Serial Port Hardware Specs

One of the Newton 2.x OS Q&As
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This document was exported on 7/23/97.

Serial Port Hardware Specs (4/9/97)

Q: What are the hardware specifications for the serial port?

A: In the Apple MessagePad 100, 110, 120, 130, 2000, the eMate 300, the Sharp ExpertPad, and the Motorola Marco, the serial port is an EIA standard RS-422 port with the following pinout (as viewed looking at the female Mini-DIN-8 socket on the side of the Newton device, or looking at the female Mini-DIN-9 on the Newton Serial Adapter):

<see diagram>

Pin 1    HSKo                /DTRPin 2    HSKi                /CTSPin 3    TxD-                /TDPin 4    GND                Signal ground connected to both logic and chassis ground.Pin 5    RxD-                /RD Pin 6    TxD+                (see below)Pin 7    GPi                General purpose input received at SCC's DCD pin.Pin 8    RxD+                (see below)Pin 9 Power out          5V@100ma.  This pin only exists on the eMate 300 and the Newton Serial Adapter.


All inputs are:
Ri 12K ohms
minimum Vih 0.2v, Vil -0.2V
maximum tolerance Vih 15V, Vil -15V

All outputs are:
Rl 450 ohms
minimum Voh 3.6V, Vol -3.6V
maximum Voh 5.5V, Vol -5.5V

No more than 40mA total can be drawn from all pins on the serial port. Pins 3 & 6 tri-state when SCC's /RTS is not asserted.

The EIA RS-422 standard modulates its data signal against an inverted (negative) copy of the same signal on another wire (twisted pairs 3/6 & 5/8 above). This differential signal is compatable with older RS-232 standards by converting to EIA standard RS-423, which involves grounding the positive side of the RS-422 receiver, and leaving the positive side of the RS-422 transmitter unconnected. Doing so, however, limits the usable cable distance to approximately 50 feet, and is somewhat less reliable.

The MessagePad 120 and the MessagePad 130 use a Linear Technology LTC902 serial line driver. This part drives +5/-5 nominally for the RS422 signals, and you can use just one half to interconnect with RS232 compatible signal levels.

The MessagePad 2000 and the eMate 300 use a Linear Technology LTC1323 serial line driver. This part drives +5/-5 nominally for the RS422 signals, and you can use just one half to interconnect with RS232 compatible signal levels.
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