Technical Q&A DV31

Speed of the Printer Port

Q: In Inside Mac:Devices, it says:

Because the serial hardware in some Macintosh computers relies on processor interrupts during I/O operations, overrun errors are possible if interrupts are disabled while data is being received at the serial port. To prevent such errors, the Disk Driver and other system software components are designed to store any data received by the modem port while they have interrupts disabled, and then pass this data to the port's input driver. Because the system software only monitors the modem port, the printer port is not recommended for two-way communication at data rates above 300 baud.

Is this still true for computers which use the Serial DMA driver? Is printer port still slower than modem port?

A: Practically speaking, the printer port is probably no less capable than the modem port of maintaining high baud rates if it has DMA on the receive channel. Note that on the 68K AV Macs, which do use SerialDMA, there is still no DMA on the receive channel, so the printer port is less capable on those machines.

On the other hand, there are internal prioritization algorithms for the SCC which dictate that channel A (modem port) takes priority over channel B (printer port), so if you're trying to do something on both ports simultaneously and channel A is completely saturated (a term which I use loosely because I am at a loss to define it accurately), then port B could theoretically be starved.