Fresenius Medical Care 4008 E Technical Manual page 352

Serial interface module
An 85C30 serial interface module SCC is mounted on the P.C.B. for the purpose of serial
communication with external peripheral equipment. This module is designed so that it can control
two serial transmission channels in the full-duplex mode.
The transmission parameters for channel A and channel B are as follows:
– Transmission rate of 9600 baud
– 1 start bit
– 8 data bits
– 1 stop bit
– No parity bit
Channel A is reserved for the communication with CPU 2 (P.C.B. LP 632). It operates with TTL
level.
Channel B is reserved for connections to external peripheral equipment. The insertion of a V.24
level converter (MAX232) into the transmitter and receiver line permits unrestricted operation on
the V.24 level.
For channel B, the XON/XOFF method is used as handshake record.
Since the processing speed of the microcontroller during the transmission sequences should not
be reduced too strongly by status requests (Polling Mode), the major part of the communication
is performed in the interrupt operating mode.
To this end, the interrupt input of the SCC is monitored by the interrupt controller provided in the
80C188 (INT0). Upon the arrival of an interrupt request, i.e. a data byte is ready for being
retrieved or the transmitter buffer is empty, the SCC unit which has released the interrupt is
determined, and the respective function will be called up.
The data transfer between the transmitter buffers in the RAM and the respective registers in the
SCC is now performed by means of the interrupt treatment function. As soon as the data transfer
has been completed, the interrupt treatment function will be terminated and the machine returns
to the program which has been interrupted.
In addition, the communication via channel A is used as technical safety device, i.e. the software
watchdog. The software watchdog operates as a complement to the hardware watchdog. It is
provided for monitoring the cooperation between the two processor boards CPU 1 and CPU 2.
Here, the communication between the two computers via a serial interface and according to a
fixed communication protocol is taken as a basis.
Communication consists of the transmission of data records (10 bytes length) and the evaluation
of a response from the receiver.
Detailed structure of a data record with the venous pressure taken as an example:
Identification Blank
(3 bytes) (1 byte)
VEN –
Alarm status Sign
(1 byte) (1 byte)
H +
Fresenius Medical Care 4008 4/09.03 (TM)
Value Checksum
(3 bytes) (1 byte)
400 X
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