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USB Synchronous to Asynchronous Interface

USB Sync Interface

The USB Synchronous to Asynchronous Interface accepts synchronous RS-232 level clock and data signals on a DB-25 connector, such as those defined in MIL-188C or MIL-114, and sends the data bits through the USB bus. Data bits are packaged eight per byte, with the LSB representing the first bit, and the MSB the last. Clock rates up to 9,600 baud are supported, and may be any value as the data is captured on edge of the clock signal. The unit is for reception of data from a modem, not transmitting.

The following pinout on the DB-25 connector is used:

Pin		Function
4		Data
7		Ground
20		Clock

A "data slicer" mode is also available, where the data line is sampled at a fixed rate, and the bits are packaged as above and sent over the USB link.

The unit is configured by sending bytes to it over the USB link. The high nibble of a byte represents the command, the low nibble is the data. The following commands are supported:

CMD		Function
0		Reset bit counter to zero
1		Sets the edge from the LSB of the data, 1 means falling, 0 means rising
2		Sets the polarity from the LSB of the data, 0 means inverted, 1 means normal
3		Sets slicer mode if LSB is 1, turns slicer mode off if LSB is 0
4		Sets low nibble of slicer divisor (16 bits total)
5		Sets second nibble of slicer divisor
6		Sets third nibble of slicer divisor
7		Sets high nibble of slicer divisor and latches new rate
8		Sets slicer rate prescaler: 0=1, 1=2, 2=4, 3=8
9		Reserved
A		Reserved
B		Reserved
C		Reserved
D		Reserved
E		Reserved
F		Reserved

The slicer rate is computed by dividing 2,000,000 by the prescaler (1, 2, 4, or 8) and by the divisor (1 to 65535). For example, With a prescaler of 8 and a divisor of 125, the slicer rate is 2000 Hz.

The USB driver makes the unit emulate a normal serial port, so that the usual operating system functions work as usual, as per the operating system.

Communications over the emulated (USB) serial port should be 38,400 baud, 8 data bits, 1 stop bit, no parity.

The DTR line over the USB link should be asserted for normal operation, it can be momentarily de-asserted to reset the microcontroller in the unit. The RTS line is ignored.

Please contact us regarding availability of this product.

Looking for a way to connect your computer to your Icom rig? Take a look at the Black Cat Systems USB Icom CI-V Interface

Application Notes for interfacing to specific equipment:

HALComm ST-8000 HF modem

Rockwell MDM-2001 HF modem Download Area:

USB Drivers (note the FTDI driver is built into Linux kernel 2.4.20 or greater):

Windows 2000/XP FTDI USB Driver

Windows 98/ME FTDI USB Driver

Mac OS X FTDI USB Driver Version 2.1.0

Intel Mac OS X FTDI USB Driver - Requires 10.4 (Tiger) Version 2.2.0

Mac OS 8/9 FTDI USB Driver

These test programs let you send commands to the interface, as well as view data coming back. They are purely meant for diagnostics purposes, to verify the unit is functional and receiving data and clock signals.

Windows Sync program - Talks to the unit over the USB bus for basic testing purposes.

Mac OS X Sync program - Talks to the unit over the USB bus for basic testing purposes.

Mac OS Sync program - Talks to the unit over the USB bus for basic testing purposes.

Linux Sync program - Talks to the unit over the USB bus for basic testing purposes.

The following program lets you use a PC running under DOS (not a DOS window in Windows but really in DOS) to send synchronous data to the unit for testing purposes. A com port at 0x3bc is assumed.

SYNCTEST program - Runs on a PC in DOS mode to test the unit via the RTS and DTR handshake lines.

Email your comments and questions to

Last modified February 8, 2008