Transmitter (UART) or equivalent buffered legacy serial port is required to support high-speed communications while reducing the CPU requirements for servicing the device. The device must be able to support 115.2K baud.

11. Legacy serial port supports flexible resource configuration and dynamic disable capabilities
Required

A legacy serial port must provide flexible resource configuration and complete dynamic disable capabilities as defined in the Plug and Play External COM Device Specification, Version 1.0.

These are the recommended resource settings for non-PCI devices:

  • Four I/O locations for each port, where the standard ISA I/O addresses are 3F8h, 2F8h, 3E8h, 2E8h. Using the standard addresses ensures the proper functioning of software that directly addresses these locations.

  • Two IRQ signals, where the standard is programmable interrupt controller-based (PIC-based) IRQ 3, IRQ 4. Using the standard IRQ signals ensures the proper functioning of software written for systems that use standard IRQ signals.

Two IRQs are required for each port. If two serial ports are implemented in the system, the IRQs can be assigned as follows:

  • For serial port A: PIC-based IRQ 4 and IRQ 11

  • For serial port B: PIC-based IRQ 3 and IRQ 10

An IR adapter port might replace a serial port in a system. In such a case, the IR port should use the resource configuration that would otherwise be assigned to the second serial port.

Notice that, as for all devices, IRQ sharing is required if the minimum resource requirement cannot be met.

12. Conflict resolution for legacy serial port ensures availability of at least one serial port

Required

In the event of an irreconcilable conflict with other serial ports on the system, a legacy serial port must be capable of being disabled by Plug and Play software. This allows at least one of the two conflicting serial ports to operate correctly.