JVL MAC050 User Manual page 279

5.9 Bus serial communication
The MAC00-xx modules translate between the 5V (TTL level) RS-232 used by the basic
motors (MAC050, MAC095, MAC140, MAC141, MAC800) and standard RS-485 and
RS232 signals
Since the RS-485 interfaces use the standard two-wire coupling, data can travel in only
one direction at a time, witch means that the line is either in a state where all nodes
'listen' or in a state where one node transmits while all the other nodes 'listen'.
Ideally the node that transmits should stop driving the lines as soon as the last (stop) bit
in the last byte it wants to transmit has been sent out. This would allow the other nodes
to start transmitting as soon as possible.
It is a classical problem that a device that translates between RS-232 and two-wire RS-
485 does not have access to a hardware signal that tells it when to change transmit/listen
direction.
On the MAC00-xx modules, the direction is controlled by monitoring activity on the RS-
232 transmit signal and drive the RS-485 line for one byte-time at 19200 baud after the
last activity was detected.
On the MAC00-xx modules, the direction is controlled by monitoring activity on the RS-
232 transmit signal and drive the RS-485 line for one byte-time at 19200 baud after the
last activity was detected.
Depending on the data in the last byte transmitted, the line may then be reased from just
after the last bit has been transmitted or not until a full byte length after the last byte has
been transmitted.
It is required that all other nodes on the RS-485 bus do not start transmitting until a full
byte-time has passed since the last byte of a telegram from a MAC00-xx module has been
transmitted.
It is a common requirement in many protocols to require a pause of 1.5 bytes-times or
more between telegrams. With the JVL MacTalk protocol, this pause can be considered
to be two byte-times at 19200 baud and six byte-times at 57600 baud.
JVL Industri Elektronik A/S - User Manual - Integrated Servo Motors MAC050 - 800 275