Difference between Turbo and Standard Mode on the EtherLink 16

This document explains the difference between the Turbo and Standard modes of operation for the EtherLink 16 and 16TP (3C507 and 3C507TP) adapters. The parameter for mode of operation is set via the 3C507.EXE program.

To understand the parameter's operations, it is necessary to understand why incompatibilities can occur between some PCs and a 3C507 adapter in Turbo mode. The ISA Bus Specification has a flaw that will not allow true 16-bit devices to function correctly when co-existing in the same 128 KB page of shared memory as an 8-bit device. The problem is that the 16-bit device will respond to any shared memory calls in this range, whether directed to the 16-bit device or the 8-bit device, causing both devices to fail. The problem is often seen when an 8-bit VGA adapter resides in the memory area ending at C7FF, and the 3C507 uses shared memory in that same 128 KB page ending at DFFF. To overcome the problem of the specification, the 3C507 and 3C507TP provide both Turbo and Standard modes of operation, which allow control of the SAD and LAD bits in the adapters' configuration registers.

STANDARD MODE: Standard mode is the 3C507's default setting. It sets the adapter to use 8-bit data transfers only, making it compatible for all ISA machines. However, Standard mode also limits the adapter to the speed and throughput of an 8-bit adapter, unless 3Com drivers are used. As of this writing, only 3Com drivers have been tuned to allow 16-bit data moves even in standard mode, by disabling the CPU interrupts during data transfer. This process is called Standard/16 mode and is transparent to the user.

TURBO MODE: Turbo mode allows the adapter to function as a full 16-bit adapter. It may be used with machines that support modified ISA bus sequencing, which most PC manufacturers do support. In Turbo mode, the 3C507 cannot reside in the same 128 KB region as an 8-bit add-in device.

Following is a list of vendors who do NOT support the modified ISA bus sequencing standard. On these computers, data corruption may result when the 3C507 is configured in Turbo mode:

IBM PS/2 Model 30/286, 25/286
Siemens computer system
Intel 25 MHz Intel386 MicroComputer Model 302
VLSI Technology, Inc. (VTI) chipset
Wyse WY-2012I