CCA2D2 Serial Interface Cabling

Both CCA2D2 interfaces, like a PC, are DTE device and so normally need a null modem cable to connect them to a computer.  They were designed this way so that they could more easily connect to handheld computers.  The CCA2D2 v1 interface uses a DB9 female connector with the same pin designations as the serial port on a PC.  This means that if you have a Palm or PocketPC serial cable for synching to a PC it will directly connect to the CCA2D2 v1 interface.

The CCA2D2 v2 interface uses an RJ-11 modular telephone jack to provide the serial connection.  We used this connector in order to make the enclosure smaller.  For the Palm we provide a short cable to directly connect the Palm to the interface.  In order to connect the interface to other computers easily an adaptor assembly is needed.  This consists of a short length of modular telephone cable connected to an RJ-11 to DB9 Male adaptor.  This adaptor will directly connect to an handheld through it's serial synch cable.  To connect to a PC you will need a DB9 Female to DB9 Female null-modem cable.  To connect to a Macintosh serial port you will need a custom serial cable described below.

Macintosh Serial Port Cable Description

The custom cable used to connect the interface to the Mac is a MiniDin8 male to DB9F with connections as follows:

DB9 Female	MiniDIN-8 Male	Mac Use
2	3	TX-
3	5	RX-
5	4	GND
8	1	Handshake Output
	*8	RX+

* MiniDIN-8 Male Pin 8 is connected to Gnd (MiniDin8 Male Pin 4) thru 2 series connected 1N4148 diodes (cathode to Pin 4). This shifts the Mac's receive threshold to about +1.2V, which is somewhere near the middle of the interface transmitter's range.

Technical Background

1. The transceiver chip we are using in the interface is a differential 3.3V RS485 chip, but we are using it in the single ended mode. This is out of spec with the RS232 or RS422 standards, so we have to hope and play some games with the various machines. So far, all the PC type RS232 ports we've tested seem happy with this.  As well as Palms and iPaq and Casio Pocket PCs.
2. Measuring several older Mac differential RS422 serial port input (7600, PowerBook G3) we found the unterminated input voltages being +3.6 V for Rx- input and +4.2 V for RX+.
3. Using this single ended device to drive a differential receiver's Rx- input entails deciding what to do with the Rx+ input.
4. RS422 is spec'ed for a ±200 mV threshold, so if we tie the MAC's Rx+ to ground (the normal procedure), the device's output would have to cover the 0V ±200 mV range to guarantee reliable operation. If we leave the MAC's Rx+ floating, the device's output would have to cover the 4.2 ±200 mV range to guarantee reliable operation. Neither situation is possible with a 3.3 V supply.
5. The problem is solved by tying the MAC's Rx+ pin to somewhere near the middle of the 0->3.3 V range using 2 diode drops and the pullup resistor in the MAC's port. This may not work on other types of MACs if the port doesn't have pullup resistors on Rx+.

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