Black Box MD1641A, MD1640A manual Introduction, Serial Port Limitations

SERIES II INTELLIGENT DATA/FAX MODEMS

3.Software Configuration

3.1Introduction

Since your communications software configuration is affected by the capabilities of your computer, this chapter begins with a discussion of the limitations of some serial ports and how to identify them. It then discusses communications configuration in general and recommends settings specifically for the Series II Modem.

3.2 Serial Port Limitations

When you configure your software, you need to consider how the hardware on both ends of the connection will affect the connection. Some serial ports, particularly those in older PC-compatible computers, may limit the performance of the Series II Modem. You should know if yours is one of them.

The limiting factor is an integrated circuit called a Universal Asyn- chronous Receiver/Transmitter, or UART. All data from your modem flows through it. The UARTs typically used in PC-compatible computers are types 8250, 8250A, 16450, and 16550AFN. The 8250 is unreliable above 9,600 bps, and the 8250A and 16450 are unreliable above 19,200 bps. If the modem sends data to the UARTs above those speeds, the UARTs may not be able to process the data fast enough to keep from losing some of it. The 16550AFN, however, can safely handle data to 115,200 bps.

When a modem communicates with V.42bis 4-to-1 compression en- abled, it sends up to four times as much uncompressed data to the serial port as it receives compressed over the telephone line. Therefore, a modem communicating at 14,400 bps may require a serial port that can reliably transfer data at four times 14,400 bps, or 57,600 bps; at 19,200 bps it may require one that works reliably up to four times 19,200 bps, or 76,800 bps; and at 28,800 and 33,600 bps it may require one that works reliably up to a maximum of 115,200 bps. If your serial port cannot handle these speeds, we recommend that you replace your present serial card with one that has a 16550AFN UART or equivalent.

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