Tandy 102 Service Manual page 53

11. RS-232C interface doesn't function
11
Check the transmit side.
Check if the switching digital signal (±5V - ±3.5V) is output to connector pin 2
during transmission. Then check if the CTS signal of pin 5 is low level.
(If not output, check M22, M24, M35 , C71, C72 and C73.)
Check the receive side.
Check if a digital signal is input to M22 pin 20 (RRI terminal) during data reception.
Check also to be sure that the RTSR signal of pin 4 is low level.
(If not emitted, check M22, M24, M33, D9, D8 and D10.)
Check the RS-232C select signal.
Check if PB3 port (pin 32) of M25 is low level.
(If not, check M25 and M34.)
Check the RS-232C connector and cable.
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